Sun Yangyang 200612 Phd

8/10/2019 Sun Yangyang 200612 Phd

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STUDY ON THE NANOCOMPOSITE

UNDERFILL FOR FLIP-CHIP APPLICATION

A ThesisPresented toThe Academic Faculty

by

Yangyang Sun

In Partial Fulfillmentof the Requirements for the Degree

Doctor of Philosophy in theSchool of Chemistry and Biochemistry

Georgia Institute of TechnologyDecember, 2006


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STUDY ON THE NANOCOMPOSITE UNDERFILL FOR FLIP-CHIP

APPLICATION

Approved by:

Dr. C. P. Wong, AdvisorSchool of Materials Science andEngineeringGeorgia Institute of Technology

Dr. Rigoberto HernandezSchool of Chemistry and BiochemistryGeorgia Institute of Technology

Dr. Karl JacobSchool of Polymer, Textile and Fiber

EngineeringGeorgia Institute of Technology

Dr. Boris MizaikoffSchool of Chemistry and Biochemistry

Georgia Institute of Technology

Dr. Z. John ZhangSchool of Chemistry and BiochemistryGeorgia Institute of Technology

Date Approved: November 8, 2006


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ACKNOWLEDGEMENTS

I would like to express my sincere gratitude to my advisor, Dr. C. P. Wong for his

guidance, inspiration, and financial support throughout the course of this research. Iwould like to extend my gratitude to Dr. Rigoberto Hernandez, Dr. Karl Jacob, Dr. Boris

Mizaikoff and Dr. Z. John Zhang for serving on my Ph. D committee as well as providing

invaluable instructions and recommendations.

I would like to thank the faculty and staff members in the National Science

Foundation Microsystems Packaging Research Center, the School of Chemistry and

Biochemistry, and the School of Materials Science and Engineering. They are Professor

Rao R. Tummala, Professor Jianmin Qu, Professor Suresh Sitaraman, Professor, Z. L.

Wang, Professor David Collard, Dr. Cam Tyson, Dr. Mira Josowicz, Dr. Leyla Conrad,

Mr. Dean Sutter, Ms. Yolande Berta, Ms. Vicki Speights, Ms. Mechelle Kitchings, Mr.

James Cagle, and Mr. Tim Banks.

My special thanks go to my fellow co-workers in Dr. Wongs group, for all the

discussions and helps I received from Dr. Lianhua Fan, Dr. Kyoung-sik Moon, Dr.

Shijian Luo, Dr. Haiying Li, Dr. Zhuqing Zhang, Dr. Jianwen Xu, Dr. Fei Xiao, Dr. Hai

Dong, Dr. Brian Englert, Mr. Suresh Pothukuchi, Ms. Lara Martin, Ms. Yi Li, Ms.

Jiongxin Lu, Mr. Lingbo Zhu, Mr. Hongjin Jiang, Mr. Yonghao Xiu, Mr. Brian Bertram,

Ms. Jessica Burger, Ms. Gusuel Yun. I would like to thank the undergraduate students

and high school intern students who worked with me during the PhD study. They are Mr.

Jonathan Peak, Mr. Jerry Grimes, Mr. David Lorang, Ms. Qian Wan, Ms. Elizabeth

Varner.Special appreciation is extended to Texas Instruments and Indium Corporation of

America for their interests and supports of this work, and also to Hexion Specialty

Chemicals, Hanse Chemie, and Lindau Chemicals for their supply of the materials. This


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work is funded by National Science Foundation through Packaging Research Center of

Georgia Tech.

Finally, I would like to thank my parents, my brother, and my friend Zhimin Song

for their continuous support and encouragement. Without them, this dissertation would

not be possible.


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TABLE OF CONTENTS

ACKNOWLEDGEMENTS III

TABLE OF CONTENTS V

LIST OF TABLES IX

LIST OF FIGURES XI

SUMMARY XVII

CHAPTER 1. INTRODUCTION 1

1.1. ELECTRONIC PACKAGING AND FLIP-CHIP TECHNOLOGY 11.1.1. Packaging technology development 1

1.1.2. Flip-chip technology 31.2. UNDERFILL MATERIALS AND NANO SIZE FILLER 5

1.2.1. Overview of underfill materials 51.2.2. Underfill classifications 71.2.3. Composition of epoxy underfill 101.2.4. Filler in the underfill 14

1.3. PARTICLE DISPERSION 181.3.1. Energy state of particle in the medium 181.3.2. Attractive force 191.3.3. Repulsive force 201.3.4. Filler stabilization in underfill 22

1.4. IMPACT OF NANOPARTICLES ON THE RHEOLOGY 251.4.1. Definition of viscosity 251.4.2. Einstein Equation for dilute suspension 271.4.3. Kreigher-Dougherty Equation for concentrated suspension 271.4.4. Particle size effect to viscosity 29

1.5. RESEARCH OBJECTIVES 31

CHAPTER 2. NANOSILICA SYNTHESIS AND MODIFICATION 34

2.1. SILICA SYNTHESIS 342.1.1. Pyrogenic silica 34

2.1.2. Sol-gel method 372.1.3. Size control of nanosilica by Stber method 402.2. SURFACE MODIFICATION OF SILICA BY SILANE 44

2.2.1. Contact angle and surface wetting 442.2.2. Silane coupling agent 47

2.3. EXPERIMENT 492.3.1. Material 492.3.2. Surface tension measurement after treatment 50


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2.3.3. Surface modification of nanosilica 512.3.4. Particle characterization 512.3.5. Underfill composite preparation and characterization 52

2.4. RESULTS AND DISCUSSION 532.4.1. Surface tension measurement of silicon dioxide after treatment 53

2.4.2. Optimal experimental conditions for nanosilica modification 572.4.3. Characterizations of treated nanosilica 642.4.4. Viscosity of nanocomposite no-flow underfill 71

CHAPTER 3. MATERIAL PROPERTIES CHARACTERIZATION OF THENANOCOMPOSITE UNDERFILL AFTER CURING 73

3.1. EXPERIMENTS 733.1.1. Materials 733.1.2. Underfill composite preparation 743.1.3. Underfill composite characterization 75

3.2. RESULTS AND DISCUSSIONS 773.2.1. Anhydride epoxy polymerization mechanism 773.2.2. Curing Behaviors and Tg of composite underfills 793.2.3. Rheological and optical behavior of composite underfills 813.2.4. Thermal mechanical properties 833.2.5. Moisture absorption and density measurement 863.2.6. Morphology 913.2.7. Wetting test 92

3.3. GLASS TRANSITION AND RELAXATION BEHAVIORS OFNANOCOMPOSITES 943.3.1. Experiments 953.3.2. Characterization 963.3.3. Results and discussion 97

CHAPTER 4. INFLUENCE OF INTERPHASE AND MOISTURE ON THEDIELECTRIC SPECTROSCOPY OF EPOXY/SILICA COMPOSITES 108

4.1. DIELECTRIC PROPERTIES OF COMPOSITE MATERIALS 1084.1.1. Theory and background 1084.1.2. Existing dielectric study for composite material 1124.1.3. Dielectric properties measurement 113

4.2. RESULTS AND DISCUSSIONS 1144.2.1. Dielectric properties 1144.2.2. TTS shifting of dielectric loss curve 116

4.2.3. Dielectric loss in composites 1194.2.4. Moisture influence for dielectric properties 122

CHAPTER 5. THE HARDENER EFFECTS TO COLLOIDAL SILICADISPERSION 127

5.1. EXPERIMENT 1285.1.1. Materials 128


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5.1.2. Dynamic rheology 1295.1.3. Dielectric constant of liquid sample 131

5.2. RESULTS AND DISCUSSIONS 1315.2.1. Rheology measurement 1315.2.2. Van der Waals interaction 137

CHAPTER 6. PHOTO-POLYMERIZATION OF EPOXY NANOCOMPOSITE FORWAFER LEVEL APPLICATION 143

6.1. PHOTO-POLYMERIZATION OF EPOXY 1436.2. EXPERIMENTS 145

6.2.1. Materials 1456.2.2. Preparation of nanocomposites 1466.2.3. Characterization 146

6.3. PREPARATION OF PHOTO-CURABLE NANOCOMPOSITES 1516.3.1. Filler size of nanosilica 1516.3.2. UV absorption of compositions in the photo-curable nanocomposite 152

6.4. REACTION MECHANISM AND KINETICS OF PHOTO-CURABLE NANOCOMPOSITE1556.4.1. Mechanism of cationic photo-polymerization 1556.4.2. Reaction process measured by real-time FTIR 1576.4.3. Two-steps curing of underfill by cationic photo-polymerization 1606.4.4. Reaction kinetics of underfill by photo-polymerization 164

6.5. MATERIAL PROPERTIES CHARACTERIZATION 1706.5.1. Optical properties 1706.5.2. Glass transition temperature 1736.5.3. Thermal degradation behavior 1746.5.4. Thermal expansion 1756.5.5. Thermal mechanical properties of photo-cured nanocomposites 176

6.5.6. Nanocomposite morphology 1796.5.7. Surface hardness 180

6.6. APPLICATION OF PHOTO-CURABLE EPOXY NANOCOMPOSITE IN WAFER LEVELPACKAGING 182

6.6.1. Novel wafer level packaging process 1826.6.2. Advantages of photo-curable nanocomposites 1856.6.3. Pattern formation with photo-curable nanocomposite 186

CHAPTER 7. CONCLUSIONS AND SUGGESTED WORK 190

7.1. CONCLUSIONS 190

7.2. SUGGESTED WORK 1957.2.1. Chemical bond between filler and epoxy matrix 1957.2.2. Molecular level reinforcement in epoxy 1977.2.3. High performance polymer matrix 1997.2.4. Nanocomposite polymeric optical waveguide 200

APPENDIX A AUTHORS AWARDS, PATENTS, AND PUBLICAITONS 201


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REFERENCE 207


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LIST OF TABLES

Table 1-1. Trend in the microelectronic manufacturing 3

Table 1-2. Coefficient of thermal expansion of major materials in flip-chip packaging 6

Table 1-3. Underfill classification 10

Table 1-4. List of Epoxy resin used in this study 11

Table 1-5. List of curing agents used in this study 12

Table 1-6. Lists of catalysts used in this study 14

Table 1-7. Bulk resistivity of underfill formulation (before curing) 23

Table 2-1. Physical, mechanical, thermal and electrical properties of silica 35

Table 2-2. Ingredients for sol-gel synthesis silica 40

Table 2-3. Chemistry structure of silane coupling agents 50

Table 2-4. Contact angles (degree) of three probe liquids and epoxy on SiO2 surfaces atdifferent treatment conditions 56

Table 2-5. Critical surface tension of SiO2 surfaces with different silane treatment 56

Table 2-6. DOE of modification condition 58

Table 3-1. Chemicals used in the underfill formulations 74

Table 3-2. Moisture absorption kinetics parameter 89

Table 4-1. Constant parameters of WLF equation for three samples 119

Table 5-1. Matrix composition of underfill with different hardener 129

Table 5-2. Summary of dynamic rheology of different systems 135

Table 5-3. Bulk material properties for various components 138

Table 6-1. Comparison between two photo-polymerization approaches 144

Table 6-2. Observed peaks of epoxy with FTIR 157

Table 6-3. Reaction heat and conversion for the nanocomposite measured by photo-DSCand thermal-DSC 163


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Table 6-4. Kinetics data for the photo-polymerization of nanocomposite underfill 167

Table 6-5. Light absorptivity and components concentration in the nanocompositeunderfill 167

Table 6-6. TGA measured for various filler loadings in the nanocomposites 174

Table 6-7. Materials Constant 179


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LIST OF FIGURES

Figure 1-1. Scheme of electronic packaging hierarchy 2

Figure 1-2. General configuration of wire bonding package 4

Figure 1-3. General configuration of flip-chip package 4

Figure 1-4. Conventional capillary flow underfill process 8

Figure 1-5. No-flow underfill process 8

Figure 1-6. Curing mechanism of primary amine 13

Figure 1-7. Scheme of underfill flow and possible filler clog between chip and substrate 14

Figure 1-8. Micron size silica entrapped between the solder and the contact pads 16

Figure 1-9. Optical microscope picture of the flip-chip assembly with nanocomposite no-flow underfill 16

Figure 1-10. Energy diagram of particle surface with distance 19

Figure 1-11. Electrostatic force in the dispersion system with ionic strength 21

Figure 1-12. Adsorption-dissociation mechanism of ions on the silica surface in theaqueous medium 21

Figure 1-13. Steric stabilization of particles by adsorbed polymer 21

Figure 1-14. Viscosity definition model 26

Figure 1-15. Viscosity of concentrated suspensions 28

Figure 1-16. Calculated viscosity at low shear rate as a function of particle diameter: (1)100nm; (2) 200nm; (3) 300nm; (4) high-shear limit 30

Figure 1-17. Viscosity of underfill with silica filler (nanosilica: 100nm; micron silica:

3m, theoretical calculation is based on the Equation 1-8) 30Figure 2-1. Synthesis of fumed silica 36

Figure 2-2. TEM picture of fumed silica structure[56] 37

Figure 2-3. Reaction process of sol-gel method for silica generation (with basic catalyst) 38


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Figure 2-4. Polymerization behavior of silica [60] 39

Figure 2-5. SEM picture of silica synthesized with different ammonia concentration (a)0.1M, (b) 0.3M, (c) 0.6M, (d) 1.0M (magnification: 20,000) 42

Figure 2-6. Particle size distribution of as-synthesized silica 43

Figure 2-7. Relation between particle size and ammonia concentration 43

Figure 2-8. Relationship between interfacial tension and contact angle 45

Figure 2-9. Wetting phenomenon of silica filler in the underfill 46

Figure 2-10. General structure of silane coupling agents 48

Figure 2-11. Scheme of surface modification for nano-size filler 49

Figure 2-12. Proposed mechanism for the silane reaction onto the glass slides 54Figure 2-13. Zisman plot to determine the critical surface tension 55

Figure 2-14. Nanosilica dispersion with different pre-treatments 59

Figure 2-15. Nanosilica dispersion with epoxy-silane treatments 59

Figure 2-16. Average size of nanosilica with different treatment conditions 60

Figure 2-17. Reaction mechanism of silane treatment to nanosilica surface 61

Figure 2-18. Nanosilica dispersion with amino-silane treatments and with sonication 62Figure 2-19. Dispersion of #004 (amino silane treated) 63

Figure 2-20. Dispersion of #007 (epoxy silane treated) 63

Figure 2-21. Dispersion of #007 (enlarged) 64

Figure 2-22. FTIR spectra of nanosilica with different surface modification 65

Figure 2-23. Physical water decreases and silanol groups condense [83] 67

Figure 2-24. Re-absorption of physical water below 400C[83] 67

Figure 2-25. Irreversible elimination of adjacent silanol group [83] 68

Figure 2-26. Weight loss of silica at different drying condition 70

Figure 2-27. Weight loss of nanosilica with different surface modification 70

Figure 2-28. Viscosity of nanocomposite underfills 71


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Figure 3-1. Reaction scheme of anhydride/epoxy polymerization with imidazole catalyst 78

Figure 3-2. Curing behaviors of base underfills and composite by DSC 80

Figure 3-3. Glass transition temperatures of composite underfills by DSC 80

Figure 3-4. Viscosity of silica filled composite underfills 81

Figure 3-5. Effect of filler size on the UV-Vis spectra of the composite underfills 82

Figure 3-6. CTE of silica filled composite underfills 83

Figure 3-7. Dynamic moduli of composite underfills with untreated nanosilica 85

Figure 3-8. Comparison of dynamic moduli of composite underfills with differentnanosilica 85

Figure 3-9. Moisture uptake evaluations for underfill with different silica: (a) 24h (b) 48h(c) 72h (d) 96h 87

Figure 3-10. Kinetics of moisture uptake for the samples 89

Figure 3-11. Density measurement for silica filled composite underfills 90

Figure 3-12. SEM photographs of nanosilica composite materials (a) untreated-30, (b)treated-30 91

Figure 3-13. Cross-section views of a quartz chip with no-flow underfill 93

Figure 3-14. Wetting picture of quartz chip with treated-30 underfill 93

Figure 3-15. Glass transition temperature of the silica composites 98

Figure 3-16. Glass transition temperature of the silver composites 99

Figure 3-17. Glass transition temperature of the aluminum composites 99

Figure 3-18. Glass transition temperature of the carbon black composites 101

Figure 3-19. TGA measured weight loss at a heating rate of 20 C/min under air 102

Figure 3-20. Dynamic loss moduli of the silica composites and the blank resin 104

Figure 3-21. Deconvolution of loss modulus of nanocomposite 104

Figure 3-22. Molecular structure of anhydride/epoxy polymer 105

Figure 3-23. Possible local motion of segments in anhydride/epoxy polymer 105


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Figure 4-1. Electrode design of the single surface sensor used in the experiment 113

Figure 4-2. Dielectric property of the control sample after curing 114

Figure 4-3. Dielectric property of the epoxy/silica micron-composite after curing 115

Figure 4-4. Dielectric property of the epoxy/silica nanocomposite after curing 115

Figure 4-5. DEA multi-frequency experiment results of nanocomposite sample (not allthe temperature listed) 118

Figure 4-6. Shift factors of TTS for nanocomposite sample 118

Figure 4-7. Master curves of loss factor for three samples after obtained by TTS shifting 119

Figure 4-8. Moisture absorption of three materials as aging time 120

Figure 4-9. Loss factor and ionic conductivity of the three samples at 1 Hz 122

Figure 4-10. Loss factor of three samples after curing, (a) 1Hz; (b) 1000Hz 124

Figure 4-11. Loss factor of three samples after aging under humidity, (a) 1Hz; (b)1000Hz 125

Figure 4-12. Loss factor of three samples after drying (a) 1Hz; (b) 1000Hz 126

Figure 5-1. Molecular structure of two hardeners used in the experiment 129

Figure 5-2. Elastic and viscous modulus as a function of frequency fornanosilica/anhydride mixture 134

Figure 5-3. Elastic and viscous moduli as a function of frequency for nanosilica/aminemixture 134

Figure 5-4. Steady-shear viscosity as a function of shear stress for nanosilica in twohardeners 136

Figure 5-5. Schematic representations of two possible scenarios that can occur in the caseof silica particles dispersed in a liquid. 136

Figure 5-6. Van der Waals potential between silica particles in different hardeners 139

Figure 5-7. Glass transition temperatures of silica composites with different hardeners 141

Figure 6-1. Molecular structure of photo-initiator 146

Figure 6-2. Scheme of real-time FITR setup 147


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Figure 6-3. Scheme of photo-DSC setup 148

Figure 6-4. Light transmittance of two kinds of nanosilica in ethanol solution 151

Figure 6-5. TEM picture of the 20 nm colloidal silica 152

Figure 6-6. UV absorbance of pure epoxy, nanosilica and photo-initiator 154

Figure 6-7. Influence of nanosilica on the UV absorption of photo-initiator in epoxy 154

Figure 6-8. Reaction mechanism of cationic photo-polymerization 156

Figure 6-9. FTIR absorption of epoxy 157

Figure 6-10. Peak intensity changes of pure epoxy with different UV exposure times (thearrow direction represents the time increases) 158

Figure 6-11. Relationship of integrated band area of epoxide peak and UV exposure time 159

Figure 6-12. Heat flow of underfill after UV exposure 162

Figure 6-13. Heat flow of UV-initiated underfill during thermal heating 162

Figure 6-14. Photo-DSC curves of the nanocomposite with different filler loading 163

Figure 6-15. DSC measured heat flow in an isothermal experiment 164

Figure 6-16. Polymerization rate versus time for the photo-polymerization of

nanocomposite underfill 166

Figure 6-17. Conversion versus time for the photo-polymerization of nanocompositeunderfill 166

Figure 6-18. Absorbance of underfill with different filler loading 168

Figure 6-19. Light transmittance of photo-cured nanocomposite with different fillerloading (particle average size: 20 nm) 171

Figure 6-20. Light transmittance of composite with the particle volume fraction, fp(particle average size: 8m)[140] 171

Figure 6-21. Comparison of light transmittance and the particle volume fraction forcomposite with different silica size 172

Figure 6-22. DSC Tg of the nanocomposite after photo-curing followed thermal curing 173

Figure 6-23. TGA graphs of the photo-cured nanocomposites 174


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Figure 6-24. Coefficient of thermal expansion of the nanocomposite with various fillerloading 175

Figure 6-25. DMA curves of the photo-cured nanocomposites 176

Figure 6-26. Tan delta peak temperature (DMA Tg) of the photo-cured nanocomposites 177

Figure 6-27. Comparison of composite modulus between the theoretical prediction andexperimental measurement 179

Figure 6-28. TEM picture of nanocomposite after photo-curing 180

Figure 6-29. A plot of load vs. displacement in a nanoindentation experiment 181

Figure 6-30. Hardness of nanocomposite films after photo-curing 181

Figure 6-31. Double ball redistribution uses two solder balls for each I/O, one beingencapsulated in epoxy. (Source: Fraunhofer IZM/Technical University of Berlin) 183

Figure 6-32. Wafer level process with laser ablation method to open the microvia onunderfill 183

Figure 6-33. Proposed wafer process with novel photo-curable nanocomposite 185

Figure 6-34. Molecular structure of SU-8, gamma-butyrolactone and propylene glycolmonomethylether acetate (PGMEA) 188

Figure 6-35. Flow chart of photolithography process for SU-8 nanocomposite 189

Figure 6-36. Photo-defined pattern of SU-8 nanocomposite containing 40 wt% nanosilica 189

Figure 7-1. Silica surface grafting of imidazolium salt as a catalyst 196

Figure 7-2. Surface initiation of epoxy curing reaction. 196

Figure 7-3. Poyhedral oligosilsesquioxane (POSS) structure 197

Figure 7-4. Synthesis route of POSS-containing underfill 199

Figure 7-5. Chemical structure of cyanate ester monomer 200


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SUMMARY

Underfill material is a special colloidal dispersion system with silicon dioxide

particles in the organic liquid. It is used to improve the reliability of integrated circuits(IC) packaging in the microelectronics. In order to successfully synthesize the

nanocomposite underfill meeting the requirements of the chip package, it is necessary to

have a fundamental understanding of the particle stability in the non-aqueous liquid and

the relationship between materials properties and interphase structure in the composite.

The results of this thesis contribute to the knowledge of colloidal dispersion of

nanoparticles in organic liquid by systematically investigating the effects of particle size,

particle surface chemistry and surface tension, and liquid medium polarity upon the

rheological and thermal mechanical properties of underfill materials. The relaxation and

dielectric properties studies indicate that the polymer molecular chain motion and

polarization in the interphase region can strongly influence the material properties of

nanocomposite, and so a good interaction between particle and polymer matrix is key.

With this study, a potential nanocomposite underfill can be synthesized with low

viscosity, low thermal expansion, and high glass transition temperature. The excellent

transmittance of nanoparticles leads to further investigation of their ability as reinforcing

filler in the photo-curable polymer.


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CHAPTER 1. INTRODUCTION

1.1.Electronic Packaging and Flip-Chip Technology

1.1.1. Packaging technology development

After the first transistor was invented in Bell Lab in 1947, semiconductor

technology has proceeded from the big, high cost single transistor to highly integrated

circuits(IC), and will continue to develop toward the low cost and high functions of the

electronic products. Today, the electronic industry is the largest and most pervasivemanufacturing industry in the developed world, which has brought profound impact onto

our life.

From the silicon chip to the final products, electronic packaging acts as the key

bridge for the transforms based on the following four major functions: 1) providing an

electrical path to power the circuits, 2) distributing signals onto and off the IC chip, 3)

dissipate the heat generated by the circuits, and 4) supporting and protecting the IC chip

from hostile environments[1].

Figure 1-1 shows the hierarchy of electronics packaging[2]. From the bare chip

fabricated from the silicon wafer, to the final product ready for use, the whole system can

be divided into three levels of the packaging. The first level packaging provides the

interconnection between an IC chip and a module. There are at least three popular

methods for interconnecting the chips on the substrates (either to the module or the

board): 1) face-up wire boding, 2) face up tape-automated bonding (TAB), 3) flip-chip

technology. Second level packaging provides the interconnection between the module to

the printed wiring board (PWB) or a card, which could be realized by the pin through

hole (PTH) technology, or surface mount technology(SMT). Third-level packaging

mainly is the process to put second-level packages onto a motherboard. With the


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requirements towards low-cost, miniaturization and high performance for the current

semiconductor devices, the bare IC chips can be connected to the integrated board using

flip-chip technology directly[3], which is called flip-chip on board (FCOB) or direct chip

attach (DCA).

Figure 1-1. Scheme of electronic packaging hierarchy

Today the electronic assembly and packaging are limiting factors in both cost and

performance for electronic systems. The International Technology Roadmap for

Semiconductor (ITRS) has predicted the main trends in the semiconductor industry

spanning across 15 years into the future. Table 1-1 shows some trends in the

microelectronic manufacturing[4]. The most frequently cited trend is so-called scaling

down, e.g. the ability for industry to exponentially decrease the minimum feature size

used to fabricate integrated circuits. It can be seen that the feature size of IC fabrication


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already shrinks into nanometer scale and will keep decreasing down to 20nm.

Correspondingly, the total I/O number and power on each chip are continuously

increasing. This has resulted in the acceleration of innovation in design concepts,

packaging architectures, materials, manufacturing processes and systems integration

technologies. Specifically, with the smaller and smaller pitch size (distance between the

metal contacting pads on chip), the high-density, high performance method is needed to

connect the IC to the substrate.

Table 1-1. Trend in the microelectronic manufacturing

1.1.2. Flip-chip technology

Flip chip is the first level IC packaging approach in which the active side (with

integrated circuit) of the silicon chip is faced down and connected to the substrate or

printed wire board (PWB)[5]. Figure 1-3 shows a general scheme of the flip-chip

package. The active sides of the chips are bumped with eutectic tin/lead, high lead, or

lead-free solders. After a thermal reflow process, the solder can melt and wet on the

metal contact pad of the substrate, and form the electrical and mechanical connections


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between the IC and the substrate after cooling down. Compared to the conventional wire

bonding technology (Figure 1-2) where the active side of the silicon chip is faced up and

interconnection is made by drawing gold, silver or copper wires from the peripheral edge

of the chip to the substrate, flip chip has many advantages. Since the full area of the chip

surface can be used for interconnection, the input/output density is much higher. It can

provide the shortest possible leads, lowest inductance, smallest device footprint, and

lowest profile. Since the interconnections on the chip can be finished in a one-time

thermal treatment, flip chip avoids the tedious process for individual wires as in wire

bonding.

Figure 1-2. General configuration of wire bonding package

Figure 1-3. General configuration of flip-chip package


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The concept of flip-chip was demonstrated about 40 years ago[6] by IBM on a

ceramic substrate, which was then called Controlled Collapse Chip Connection (C4).

Although the ceramic substrate has low coefficient of thermal expansion (CTE) that

matches the CTE of silicon chip, it entails high temperature and expensive process, as

well as the high dielectric constant that aggravates the signal delay. Recently, the desire

for low cost and mass production has led to increased use of organic substrate. Organic

substrate is favored in terms of its low dielectric constant and low cost. But the high CTE

difference between the organic substrate and the silicon chip exerts great thermal stress

on the solder joints during temperature cycling. This thermal stress is proportional to the

Distance to the Neutral Point (DNP). The larger the chip, the higher the stress, hence, the

worse the solder joint fatigue life. So the organic substrate was inapplicable to flip-chip

technology until underfill was invented in the late 80s[7].

1.2.Underfill materials and nano size filler

1.2.1. Overview of underfill materials

In the early stage of flip-chip technology, the substrates were limited to the high-

cost ceramic or silicon materials because the great concerns of the thermo-mechanical

fatigue life of the solder joints. Table 1-2 shows the CTE of major materials used in the

flip-chip packaging. Obviously, the CTE mismatch between chip and organic substrate is

much higher than that between solder and ceramic board, which can cause significant

stresses in the solder joints during the product use and leading to fatigue failure.

Therefore, the low-cost organic substrate such FR-4 board and polyimide could not beused extensively until the reliability issue of solder joints can be solved. Some improved

methods such as optimizing bump distribution design and joint geometry[8, 9], using

highly strong solder composition[10], or matching the CTE of circuit board to that of

silicon[11], have been explored. However, since they are expensive processes and


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provide limited improvement, above methods are still not fully satisfactory. The so-

called underfill, which is placed under the chip to fill the gap between the chip and

substrate, was discovered and became one of most innovative development to enable the

use of low-cost organic substrate in flip-chip packages.

Table 1-2. Coefficient of thermal expansion of major materials in flip-chip packaging

Materials CTE (ppm/C) Application in flip-chip

Silicon 2.5 Chip or substrateSolder 18-22 interconnects

Alumina 6.9 Ceramic substrateFR-4 board 16 Organic substrate

Polyimide 45 Flexible organic substrateEpoxy 55-75 Underfill polymerSilicon dioxide 0.5 Filler of underfill

Underfill is a liquid encapsulant, usually based on un-cured epoxy resin monomer

heavily filled with SiO2 (fused silica). It can be applied to the assembly before or after

chip reflow. Then the liquid underfill can be thermally cured to form a cross-linked

network and converted to a thermoset polymer. With the highly filling of inorganic filler,

the cured underfill shows high modulus, low CTE matching that of the solder joint, as

well as good adhesion to mechanically couple the chip to substrate to restrain most of the

lateral movement between two interfaces. Thermal stresses on the solder joints are

redistributed among the chip, underfill, substrate and all the solder joints, instead of

concentrating on the peripheral joints. The hardened underfill can reduce the solder strain

level to 0.10-0.25 of the strain in joints which are not encapsulated[12, 13], and increase

the fatigue life of the solder by a factor of 10-100. Besides dissipating the thermal stress,

the underfill also provide the environmental protections to the solder joints as the

encapsulant. With the superior advantage mentioned above, underfill products are now

available that deliver on the promise of providing the reliability required for 2nd

generation flip chip on organic platforms. Millions of flip chips are now being assembled


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on FR4 and BT laminate for a wide range of products like cellular phones, pagers, disk

drives, memory modules and much more.

1.2.2. Underfill classifications

The development of underfill technology is always driven by the advances of the

flip-chip technology and advanced in the both directions of underfilling processes and

underfill materials. Generally, the development of the underfilling process pushes the

development of new underfill materials. According to the different processing

procedures, the underfill can be dividend into capillary underfill, molded underfill, no-

flow underfill and wafer level underfill[14].

The capillary underfill (conventional underfill) is the most mature and

predominant underfill technology in industry manufacturing. It relies on capillary forces

to draw liquid underfill into the gap between the IC and the substrate, as shown in Figure

1-4[14]. Currently, this method faces many problems due to its intrinsic weakness. The

incomplete capillary flow can cause voids and non-homogeneity in the resin/filler system.

The curing of the underfill takes a long time in the oven, consuming additional

manufacturing time. The flux cleaning and flux residue incompatibility create the voiding

problems in the packaging. Decreasing bump pitch and chip height, and increasing bump

density and chip size will eventually push the limits of capillary flow underfill materials.

In order to address the problems associated with conventional underfill and satisfy

the needs of future generations of products, there are several alternative underfill

technology options have been invented. One method is to combine the process of

underfilling and transfer molding into one step and creates the molded underfill[15].

Molded underfill can be applied to the FCIP via a transfer molding process, and it not

only fills the gap between the chip and the interposer/substrate but also encapsulates the

whole chip. In order to easily flow through the gap between chip and substrate, the

molded underfill requires smaller filler size than conventional molding compound. On the


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other hand, the injection-molding process allows high filler loading materials with low

CTE, high modulus and low moisture absorption to be used[16]. Therefore, the molded

underfill can be used in the packages with smaller bump pitch and die standoff height, for

that the capillary underfill usually could not work well.

Flux

Board

Bond pad

Fluxing Dispensing

Chip

Solder ball

Chip Placement

Heated

Solder Reflow

Solvent spray

Flux Cleansing

Underfill

Underfill Dispensing

Heated

Underfill Cure

Flux

Board

Bond pad

Fluxing Dispensing

Chip

Solder ball

Chip Placement

Heated

Solder Reflow

Solvent spray

Flux Cleansing

Underfill

Underfill Dispensing

Heated

Underfill Cure

Figure 1-4. Conventional capillary flow underfill process

Chip PlacementUnderfill Dispensing

No-flow Underfill

Heated

Solder Reflow &

Underfill Cure

Chip PlacementUnderfill Dispensing

No-flow Underfill

Heated

Solder Reflow &

Underfill Cure

Figure 1-5. No-flow underfill process

Besides the molded underfill, another process called no-flow underfill was also

invented to solve the limitations of capillary underfill. The idea of no-flow underfill was


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first patented by Pennisi et al. in 1992 [17], and the first working material developed was

patented by C.P. Wong and S.H. Shi in 2001 [18]. In the no-flow underfill process, as

shown in Figure 1-5, the underfill material is dispensed on the substrate before the chip is

assembled onto it. After chip placement, the whole assembly is subjected to the solder

reflow. The underfill materials are cured simultaneously during the reflow process.

Sometimes the subsequent post-curing is also needed after assembly to fully cross-link

the underfill. This technique simplifies the flip-chip underfill process by eliminating

separate flux application and cleaning steps before assembly, and avoiding underfill

capillary flow. Thus, the no-flow underfill process can greatly improve the flip-chip

production efficiency.

Although no-flow underfill eliminates the capillary flow and combines fluxing,

solder reflow and underfill curing into one step, it still has some inherent disadvantage,

including individual underfill dispensing step and not totally transparent to standard

surface mount technology (SMT). An improved concept, wafer level underfill, was

invented to improve no-flow underfill and achieve low cost and high reliability[19, 20]. In

this process, the underfill is applied onto a wafer using a proper method, such as printing

or coating. Then the underfill is partially cured to B-stage and the wafer is diced into

single chips. The individual chips will go for further chip assembly and reflow by

standard SMT equipment with no additional process steps required. During the reflow,

the pre-applied material will melt first in order to allow solder wetting and then cure as

solid underfill. This innovative wafer level process eliminates the underfilling and curing

step for each individual die during assembly, and makes the direct die attach process truly

transparent to the assembly line.

Table 1-3 compares the four different underfill processes. It is indicated that not

only the materials chemistry and rheology are different, but also the process and

application steps are varied for these underfills. Therefore, a successful underfill


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approach needs close collaborations between the materials suppliers and assembly

designers.

Table 1-3. Underfill classification

Name Dispense Stage Application

Location

Fluxing

ability

Material Form

Wafer levelunderfill

After IC fabricationand before wafer dicing

On the wafer Yes semi-solid (afterB-stage)

No-flowunderfill

Before chip assemblyand reflow

On the substrate Yes liquid

Moldedunderfill

After chip assemblyand reflow

Between chip andsubstrate,

overmolding the chip

No solid

Capillaryunderfill

After chip assemblyand reflow

Between chip andsubstrate

No liquid

1.2.3. Composition of epoxy underfill

Different kinds of materials can be used as underfills. However, most underfills

are based on epoxy. The material system is generally composed of an epoxy resin

monomer or epoxy mixture, a curing agent, a catalyst, SiO2 filler, and other necessary

additives depending on the specific application, such as fluxing agent, toughening agent,

adhesion promoter, dispersant agent, etc.

Epoxy resin

The organic compound that contains oxirane groups can be called as epoxy. The

commonly used epoxy resin monomers can be classified in three large groups: diglycydyl

ether type[21], cycloaliphatic type[22] and epoxy novolac resin[23]. The selection ofbase epoxy resins is of critical importance to a successful underfill since the many desired

material properties such as viscosity, toughness, and moisture uptake were mainly

determined by the base epoxy resins. With the different polymerization degree and

molecular weight, the epoxy before curing can be low viscosity liquid, high viscosity


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liquid, semi-solid and solid. Depending on the application method, the epoxy resin can be

modified with solvent to adjust the viscosity for underfill application. Table 1-4 lists the

epoxy resins which have been use in the thesis study.

Table 1-4. List of Epoxy resin used in this study

Molecular structure Synonym Name

C O CH2O

CH3

CH3O

CH2O

EPON 828 diglycidyl etherof bisphenol-Aepoxy resin

CH2 O CH2O

O

CH2O

EPON862 diglycidyl etherof bisphenol-Fepoxy resin

O

CH2 O

C

O CH2

O

CH2[ ]

CH3

CH3

n

EPON SU-8 Epoxy phenolnovolac resin

O

O

O

O

ERL4221 cycloaliphaticepoxy resin

Curing agent

Although epoxy resin can be initiated by a catalytic initiator and cross-linked by

homo-polymerization, it is necessary to use a curing agent, also known as hardener, to

promote the cross-linking reaction or curing of epoxy resins in the practical application in

order to obtain good material properties. Many organic compounds, including amines,

acid anhydrides, and phenol-formaldehydes, have been used as curing agents[24].

Table 1-5 shows the curing agents used in the thesis study. These three curing

agents can cure epoxy resins through polyaddition reactions by the active hydrogen[25].

For underfill application, the decision among curing agents should consider the viscosity

and flow ability, curing mechanism, gelation behavior[26], wetting ability to the metal


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before curing, as well as the chemical structure and material properties after curing. The

compatibility of curing agents to the SiO2is also an important issue because the filler is

the largest composition in the underfill materials. The polarity and hydrophilicity of

curing agents will influence the filler dispersion and the rheology of the underfill.

Table 1-5. List of curing agents used in this study

Molecular structure Synonym NameOH

H2C CH2

OH OH

n

LBR-6 Phenolic resin

O

O

O

HMPA Methyl tetra hydrophthalic anhydride

H2N

H2N or

NH2

NH2

DETDA Diethyl toluenediamine

Catalyst

Latent catalyst is another component of critical importance in a successful

underfill since the pot-life, curing temperature and time, and processability of an underfill

is mainly determined by the latent catalysts. To provide convenience to the end user, the

underfill materials are usually formulated as a one-pot composition, by which user just

need dispense the materials without further materials processing such as mixing the

catalyst before usage. This one-pot formulation brings great challenge to the materials

shipping and storage process. Those catalysts in the epoxy which provide an efficient rate

of curing at high temperature are generally not stable enough to be stored for any


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appreciable periods. The catalyst tends to gel the epoxy resin prematurely at normal room

temperature, or at temperatures which may be encountered during storage. Thus, it has

been necessary to ship and store the epoxy formulations under the frozen environment,

usually -40C, to prevent the polymerization reaction before material application[27]. For

the no-flow underfill and wafer level underfill, the successful formation of solder joints is

dependent on the curing kinetics of the underfill, which should maintain low degree of

reaction at the solder melting point. Latent catalyst is the key to control over the curing

temperature.

+R'NH2

CH2

CHR

O

R'NH CH2

CHR

OH

Figure 1-6. Curing mechanism of primary amine

For the underfill formulations with primary and secondary amine curing agents,

the catalyst is usually not necessary because these amines contain the active hydrogen

which can add to the epoxy group. Generally, primary and secondary amines are used at

mix ratios that provide one amine active hydrogen for each epoxy group, i.e. the

stoichiometric amount. Figure 1-6 illustrates the initial step which involves the primary

amine reaction. This is followed by the resulting secondary amine adding to another

epoxy group. Sometimes the catalyst also can be added to cure the epoxy more

effectively. Tertiary amines are usually used as a catalyst with trace amount in the

formulation.

There are at least four categories of latent catalysts that have been investigated in

patents and literature. They are: (1) imidazoles and their derivatives[28-30]; (2)

quaternary phosphonium compounds[31]; (3) metal acetylacetonates[32]; (4) some

photoliable onium salts[33]. Table 1-6 lists the catalyst used in this study.


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Table 1-6. Lists of catalysts used in this study

Molecular structure Synonym Name

N

NH

N

2E4MZ-CN

1-cyanoethyl-2-ethyl-4-methylimidazole

N N COOHHOOC

COOHC11H23

CN CH2 CH2C11Z-CNS 1-(2-Isocyano-ethyl)-2-

undecyl-1H-imidazole

1.2.4. Filler in the underfill

Figure 1-7. Scheme of underfill flow and possible filler clog between chip and substrate

Among the material components in the underfill formulation, filler plays an

important role in reducing overall CTE of the underfill material, minimizing moisture

uptake, and eventually improving device reliability. As a general rule of thumb, the

maximum filler particle size should be less than one third of the gap height between chipand substrate[34]. Otherwise, the probability of particles getting trapped, shown in Figure

1-7, is very high. Currently, the flip-chip gap size has reached to 50 micron and will

target to 15 micron in the future [35]. The shrinking gap in the flip-chip package

continues to demand the underfill material with smaller and smaller particle size. The


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filler size has been quickly decreased to below single digital micron in diameter in most

advanced underfill formulation. The nanosilica ranging in diameter from 20nm to 550nm

has been investigated as the filler for underfill application[36-39].

Another important phenomenon associated with filler is filler particle settling.

Filler settling could occur at different stages of underfill processing, such as during

dispensing, after dispensing, and during curing, resulting in a non-homogeneous filler

content distribution along the Z direction of underfill layer. As many properties of

underfill are function of filler content, filler settling modulates Tg, CTE, toughness and

adhesion of underfill[40]. Severe filler settling can cause cracks, and deteriorate the

potential reliability performance of the underfill materials[41, 42].

The underfill material is a fluid with very small Reynolds number (


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Figure 1-8. Micron size silica entrapped between the solder and the contact pads

Figure 1-9. Optical microscope picture of the flip-chip assembly with nanocomposite no-flow underfill

The reducing of filler size is important for underfill processes to address theconcerns about the shrinking of pitch size and gap height, and filler settling problems.

Moreover, the fine size filler is also the most critical factor to ensure the solder wetting

and interconnect formation during a no-flow process. Since the no-flow underfill is

applied to the substrate prior to the placement of the IC chip, conventional micron size


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fillers have a great probability of being entrapped between the solder bumps on the chip

and the contact pads on the substrate[44], as shown in Figure 1-8. The trapped fillers

prevent solder wetting on contact pads and thus significantly reduce the solder joint yield

and the electrical continuity[45]. Thus, the micron size fillers have to be replaced by the

nano size fillers in the no-flow process. It was found that silica in the size region of 100

to 150 nm were less likely to be trapped in the flip-chip assembly and a nanocomposite

no-flow underfill with 50 wt% silica filler of 120 nm size has been demonstrated to offer

both high solder joint yield and package reliability in air-to-air thermal cycling test[46].

Figure 1-9 shows an optical microscope image of the flip-chip assembly with the

nanocomposite no-flow underfill.

The wafer level underfill also faces to many challenges regarding the filler

addition. Besides the similar problems related with the solder wetting as no-flow

underfill, the vision recognition in the wafer level underfill process becomes a new issue

because the wafer is covered with underfill[20]. During the pick-and-place vision process

of wafer level packaging, the solder bumps are used as locating features in place of

fiducials. In the presence of underfill, the apparent size and shape of the bumps may be

altered. Especially, the highly filled underfill with large filler particles makes the bumps

almost invisible due to the light scattering. In addition, the three-dimensional topography

of an uncoated bumped die aids vision recognition with the formation of shadows around

the bumps. These shadows enhance the cameras ability to sense the bright bump against

the darkness of the shadows. With a coated die, the topography is flattened, further

complicating the recognition step. The superior optical transparency of nanocomposite

underfill will be helpful to solve this problem[38]. It was found that the underfills with

nanosilica were almost as transparent as pure epoxy in visible region (400~700 nm)

because the filler has a particle size smaller than the wavelength of the visible light.

The nano size fillers show superior properties in the underfill applications and

have the potential to solve the problems associated with the large size filler, such as


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clogging flow in the fine pitch package, filler setting down, hindering the solder wetting,

and disturbing the vision recognition during assembly. Nanocomposite underfill will

provide significant reliability improvement for the large-area flip-chip packages.

However, challenges remain and some fundamental problems need to be addressed

before the successful implementation of the nanocomposite underfill to meet the

requirements of low cost, high yield and high reliability for flip-chip assembly.

The filler particle dispersion becomes a dominated factor for the nanocomposite

underfill application. The nanoparticle filled underfill can be considered as a solid-liquid

colloidal dispersion with silica as colloidal phase and epoxy monomer as the liquid

medium. The degree of dispersion, the interaction between the dispersed phase and the

dispersion medium, and the interaction between the particles can influence the materials

properties. The silica colloidal stability in the liquid underfill means the particles have no

tendency to aggregate. However, the filler dispersed state is not the lowest energy

condition; in other words, there is natural tendency for particles to aggregate. To maintain

the silica stable in the underfill, we must use the correct conditions to disperse the particle

and overcome the particle-particle attractive force.

1.3.Particle dispersion

1.3.1. Energy state of particle in the medium

The colloidal domain of matter can be broadly defined as particles of size ranging

from 1 nm to 1m. The stability of dispersion against flocculation is highly dependent

upon the total interaction energy between particles. If the particles can

thermodynamically seek the lowest free-energy state, a stable colloidal suspension can be

formed. The net, or total, interaction energy is obtained by the summation of the

individual attractive and repulsive energy terms. DLVO (Derjaguin-Landau-Verwey-


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Overbeek) theory considers the case in which the attractive energy is due to van der

Waals interactions balanced by the repulsive energy. Now, we will analyze the factors

which influence the silica colloids in the underfill systems.

Figure 1-10. Energy diagram of particle surface with distance

1.3.2. Attractive force

Without any stabilization effects, the particles which are dispersed in a liquid can

attract with each other naturally and tend to fairly unstable[47]. This attraction is

attributed to the fact that the atoms or molecules forming the dispersed particles can

generate transient dipoles because electron density moves about a molecule

probabilistically. There is a high chance that the electron density will not be evenly

distributed throughout a non-polar molecule. This leads to a so-called London-van der

Waals attraction between molecules. The interaction energy between two particles invacuum can be found by summing the attractive energies of atom pairs over all atoms of

both particles, which is given by:


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d

rA

12=

Equation 1-2

Where A is the Hamaker constant, d is the distance between two particles, and r is the

diameter of the particle.

When the particles are immersed in a medium (e.g. silica filler in the liquid epoxy

monomer for the underfill case), an effective Hamaker constant is substituted in the

Equation 1-2 in order to determine the effective van der Waals interaction energy, which

is given by:

22112211 2 AAAAAeff += Equation 1-3

Where A is the Hamaker constant for particle-particle interaction in vacuum, D is the

Hamaker constant for medium-medium interaction in vacuum.The effective Hamaker constant effA is always positive for particles of type 1

immersed in medium of type 2. Therefore, the van der Waals interaction energy is always

negative (i.e. attractive energy). The higher Hamaker constant, the stronger attractive

interaction occurs between particles. Because of this attraction, dispersed particles

consisting of equal material will form aggregations (flocs), unless there are factors that

retard the aggregation formation. Such factors will be discussed in next section.

1.3.3. Repulsive force

Electrostatic stabilization:

The particles immersed in the liquid can develop an electrical surface charge due

to the interactions of the ions in the liquids, especially in an aqueous system with ions.

With absorbing the ions on the surface, the colloidal particles are charged (Figure 1-11).

In order to maintain overall electro-neutrality of the system, the charged particles are also

accompanied by a surrounding cloud of ions of opposite sign (called counter ions) in the

medium. The formation of charge on the oxide surface is usually controlled by two

important processes[48].


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Figure 1-11. Electrostatic force in the dispersion system with ionic strength

OH

OH

OH

Silica

Particle O

O

O

SilicaParticle + nH (aq)

OH

OH

OH

SilicaParticle

OH2

SilicaParticle

nH (aq)+ OH2

OH2

OH

OH

OH

SilicaParticle

nOH (aq)+O

O

O

SilicaParticle

+ H2O

Figure 1-12. Adsorption-dissociation mechanism of ions on the silica surface in theaqueous medium

Figure 1-13. Steric stabilization of particles by adsorbed polymer


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(1) Adsorption of water molecules on the oxide followed by dissociation of the

hydrolytic product formed. The fully hydroxylated solid surface has an amphoteric

character and is positively or negatively charged depending on the pH. Figure 1-12 shows

the charging mechanism of silicon dioxide in the aqueous medium.

(2) Adsorption of referentially released ion species from the soluble salts. Salts

such as AgI, CaF2, BaSO4can be added into solution and ions can form after hydration.

Steric stabilization:

For polymer adsorbed or anchored to the particle surface at large surface

coverage, a polymer-mediated force that can extend 10-100nm will arise due to polymer-

polymer interactions across the gap between particles. These can be either repulsive or

attractive depending on the solvent quality and the extent of the polymer excluded

volume effect. Such force, arising from the proximity and overlapping of polymer chains,

have been referred to as steric forces.

In a word, the DLVO theory demonstrated that the stability of charged colloidal

systems was governed by the competition between the attractive van der Waals forces,

and the repulsive electrostatic forces and polymer chain mediated forces. Both of these

forces are long range in nature, extending to a few tens of nanometers[49]. Many well-

defined analytical forms have been developed to characterize the particle separation

characteristic and flocculation phenomena.

1.3.4. Filler stabilization in underfill

Electrostatic stabilization and steric stabilization are the most common methods to

stabilize the colloidal particles in the medium. Nevertheless, underfill materials which

consist with the epoxy monomers and silica colloidal particles could not use these two

mechanisms to stabilize the filler particle and achieve good dispersion, because of the

special composition of underfill. Unlike the common colloidal dispersion such as


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pigment, polyelectrolyte, or polymer solution, underfill material are quite unique

colloidal dispersion system.

Table 1-7. Bulk resistivity of underfill formulation (before curing)

name chemical Bulk resistivity

(ohm-cm)

Epoxy monomer Bisphenol A 2.67E+10amine 2.31E+09Curing agent

anhydride 1.66E+08Bisphenol A -amine 7.72E+09Underfill

Bisphenol A -anhydride 1.15E+09De-ionized water water 1.80E+07

Firstly, underfill is a non-aqueous/non-solvent system, and the ionic concentration

inside the underfill is limited to extremely low level in order to avoid current leakage and

break-down during the application in the electronic device. Therefore, it is not possible to

form ions or charge on the silicon dioxide particle with the neutral surroundings of epoxy

monomer. Typical particle surface charge densities in aqueous dispersions may be ~0.2

C/m2, but they can be 2 or 3 orders of magnitude lower in the non-aqueous systems[50].

Table 1-7 lists the resistivity of underfill and its components. Comparing to water, these

liquids are quite insulating, and the ion concentration is much lower. From the colloid

point of view, sufficient ions are needed to charge the particle surface and maintain the

overall colloidal stability of the system from a charge stabilization mechanism, which is

lacking in the underfill systems.

Secondly, the silica particles in the underfill system only contact with epoxy

monomer and other small organic molecules (such as curing agents) whose molecular

weight is usually less than 400 g/mol. The organics absorbed on the silica surface are not

long enough to form steric interaction and stabilize the particle. The overlapping and

entanglement of polymer long chains, as in a polyelectrolyte-colloidal system stabilized

by the long chain polymer surfactant, will not happen in the underfill system. It is found,


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in fact, that a number of inorganic particles dispersed in the non-aqueous system could

not be stabilized by non-ionic surfactant[51]. Hence, the steric mechanism could not

provide sufficient protection to the colloidal silica particles in the underfill.

As we mentioned before, the DLVO theory is used to describe the surface energy

potential on the particle surface and depict the equilibrium between attractions and

repulsions as a function of the separation distance between two particles. However, the

traditional DLVO treats the liquid medium, in which the particles are immersed, as a

structureless continuum. This approach may be valid when the separation distance

between the colloidal particles is great. When two surfaces of particles approach closer

than a few nanometers, this theory fails to describe interactions between the hydrophilic

particles. Aggregation of hydrophobic colloids has been observed to occur in the

presence of electrostatic repulsion, whereas hydrophilic colloids are stable even without

electrical charge on the surface[48].

For nanosilica underfill system, the filler size has fallen within a few tens of

nanometers, and the distance between each other is very close at the high filler loading.

The non-DLVO forces come into play when the particles are too close to be separated.

For the case of nanosilica dispersed in the underfill, the surfaces of silica particles are

surrounded by the epoxy monomer (and hardener), which act as liquid phase in the

colloidal dispersion. Within the short range, the thin layer of epoxy liquid is not

continuous to the bulk epoxy phase, but has a discrete structure that differs significantly

from that of the bulk phase. Some people refer this region as the interphase, while these

short-distance interactions are usually referred to as solvation forces or as a structure

force. If the solvent (organic liquid) can spread and have good wetting on the particle

surface, the nanoparticle can be stabilized by solvation force. In the other word, if

interaction between the particle surface and liquid is much stronger than that of the

particle and particle, it is possible to stabilize the nanosilica and form a stable colloidal

dispersion. In this scenario, questions like, how the physical and chemical properties of


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the solvent and the particle surface affect the free-energy balance and phase behaviors of

the colloidal nanoparticles, and how the properties of stabilized colloidal dispersion

compares with the flocculated one, need to be answered.

1.4.Impact of nanoparticles on the rheology

The application and flow of underfill is governed by its rheology properties. For

the nanosilica filled underfill, the rheology is highly dependent with the filler dispersion

condition. In another word, we also can use the rheological measurements to assess the

state of dispersion in suspensions. Rheology is the science dealing with flow and

deformation of materials. The rheological behavior of particle/liquid systems is important

in most processing operation, including powder and batch preparation, materials

transport, coating and deposition, and shape forming. For underfill application, the

rheology properties are the key to determine the processing condition for underfill flow.

Rheological properties are highly dependent upon the physical structure of the

particle/liquid systems. Structure is governed by factors such as the particle size and

shape distributions, solid/liquid volume ratio, and interparticle forces. Rheological

measurements can often be used to deduce information about the state of particulate

dispersion in the suspension.

1.4.1. Definition of viscosity

Consider a model situation in which a liquid is confined between two parallel

plates (Figure 1-14). One plate is movable and one is held stationary. The plates are

separated by distance y. A force, F, acts on the top, movable plate of area, A, in a

tangential direction, so that the plate moves sideways with a velocity, , relative to the

bottom, stationary plate. The layers of liquid also move in a sideways direction. The top

layer moves with the greatest velocity (i.e. ) and the bottom layer moves with the


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smallest velocity(i.e. zero). However, the velocity gradient or shear rate, , is constant

and given by :

dy

d=

Equation 1-4

The shear stress, , acting on the top plate is given by:

A

F=

Equation 1-5

The viscosity, ,is defined as the ratio of the shear stress to the shear rate:

dy

d

==

Equation 1-6

Figure 1-14. Viscosity definition model

As the proportionality factor between shear stress and shear rate, the viscosity of a

liquid is an index of the resistance to fluid flow (or alternatively, the rate of energy


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dissipation in a flowing liquid depends on the viscosity). The presence of particle in a

liquid results in perturbations of the streamlines during laminar shear flow and, therefore,

an enhanced rate of energy dissipation. In other words, a larger shear stress is required to

maintain the same shear rate as in the pure liquid. Hence, the presence of particles results

in an increased viscosity for the suspension relative to the pure liquid.

1.4.2. Einstein Equation for dilute suspension

Einstein derived an equation which relates the viscosity of a particle/liquid

suspension to the viscosity of the liquid and the volume fraction of solids in the

suspension[52]. It was assumed that particles are: 1) spherical, 2) rigid, 3) uncharged, 4)

very low in the concentration (i.e., hydrodynamic interactions between particles are

ignored), 5) small compared to the dimensions of the container (i.e., wall effects are

ignored), and 6) large compared to the size of liquid molecules (i.e. the liquid medium is

treated as a continuum). Furthermore, the flow rate of the liquids should be low (i.e.,

laminar flow under Stokesian conditions). Under these conditions:

)5.21(0 += Equation 1-7

Where is suspension viscosity, 0 is pure liquid suspension, and is the volume

fraction of solid/particles

This viscosity of colloidal dispersion is the low shear limiting behavior so that the

spatial arrangement of the particles is not perturbed by the shear rate.

1.4.3. Kreigher-Dougherty Equation for concentrated suspension

Although the Einstein equation provides a simple way to estimate the viscosity of

a filler/liquid system, it is only useful for the ideal situation with all the conditions

mentioned above should be satisfied. However, in practical systems, the flow and

viscosity of filler-filled medium are affected by both the decreased spaces between the

particles and particle-particle interactions, which result in attraction and structure


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formation. For highly concentrated suspensions with high filler loading, there is not a

rigorous hydrodynamic equation which can describe all the situations accurately due to

the difficulties in handling multibody interactions. Many empirical or semi-empirical

equations have been proposed. The Kreigher-Dougherty Equation[53], as below:

mm

mm

= 5.20][

0 )1()1(

Equation 1-8

Where m is the maximum packing fraction of filler in the system

The effect of fillers concentration on suspension viscosity is illustrated

schematically below. Deviations from the Einstein relationship occur at low filler

loading, as the suspension viscosity increases rapidly with increasing solids content. At

high filler loading, (i.e. m ), the particle-particle interlocking occurs and the viscosity

become infinite. The filler loading at which a rigid particulate structure develop is

strongly dependent upon the particle characteristics (shape and size distribution) and the

nature of the interparticle forces.

0

1 Einstein Eq.

Re

lativev

iscos

ity

Volume fraction of filler

A

maxD

max

DispersedAgglomerated

0

1 Einstein Eq.

Re

lativev

iscos

ity

Volume fraction of filler

A

maxD

max

DispersedAgglomerated

Figure 1-15. Viscosity of concentrated suspensions


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1.4.4. Particle size effect to viscosity

The viscosity of colloidal dispersion systems is also influenced greatly by the

colloidal particle size. As we know, the colloidal particle in the liquid medium could

move around with a random motion, which is called Brownian motion, due to the thermal

motion and collision. As the filler size decreasing, the colloidal forces between the

particles become significant and the stress required to move them relative to each other

increases. This means that the viscosity at zero-shear condition is controlled by Brownian

term. In order to calculate the maximum packing density, the particle size need be

replaced by the effective radius which is the collision radius of the particles during a

Brownian motion encounter. Due to the increase in the excluded volume of the smaller

size particle, the maximum volume fraction ( m ) is also modified. Hence, Equation 1-8

should be revised for colloidal dispersion with nanoparticle [54]:

'5.2

'0)1( m

m

Equation 1-9

Where

3

0

'

)

2

(495.0 r

am = , is the radius of the particle, 0r is the closest distance between

particle centers, or the value of the effective hard sphere diameter.

Russel et al. has calculated the 0r for a colloidal dispersion system which is

equilibrated by the electrostatic repulsive force. It is found that the small particles have

large excluded volume. Based on the revised Kreigher-Dougherty Equation, Figure 1-16

calculates and plots the viscosity increasing versus filler volume fraction for different

filler size. According to this theoretical calculation, the viscosity of dispersion system

with 500nm filler is only about 3 times higher than that of the pure liquid (assuming

volume fraction 0.25). Viscosity increases to 7 times higher for 200nm filler and reaches

to infinite for the 100nm filler. It is shown that the viscosity is very sensitive to the filler

size. The high shear will helpful to reduce the viscosity by the shear-thinning force.


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Although this modeling work is not the same as the piratical cases, and the particle size

dependence for other dispersion systems may be different, there are many common

origins so that we can see the general trends for the filler size effect to viscosity.

Figure 1-16. Calculated viscosity at low shear rate as a function of particle diameter: (1)100nm; (2) 200nm; (3) 300nm; (4) high-shear limit

0.0 0.1 0.2 0.3 0.40

5

10

15

400

600

800

1000

Viscosityofcomposite(Pa.s

)

Filler volume fraction

nano silica

micron silica

Theoretical calculation

Figure 1-17. Viscosity of underfill with silica filler (nanosilica: 100nm; micron silica:3m, theoretical calculation is based on the Equation 1-8)


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We know that filler size reduction can lower the maximum packing density due to

the fast Brownian motion; therefore, the viscosity for nanoparticle filled dispersion is

inevitably higher according to Equation 1-9. Moreover, the particles with nanoscale size

possess potentially large excess interfacial free energies due to the high specific surface

area. The Van der Waals attractive potentials would lead to bare (un-stabilized)

nanoparticles having very strong net attractive interaction comparing to the large

particles. The overall effect is that in the absence of methods to lower the interfacial

excess free energy (interfacial tension) and influence the attractive potential, irreversible

flocculation of particles occurs. Therefore, the reduction of particle size causes

extremely difficulty for particle dispersion and liquid flow, especially for the underfill

systems in which the filler loading is usually very high.

1.5.Research objectives

Underfills are composite materials mainly based on epoxy chemistry and silica

reinforcement. Most of the underfill applications have strict requirements for the

rheological properties of the materials. The nanocomposite underfill presents a novel

solution to the filler entrapment issue in the no-flow underfill process and potentially can

provide significant reliability improvement for the large-area flip-chip packages.

However, challenges remain and some fundamental problems need to be addressed

before the successful implementation of the nanocomposite underfill to meet the

requirements of low cost, high yield and high reliability for flip-chip assembly. The key

issue in nanocomposite underfill and chemical processing is to find the stability

mechanism of colloidal particle in the polymer dispersion, where the traditional DLVO

mechanism by electrostatic repulsion and steric repulsion is absent. The present thesis

will pursue the following research objectives:

(1) To investigate the nanoparticle stability and dispersion in the underfill by

particle surface modification. Silane coupling agent with different functional groups will


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be characterized of their ability to alter the filler surface tension and change the liquid

wetting. Solder wetting test will be performed to prove experimentally that the nanosilica

can be used as filler in underfill and will not increase significantly the viscosity if the

optimal modification condition can be found.

(2) To measure the mechanical, thermo-mechanical, and physical properties of the

no-flow underfill with nanosilica. Apart from the impacts on viscosity of underfill, the

filler-filler interaction and filler-polymer interaction also affect the materials properties

due to the high interface area in the nanocomposite. Experiments will be performed to

assure that nanocomposites possess critical material properties for a successful no-flow

underfill process.

(3) To characterize the interphase properties of nanocomposites. Filler

agglomeration and poor filler-polymer compatibility result in deterioration of material

properties such as a lower Tg, higher moisture absorption, and increased viscosity[38].

Despite the large amount of research on the nanocomposites in recent years, the lack of

effective methods or techniques to characterize the interphase property has restricted our

understanding of the nanocomposites properties. The different structure and mobility of

interphase from the continuous bulk polymer phase will be detected, and the effect of

microscopic interphase properties will be studied on the macroscopic material properties

including Tg, dielectric, etc.

(4) To identify the liquid medium properties of the underfill materials. Control

over the interaction between colloidal fillers can be achieved not only by chemical or

physical modification of filler surface properties, but also by tuning the solvent (underfill

liquid) in which colloidal particles are dispersed. By changing the hardener compounds,

we easily can adjust the underfill liquid. A suitable underfill matrix will be found for

nanosilica dispersion.

(5) To study the cationic photo-polymerization of epoxy reinforced by the

nanosilica. The nanoscale size gives the filler unique optical properties and makes it


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possible to add filler into the photo-sensitive epoxy without blocking the UV absorption.

The novel photo-curable material with good thermal-mechanical properties can by

synthesized and studied. Understanding the effects of nanoparticles on curing kinetics,

reaction mechanism, and bulk material properties can provide guidelines for the design

and process of photo-curable nanocomposite for microelectronic packaging technology.


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CHAPTER 2. NANOSILICA SYNTHESIS AND MODIFICATION

The silicon dioxide particles (silica) are the most important composition in the

underfill to reduce the thermal expansion of epoxy polymer and reduce the cost of

material. As the reduction of flip-chip feature size, the filler inside the underfill is also

forced to shrink to the nanometer region. The synthesis of large scale nanosilica will be

different from micron size silica. How to add the nanosilica into the epoxy matrix without

significantly increasing the viscosity brings great challenges for the material development.

A fundamental understanding is needed for the surface chemistry of filler, filler

dispersion, solid-liquid interaction and composite liquid viscosity, etc, which will beinvestigated in this Chapter.

2.1.Silica synthesis

2.1.1. Pyrogenic silica

The name of silica comprises a large class of products with the general formula

SiO2. Some silica is a natural material, such as flint and quartz. Most of the silica used inthe industry is the synthetic amorphous silica. There are many different synthesis

methods to prepare silica, mainly divided into two categories: wet chemical route to

prepare the silica in the liquid phase and dry chemical route to form silica at high

temperature. A more profound discussion of the wet chemical route will be given below.

Here we are talking about the dry chemical route with pyrogenic condition.

There are two kinds of pyrogenic silica, fumed silica and fused silica. The fused

silica is made by the fusion of high purity sand in electric arc or plasma arc furnace at

temperatures of around 2000C. It is the noncrystalline form of quartz. As a typical

glassy state material, it lacks long range order in its atomic structure. But the highly

cross-linked three dimensional structures give rise to its high use temperature and low


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thermal expansion coefficient. Table 2-1 lists the physical, mechanical, thermal and

electrical properties of quartz and fused silica. Fused silica has comparable properties as

the crystal quartz in the terms of thermal conductivity, mechanical and electrical

properties, and furthermore, it has much lower coefficient of thermal expansion (CTE),

which is around 0.5 ppm/K. Therefore, the fused silica particles have been used as filler

in the underfill or encapsulant in the microelectronics application in order to reinforce the

epoxy and reduce the thermal expansion of the composite.

Table 2-1. Physical, mechanical, thermal and electrical properties of silica

Material Quartz Fused silicaDensity (g/cm3) 2.65 2.2Thermal conductivity (Wm-1K) 1.3 1.4

Thermal expansion coeff. (10-6K-1) 12.3 0.5Tensile strength (MPa) 55 110

Compressive strength (MPa) 2070 690-1380Poisson's ratio 0.17 0.165

Fracture toughness (MPa) - 0.79Melting point (C) 1830 1830

Modulus of elasticity (GPa) 70 73Thermal shock resistance Excellent Excellent

Permittivity (') * 3.8-5.4 3.8Tan (x 104) * 3 -

Loss factor ('') * 0.0015 -Dielectric field strength (kV/mm) * 15.0-25.0 15.0-40.0

Resistivity (m) * 1012-1016 >1018* Dielectric Properties at 1 MHz 25C

The fused silica formed by arc or plasma shows a greater variation in particle size.

The primary particles do not form chains but form dense, hard, non-microporous

secondary particles in the micron meter range. In the real application, fused silica needs

to be sieved to get well-controlled filler size. The silica used in the epoxy reinforcement

and thermal stability improvement for underfill and encapsulant applications are the

discrete spherical particles with defined shape and geometry, which is usually the fused

silica with micron range size. For even finer particle sizes, fused silica will be replaced


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by the nanosilica synthesized by the sol-gel process by precipitating and drying the

colloidal silica particle. A more profound discussion of this process will be given below.

Fumed silica is another kind of widely used silica additive, which is also

synthesized from the pyrogenic condition. Flame is used to burn the SiCl4with hydrogen

and oxygen, and following reactions take place (Figure 2-1):

2H2 + O2 2 H2O

SiCl4 + 2 H2O SiO2 + 4 HCl

2H2 + O2 + SiCl4 SiO2 + 4 HCl

Figure 2-1. Synthesis of fumed silica

Although with the same chemical composition, the fumed silica has quite

different structure and application from the participated silica or fused silica. The fumed

silica has a random network structure formed by SiO4 tetrahedrons, which are highly

disordered. Even with heated up to 1000C, the fumed silica dose not change its

morphology and will not crystallize neither. The precipitated silica differs considerably

from fumed silica, which are completely crystallized after only 20 minutes at 1000C[55].

Visually, the fumed silica is identified as a loose, bluish-white powder with about

98vol% air. The tapping density of fumed silica is around 0.05-0.12g/cm3, which is very

less than the silica density (2.2g/cm3). The primary particles of fumed silica are

extremely small, around 7-40 nm. They contact with each other by hydrogen bonds or

van deer Waals force and build up a loose, non-isolated network. Figure 2-2 shows the

TEM picture of the fumed silica.

With this special structure, the fumed silica, also known by their brand name Cab-

o-sil (Cabot) or Aerosil (Degussa), is usually used as a thixotropic additive which

when dispersed into the epoxy resin or other organic system increases viscosity, imparts

thixotropic behavior and adds anti-sad and anti-settling characteristics before and during


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the potlife or curing of an epoxy systems. Although the primary size of fumed silica is

small, it could not be used as added filler for epoxy reinforcement and CTE reduction

because of its loose structure.

Figure 2-2. TEM picture of fumed silica structure[56]

2.1.2. Sol-gel method

The sol-gel process is a wet chemical method to make ceramic or glass materials.

The sol is made of solid particles of a diameter of few hundred nanometers, suspended in

a liquid phase. In a typical sol-gel process, the precursor is subjected to a series of

hydrolysis and condensation (polymerization) reactions to form a colloidal suspension,

then the particles either condense in a new phase, the gel, in which a solid macromolecule

is immersed in a solvent, or grow up to large particle. For the silica sol-gel synthesis, the

alkoxysilane such as tetramethoxysilane (TMOS) or tetroethoxysilane (TEOS) are used

as starting compounds.

The alkoxysilane can hydrolyze into monomer Si(OH)4in the polar solvent such

as ethanol. In order to get a rapid and complete hydrolysis, a base or acid catalyst and

trace amount water may be used. The reaction occurs by a nucleophilic attack of the

oxygen contained in water to the silicon atom[57]. For the basic catalyzing reaction, the

50nm


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leaving alkoxy group is rapidly protonated. After forming a pentacoordinate intermediate,

and a hydroxyl group is formed on the silicon atom (Figure 2-3 A). The four alkoxy

groups on the Si atom will be hydrolyzed one by one with enough reaction time.

In the condensation step, the silicic acid Si(OH)4 molecules condense to form

siloxane bonds, with release of water. The condensation reaction may also occur between

the alkoxysilane and the silanol group, releasing an alcohol (Figure 2-3 B). The as-

formed siloxane molecule can be considered as the monomer which can further cross-link

to difference structure. The condensation reaction may also be acid or base catalyzed.

H2O + B BH Si OROH

Si B+ +R OHOH

Si OR+

Si OH + Si OH Si O Si +H2O

Si OH Si OR+ Si O Si +ROH

hydrolysis

condensation

A:

B:

Figure 2-3. Reaction process of sol-gel method for silica generation (with basic catalyst)

The hydrolysis and condensation occurs simultaneously. The relative rate of both

processes determines the products structure[58, 59]. In acidic conditions, hydrolysis is

faster than condensation. The rate of condensation slows down with increasing number of

siloxane linkages around a central silicon atom. This leads to weakly branched polymeric

network and further evolution will give the gel structure. In the basic condition, on thecontrary, condensation is accelerated relative to hydrolysis. The rate of condensation

increases with increasing number of siloxane linkage. Thus, highly dense network with

ring structure are formed and gives sol structure. In the initial state, the primary particle

size is limited to about 2 nm. Under the basic environment, the particles are charged by


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ionization so the aggregation and bridge formation between particles are limited. The size

growth occurs by monomer deposition and Ostwald ripening. Figure 2-4 illuminates the

growth process an

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Sun Yangyang 200612 Phd