I N N O V A T E. C R E A T E. M A K E T H E D I F F E R E N C E.TM����� ����� ����6th TRC Oct. 27-28, 2003 Austin, TX 1

Influence of Solder Reaction Across Solder Joints

Kejun ZengFC BGA Packaging Development

Semiconductor Packaging DevelopmentTexas Instruments, Inc.

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Outline

� Introduction

� Solder reactions of metals

− Dissolution of metals in molten solders

− IMC formation during reflow

− IMC formation during solid state aging

� Solder reactions across joints

– Cu6Sn5 appears on Au/Ni(P) pad of substrate

– AuSn4 forms throughout solder joint

� Influence of metals on reliability across joints

– Au enhances consumption of Ni(V)/Cu UBM

– Ni plating is consumed by Cu6Sn5

� Summary

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Introduction

� With the electronic devices being continuously scaled down, solder reaction is becoming one of the major concerns for packaging reliability.

� Due to the Pb-free requirement, new surface finishes have appeared in the market and more are being studied.

� Persistence of the Black Pad problem results in the application of OSP-Cu or bare Cu.

� Local effects of solder reaction on joint reliability has been extensively studied, but its effects on the other side of the joint is relatively new to the industry.

� Theories for the study of solder reactions:

− Local equilibrium− Reaction path− Dominant diffuser

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Dissolution of metals in molten solders

W

Stable phase diagram

Metastable phase diagram

StableMetastable

Actual concentration of Cu in molten solder at the interface can be higher than what the stable phase diagram indicates, depending on the surface condition of Cu and the time above the eutectic T.

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0.0001

0.001

0.01

0.1

1

1.4 1.5 1.6 1.7 1.8 1.9 2 2.1

1000/T(K)S

olub

ility

(Mol

e fr

actio

n)

Au

Ni

Pd

Cu

0.0001

0.001

0.01

0.1

1

1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2

1000/T(K)

Sol

ubili

ty (M

ole

frac

tion)

Cu

AgAu

Ni

Pd

Metal solubility in Sn-Pb Metal solubility in Sn-Ag

Thermodynamically calculated equilibrium saturation solubility of metals in molten solders. (1) Ni has the lowest solubility in solders. (2) Solubility of metals in Sn-Ag, SnCu, and Sn-Ag-Cu solders is higher than in Sn-Pb solder.

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Dissolution rates of various metals in the 60Sn-40Pb solder as a function of T [Bader, 1969]. Note that the latest experimental results showed that the solder reaction of Pd was very fast [Wang & Tu, 1995].

Schematic concentration profile of metal in moleten solder. Higher solubility � greater gradient � higher diffusion rate � higher dissolution rate.

Solder

Metal

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IMC formation during reflow

350°C

Reaction path (arrows exaggerated for readability). After saturation solubility is reached, Cu6Sn5 forms at interface. Because Cu6Sn5 is not in equilibrium with Cu, Cu3Sn appears between Cu6Sn5 and Cu.

400°C

Following the same procedure, it is predicted that in high Pb solder joint Cu3Sn is the first IMC to form. This is in agreement with experimental results by Grivas et al., 1986.

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220°C

After reflow process, Cu6Sn5 is usually observed in the eutectic solder joint. A thin layer of Cu3Sn forms but is not easy to see.

Eutectic SnPb/Cu joint after 1 reflow. SEM image (Texas Instruments).

Cu6Sn5

Cu3Sn

Cu

Eutectic SnPb/Cu/Ni(V)/Al joint after 3 reflows. Voids are marked by red arrows. TEM image. (Courtesy of K. N. Tu, UCLA)

1µm

Cu6Sn5 Ni(V)

Al AlSiO2

Si

Cu3Sn

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IMC formation during solid state aging

175°C

During solid state aging, both Cu6Sn5 and Cu3Sn grow thicker.

SnPb/Cu joint after 40 days at 150°C. Since Cu is the dominant diffuser in Cu6Sn5, Kirkendall voids form in the Cu3Sn layer. SIM image (Texas Instruments).

Cu6Sn5

Cu3Sn

Cu

Solder

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SnPb/Au/Ni/Cu BGA joint after 500 hours at 160°C. During reflow, Au plating was totally dissolved away. After aging, Au diffused back to the interface and form (Au,Ni)Sn4. SEM image. (Courtesy of C. R. Kao, National Central University, Taiwan).

Ni can be dissolved into AuSn4 by replacing gold atoms. This is because the dissolution of Ni can lower Gibbs energy of AuSn4. Thermodynamic calculation shows that the max. solubility is 10 at.% or 4.9 wt.% and the max. energy change is 3 kJ/mole of atoms.

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Cu6Sn5 appeared on other side

SnPb flip chip bumps on Ni/Au pad. Cu in the interfacial compounds came from Cu UBM on the die side. (Cu,Ni)6Sn5 composition (at.%): 46.7Cu, 8.2Ni, 45.1Sn. (Texas Instruments)

This is an indication that Ag and Pd can also reach the other side of a joint after reflow.

Cu6Sn5

Ni3Sn4

Ni

Cu

Solder

(Cu,Ni)6Sn5

Cu

Ni

a) After 3 reflows b) After 150°C/1000 hour baking

Solder

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AuSn4 formed throughout joint

Eutectic Sn-Pb solder cap on Au at 200°C. a) after 5 seconds, b) after 60 seconds. AuSn4compound has extended all the way to the surface of the cap. With a diffusivity of 10-5 cm2/s, Au atoms can diffuse a distance of 100 µm in 5 seconds to the cap top. (Kim & Tu, 1996).

Au

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Au enhances consumption of Ni(V)/Cu UBM

Ni(V): 400 nm

Passivation layer

SiO2

Cu: 300 nm

Al: 400 nm

Au: 0.125 µm

Ni(P): 10 µm

Solder

Si

FR4

Cu pad

50% after 1 reflow; 100% after 3 reflowsSn-3.5Ag-1.0Cu

30% after 10 reflows; 100% after 20 reflowsSn-37PbNi(P)/Au/solder/Cu/Ni(V)/Al

60% after 10 reflowsSn-3.5Ag-1.0Cu

No dissolution after 20 reflowsSn-37PbSolder/Cu/Ni(V)/Cu/Al

Fraction of Ni(V) dissolved by molten solderSolderJoint structure

(See Zeng & Tu, Mater. Sci. Eng. Reports, 2002)

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Au/Ni(P) pad

Cu/Ni(V)/Al UBM

SnAgCu bump

(a)

Au/Ni(P) pad

SnAgCu bump

Cu/Ni(V)/Al UBM(b)

SEM images of cross-section of a eutectic SnAgCu flip chip joint. (a) As-bonded. (b) After 10 reflows, IMC crystals have been spalled into bulk solder. Ni(V) layer has been completely dissolved. The presence of Au has enhanced the dissolution of Ni(V) layer. (Courtesy of M. Li, Institute of Materials Research and Engineering, Singapore)

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Cu6Sn5compound

Cr

Si5 µm 1µm

(a) SnPb on Au/Cu/Cr UBM. Cu6Sn5scallops were decorated with small particles. [Liu&Tu, 1996]

(b) SnPb on Cu/Ti UBM. Surface of Cu6Sn5 scallops were smooth. [Kim&Tu, 1996]

Ni(V)

r

Cu6Sn5

Ni(V)

RCu6Sn5

When Au is present, Cu6Sn5 crystals become spherical, exposing Ni(V) to the solder.

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Ni plating is consumed by Cu6Sn5

If one side of a joint has Ni plating and the other side has bare Cu or OSP-Cu, the Cu may decrease the reliability of the Ni plating. Here is a such an example. After 1000 hours of baking at 150°C, Cu6Sn5 had dissolved a great amount of Ni from the Ni plating so that the Ni plating became porous.(Texas Instruments)

Ni

(Cu,Ni)6Sn5

Solder

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Thermodynamically calculated phase diagram showed that the maximum solubility of Ni in Cu6Sn5 is 6Cu:4Ni, corresponding to 21.8 at.% Ni. Experimentally, the Ni content in the interfacial (Cu,Ni)6Sn5 layer in page 16 was measured by EDX to be 21 at.%. The driving force for the dissolution of Ni into Cu6Sn5 is the Gibbs energy change.

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Summary

� Ranking of dissolution rate of metals corresponds to that of saturation solubility of the metals, with Au being the fastest and Ni the slowest.

� Dissolution rate of metals in the eutectic SnAg solder is higher than in the eutectic SnPb solder.

� Dissolved metals may diffuse across the solder joint during reflow process, altering the solder reactions on the other side of the joint and thus influence the joint reliability.

� When assessing solder joint reliability, influence from the other side of the joint should also be considered if the surface coatings are different on the two sides.

� Interfacial reaction is controlled by thermodynamics (Gibbs energy change) and diffusion kinetics.

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References

� W. G. Bader, Weld. J. Res. Suppl., 1969, 28, 551s-557s.

� D. Grivas, D. Frear, L. Quan, and J. Morris, J. Electr. Mater., 1986, 15, 355-359.

� Y. Wang, H. K. Kim, H. K. Liou, and K. N. Tu, Scr. Metall. Mater., 1995, 32, 2087-2092.

� P. G. Kim and K. N. Tu, J. Appl. Phys., 1996, 80, 3822-3827.

� H. K. Kim, K. N. Tu, and P. A. Totta, Appl. Phys. Lett., 1996, 68, 2204-2206.

� A. A. Liu, H. K. Kim, K. N. Tu, and P. A. Totta, J. Appl. Phys., 1996, 80, 2774-2780.

� C. Y. Liu, K. N. Tu, T. T. Sheng, C. H. Tung, D. R. Frear, and P. Elenius, J. Appl. Phys., 2000, 87, 750-754.

� C. E. Ho, W. T. Chen, and C. R. Kao, J. Electr. Mater., 2001, 30, 379-385.

� K. Zeng and J. K. Kivilahti, J. Electr. Mater., 2001, 30, 35-44.

� K. N. Tu and K. Zeng, Mater. Sci. Eng. Reports, 2001, 34, 1-58.

� K. Zeng and K. N. Tu, Mater. Sci. Eng. Reports, 2002, 38, 55-105.

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Acknowledgements

The thoughts on this topic and the approaches taken in the studywere initiated and developed when the author was with

• Lab of Electronics Production Technology, Helsinki University of Technology, Finland

• Electronic Thin Film Lab, University of California at Los Angeles, USA

The author would like to thank Prof. J. Kivilahti and Prof. K. N. Tu for inspiring discussions. Support from Texas Instruments is acknowledged.