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SMBS, VIT University, Vellore , India 40

Design, Manufacture and Static Reliability Analyses

of a 56L Thin Quad Leadless Moulded Package

(TQLMP)K. Padmanabhan S. Subeesh and G. Saravanaram

Abstract-This investigation is a part of the ongoing activities in thejoint project with SPEL Semiconductor Ltd. on advancedsemiconductor materials and packaging. Design of the entire 56LThin Quad Leadless Moulded Package (TQLMP) was done in Pro-EWildfire 3.0 and the performance analysis accomplished using the

ANSYS v.11 FEM ( Finite Element Method) software. The uniquefeature of the 56L IC is that it is rectangular and not a square designlike the 48L TQLMP. Reliability plays a major role at every stage inthe manufacture, testing and use of integrated circuit packages. The

56L ICs were encapsulated using a nano-composite mould compoundand the Resin Transfer Moulding method (RTM). The electro-hygrothermo-mechanical reliability of the IC packages wereinvestigated and reported with the aid of finite element coupled field

analysis and corroboration of the results with experimental results. Inorder to improve reliability, suggestions were made based on thefeed back from the analyses. The influence of operating voltages and

joule heating on the mechanical reliability of ICs were investigated.

Die shear tests were conducted on the ICs and the shear strengthcompared with the actual values obtained from the finite elementresults during operating temperatures. Thermal analyses with FEMand thermal tests in a chamber carried out on the ICs were later

inspected under a Scanning Acoustic Microscope (SAM) for possibledelaminations The influence of moisture at elevated temperatureswas also studied for the aforementioned IC. in order to ascertain thereliability of the IC and its encapsulation despite hygrothermalattack.. Finally all the results of the electro- hygrothermo-mechanical

analysis were analyzed and presented. Failure theories were appliedto verify the reliability of the IC from a knowledge of the obtainedstresses and strains from FEM simulations and design or materialsdata.. Finally, the overall reliability of the IC was established.

Key words : 56 L TQLMP, Reliability, FEM, SAM

I. INTRODUCTIONThe joint project with an electronic industry SPEL

Semiconductor Ltd [1] to improve the understanding and

exploitation of advanced semiconductor materials and

electronic packaging of ICs is unique as it caters to the needs

of customers all of whom are from the export market. This

project assumes great significance as the entire 3D design and

K. Padmanabhan, Centre of excellence in Nano-composites. School ofMechanical and Building Sciences, VIT-University, Vellore-632014.

S. Subeesh Spel Semiconductor Ltd. Maraimalai Nagar, Chennai

G. Saravanaram, Sri Venkateswara College of Engineering , Sri Perumbudur

analyses for the ICs developed , are carried out with the

facilities available with the the joint collaborators .

Among the electronic packages the quad packages are

relatively new and have high design complexities coupled

with superior function at a small size. A detailed account of

the types of packages is given in Microsystems Packaging by

Tummala Rao [2]. As we have ventured in to manufacturing

the quad packages like the Thin Quad Leadless Moulded

Package ( TQLMP) that are increasingly being used in mobile

phones, digital cameras, PDAs and automobile control

systems , there is enormous customer curiosity on the

reliability of the miniaturized products manufactured with

these ICs. Hence there is an imperative need to address these

issues and demonstrate the evaluation techniques and

conformance of these packages to user requirements. Figure 1

shows the architecture of a simplified plastic IC package.

Plastic packages are non-hermetic meaning permeable but

flexible and cost affective at operating temperatures below

200 C.

Figure 1: Two-dimensional cross section model of an IC package

Plastic based non-hermetic packages are more popular with

the manufacturers of electronic packaging. Their maximum

operating temperature is low and they may face a hoard of

problems that may lead to premature failure. The causes of

such failures are mostly due to joule heating due to resistive

heating ( I2 R losses), electrostatic discharge (ESD),

hygrothermal fracture , electro-thermo-mechanical fatigue,

manufacturing defects like delaminations, soldering defects

and material defects.

In the present investigation the complete architecture of a

56L TQLMP was designed in Pro-E modelling software and

analyzed for electro-hygro-thermo-mechanical static reliability

using the ANSYS version 11 finite element modelling

software [3] and reliability tests. As the IC packages

experience joule heating during operation due to the

application of voltage difference and current they cause

thermal stresses to be induced in the package. As thermal

stresses cause warping, bending, twisting and other type of

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deflections, the mechanical stresses develop in the package

that is fixed to a Printed Circuit Board (PCB) or a Printed

Wiring Board (PWB) in the leads and the package as whole.

The reliability and durability of some of the quad packages

have been investigated earlier by Padmanabhan et al [4,5,6].

Here, the results from the simulation studies and calculations

based on actual operating conditions are compared with the

experimental results from the die shear tests , thermal pre-

conditioning tests and delamination observation under thescanning acoustic microscope (SAM). Comparison reveals

that the operating stresses are lower than the failure stresses

with the factor of safety included. The significance of this

investigation is in the consideration of the actuarian multiple

effects that the IC packages go through. The Design for

Manufacturing, Assembly and Environment (DFMAE)

approach is emphasized here as concurrent design, analysis,

manufacturing and testing feedback principles are employed

keeping cost in mind. The ICs fabricated by SPEL

semiconductors are also lead free thereby addressing the issue

of environmental impact.

II. MODELLING AND FINITE ELEMENTANALYSIS :

The 3 Dimensional model was created using the Pro-E

wildfire 4.0. For analytical requirements the model was

created with and with out the IC plastic encapsulation as the

property evaluation had to be done for both the conditions in

order to clearly identify the influence of the mould compound.

The copper lead frame shown in figure 1 forms the skeletal

frame on which the silicon die is pasted using a conductive

silver epoxy paste. Gold wires with a diameter of 25 microns

are bonded from the leads on to the silicon die in order to

enable the IC to receive and transmit signals. The entire micro

assembly is encapsulated with a mould compound consistingof epoxy resin, phenolic resin, carbon black powder and fused

silica powders through the Resin Transfer Moulding (RTM)

technique. Here, in the 3D model the IC with the mould

compound encapsulation is shown to be transparent in order to

provide visual details keeping in mind the material properties.

Figures 2 (a) and (b) show the design of 56L TQLMP

packages with encapsulation . The model was imported in to

the ANSYS in the Initial Graphics Exchange Specification (

IGES) format. A coupled field electro- thermo-mechanical FE

analysis was conducted using the materials properties given in

Table 1 for the various materials used in the design . The

coupled field analysis was carried out using a solid element

with 8 nodes and capable of two degrees of freedom ,i.e.

temperature and voltage. The element is also capable of three

dimensional thermal and electrical conductivity. So it was

used for the electrical thermal coupled field analysis

involving joule heating and another solid element was used

for the thermal-mechanical coupled field analysis involving

the von mises equivalent stresses and shear stresses. The

thermal results from the first coupled field analysis were fed in

to the next analysis through an `.rth file and the thermal-

structural analysis performed using another solid element that

is used for this coupled field analysis. The results from the

thermal-structural analyses were stored in an `.rst file. Thus it

was possible to evaluate the electrical influence on the thermal

characteristics and then the thermal influence on the

mechanical characteristics of the IC package.

Figure 2 (a) 56L TQLMP IC with encapsulation

Figure 2 (b) 56L TQLMP IC gold wire bonding.

Meshed model of the 56L IC without encapsulation is

shown here in figure 3. According to the properties to be

derived the models were meshed with and without

encapsulation.. The models were constrained at the lead lines

along the four outer edges of the copper lead frame as it would

be during operation. A manufacturer recommended potential

difference of 5 volts was applied to the IC at the designated

leads. The results for the 56L IC obtained from the two

coupled field analyses are shown in figures 4 to 6. As the IC

is supplied with a voltage it takes a few minutes to reach a

steady state due to joule heating. Hence all the results

presented are from steady state electrical- thermal coupled

field analysis. No transient analysis is presented here due to

the practicality of the approach of this investigation. Figure 4

shows the maximum temperature achieved in the

encapsulated package due to joule heating which is 33.4 C.

Figures 5 and 6 indicate the XY shear stresses developed in

the entire package and the XY plane shear stress developed at

the silicon die and the copper lead frame interface bonded

with a conducting silver epoxy adhesive. The XY shear stress

is of significance as the stresses developed in operation can be

compared with the actual shear strength of the interface. The

shear stresses developed should be at least 2.5 times ( factor

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of safety) lower than the shear strength of the silicon die-

copper interface in order to qualify the IC for reliability.

Figure 3: Meshing of IC without encapsulation

Figure 4: Joule heating distribution of IC

Figure 5: XY Shear stresses with encapsulation

Figure 6: XY shear stress at Si-Cu bonding

TABLE IPACKAGING MATERIALS PROPERTIES

MATERIALSYOUNGS

MODULUS(N/mm2)

CTE

(1/C)

ELECTRICAL

RESISTIVITY( - mm)

THERMAL

CONDUCTIVITY(W/mm- C)

POISSONS

RATIO

Copper 110.31e3 16.50e-6 1.673e-5 401e-3 0.22

Silver epoxy 11.5e3 30e-6 8e-4 546e-3 0.35

Silicon 200e3 2.60e-6 1 130e-3 0.25

Gold 82.737e3 14.20e-6 2.350e-5 317e-3 0.44

Filled epoxy for 36L (M1) 26e3 7e-6 - 0.0012e-3 0.25

Filled epoxy for 64L (M2) 33e3 1.5e-5 - 0.0012e-3 0.25

III. EXPERIMENTAL DETAILS:The die shear test apparatus to measure the shear strength

of the silicon die-copper lead frame interface adhesively

bonded with a silver epoxy paste ( with out encapsulation),

shown schematically in figure 7, shows the silicon die

adhesively bonded to the copper lead frame clamped to the

apparatus. A shear beam with a wedge is tightened laterally

until the die is sheared of the copper lead frame. The average

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load from about five tests, required to shear the die is noted

and the shear strength is calculated based on the surface

contact area of the die with the lead frame. Here, temperature

rise , the shrinkage pressure due to the encapsulation

moulding and the thermal stresses developed in the

encapsulation while in operation have a role to play in the

origin of XY shear stresses , the magnitude of which is ~ 2

MPa ( Fig 6). The die shear strength evaluated through the

experimental setup was 24.51 N/mm2 or MPa which ishigher than the XY shear stresses developed during operation

by at least a factor of safety (FOS) of 10.

Figure 7: Silicon- copper lead frame die shear test

Figure 8 : SAM showing delaminations

(as red areas) in the IC

The manufactured ICs are subjected to a preconditioning test

(Thermal Shock Test (JESD22 A106B) [7] involving

temperature cycles. As the temperature of the ICs is raised

beyond 125 C which is near the glass transition temperature

of the encapsulant resin and the silver epoxy adhesive, to up

to 260 C for lead free products , delaminations were

observed in the encapsulated ICs .This was confirmed

through Scanning Acoustic Microscope (SAM) observation

of the conditioned specimens. Figure 8 provides information

about the extent of delaminations after preconditioning at the

aforesaid temperatures for 24 hours or more. As the IC

temperature during operation is not expected to go beyond 34 C as evinced through the joule heating analysis it is clear

that delaminations occur only when the IC is treated near and

above the glass transition temperature (Tg) of the epoxy for

long durations, due to the differences in the CTE of the

different materials, as discussed above. As there is a clear

margin between the maximum temperature reached during

operation and the failure temperature due to the development

of thermal stresses, it is found that the IC will not reach its

failure limits during operation. According to the ASTM STP

5229 M rule [8] the MOT ( Maximum Operating

Temperature) of the material, device/component should be at

least 25 Celsius lower than the lowest wet Tg of the material.

In this case the mould compounds qualify this clause. This

investigation deals with more reliability analyses based on

failure theories and hygrothermal influences . The 56L

TQLMP fails at around 110 C by using mould compound

Mould 2. By using the mould compound Mould 1 the design is

more robust as the wet Tg is about 121C.The ensuing

sections present thehygrothermal and the failure analyses ofthe 56 L ICs.

IV. HYGROTHERMAL ANALYSISThe packages involved in this investigation are polymeric

epoxies that are non-hermetic, meaning permeable to moisture

and transport of other ingredients. Among the many

environmental conditions that may influence polymer

mechanical behaviour, changes in temperature and moisture

contents are important for discussion. Effects of temperature

areusually referred to as thermal effects, whereas those ofmoisture are referred to as hygroscopic effects. The word

hygrothermal is used to describe the combined effects oftemperature and moisture. There are two principal effects of

changes in the hygrothermal environment, on mechanical

behaviour of polymers.

Stiffness and strength are altered. Increasing temperature

causes a gradual softening of the polymer material up to a

point. If the temperature is increased beyond the so called

glass transition region (indicating a transition from glassy

behaviour to rubbery behaviour) the polymer becomes too soft

for use as a structural material. Plasticization of the polymer

by absorbed moisture causes a reduction in glass transition

temperature (Tg). Thus the wet Tg is always lower than the

dry Tg and a corresponding degradation of polymer propertiesresults. Recent attention by Andrew Tay [9] and Ji Hyuck

Yang and Kang Yong Lee [10] are on the hygrothermal

reliability aspects of various IC packages.

The hygrothermal degradation of the polymer encapsulant

strength or stiffness can be estimated by using an empirical

equation given in Ronald Gibson [11], that is -

--- (1)

Tgw= (0.005Mr2-0.1Mr+1.0)Tgo

Where,

Fm = Mechanical property retention ratio, P =

Strength or stiffness after hygrothermal degradation, Po =

Reference strength or stiffness before degradation,T =

Temperature at which P is to be predicted (0C), Tgo= Glass

transition temperature for reference dry condition , Tgw

=Glass transition temperature for reference wet condition at

==

TT

TT

P

P

F

go

gw

m

0

2/1

0

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moisture content corresponding to property P, To = Test

temperature at which Po was measured, Mr = Weight percent

of moisture at equilibrium.

In our package the encapsulation is made up of a filled

epoxy material which absorbs moisture. Two types of

encapsulation moulding materials were used by SPEL

Semiconductor Ltd. Figures 11 to 13 show the retention ratios

and the strength degradation for the two moulding compoundsat different temperatures after saturation moisture

conditioning. The maximum moisture concentration for the

two moulding compounds as determined was 0.23 wt %

(Mould compound 1) and 0.3 wt % (Mould compound 2)

respectively.

Figure 11 Temperature Vs Retention Ratio plot

Figure 12 Temp Vs strength plot of Mould Compound 1

Figure 13 Temp Vs Strength plot of mould Compound 2

used in encapsulation Figure 14. 56L

TQLMP - XY stress

The actual strength of the encapsulant resin is reduced due

to the moisture absorption especially at higher temperatures.

The strength degradation versus temperature plots for a

saturation moisture conditioned mould compound 1 andmould compound 2 are shown in figures 12 and 13.The XY in

plane stresses of the encapsulation alone at 34 C are shown

in Figure 14.

V. FAILURE THEORIES AND ANALYSESThe failure of composites has been investigated extensively

from the micromechanical and the macromechanical points of

view. As the encapsulant mould compound is a filled

composite , failure theories for composites are required to

describe and predict failure. Failure predictions based on

micromechanics, even when they are accurate with regard to

failure initiation at critical points, are only approximate withregard to global failure with respect to orthotropy. For these

reasons a macromechanical approach to failure analysis is

preferred.

Numerous failure theories have been proposed exclusively

for composite materials and are available to the composite

structural designer. They are classified into three groups, limit

or non interactive theories (maximum stress, maximum strain)

, interactive theories by Azzi-Tsai-Hill [12] and Tsai-Wu [13]

and partially interactive or failure mode based theories . The

M 1

M2

Mould 2

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validity and applicability of the above theories depends on the

phenomenological events like interaction between the normal

and shear stresses and agreement with experimental results.

At the maximum operating temperature i.e at ~ 34 C , the

following values were obtained for the stresses actually

developed and the mean strength of the 36L package

encapsulation using mould compound 1. The values of the

tensile , compressive and shear strengths for the two mpuldcompounds at a designated temperature can be read off from

figures 12 and 13. The actual stresses developed can be

extracted from the FE analyses. These values correspond to

the encapsulation material only and not the operative part of

the package. For the total shear stress developed in the

package refer figure 5.

For Mould compound 1 the following are the stress details.

Maximum operating conditioning i.e at 34 C ( lengthwise)

X stress in MPa = 35.303

Y stress in MPa = 3.31XY shear stress in MPa = 24.26

Tensile strength in MPa =180

Compressive strength in MPa = 190

Shear strength in MPa = 90

When the failure theories are applied the parameters on the

left hand side are lesser than 1 or the failure index. This is

shown below

Tsai Wu Failure theory: 0.116 < 1

Azzi-Tsai-Hill theory: 0.107(Tensile) / 0.104(compressive)

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ICAMB - 1019