Cure Shrinkage Analysis of Green Epoxy Molding Compound with Application to WarpageAnalysis in a Plastic IC Package

Guojun Hu, Spencer Chew and Bawa SinghCookson Semiconductor Packaging Materials, 12 Joo Koon Road, Singapore, 628975

Email: ghugcooksonelectronics.com, schewgcooksonelectronics.com

AbstractFinite element analysis (FEA) is widely used throughout

the electronics industry to understand the thermal and thermo-mechanical behavior of electronic packages and interconnectduring their manufacture and lifetime. After encapsulation,thermo-mechanical stress builds up within the package due totemperature coefficient of expansion mismatch between therespective materials within the package as it cools to roomtemperature. Due to the complexity and time consuming ofthe calculation, it is almost impossible for industry to carryout the numerical simulation using viscoelastic propertywhich is the most close to the real material property ofpolymer material. However, FEA using temperature-dependent elastic property, temperature-dependent thermalexpansion coefficient and accurate chemical cure shrinkagecan help to improve the accuracy on the stress and warpageprediction. This study has developed an evaluation method forthe chemical cure shrinkage based on the measurement of thewarpage of bimaterial model. The results show that FEAsimulations without chemical cure shrinkage fail to accuratelypredict the package warpage. On the other hand, FEAsimulations with chemical cure shrinkage is outlined whichshow fair agreement with experimental measurements ofpackage warpage over a range of temperatures.

IntroductionOwing to its low cost, high performance and good

reliability, plastic encapsulation is employed for a significantpercentage of IC packages. Warpage of IC packages afterencapsulation is a major concern in package developmentbecause of the risk of die cracking if a tensile stress is built upin the die particularly in thin packages. Also excess warpagecan cause problems with mounting the package onto a PCB oranother package such as POP, so it is important to be able topredict residual warpage of plastic IC packages. Realisticmodeling and analysis of the mechanical performance andreliability of electronic packages requires sophisticatedconstitutive models for the many complex, non-traditionalengineering materials that compose these intricate devices.The proper modeling of the behavior of such plastic materialsis becoming important in the reliability studies of these ICpackages.

The prediction of warpage and stress in plastic ICpackages can be true only if the complex time andtemperature-dependent property of molding compound aretaken into account. In the analysis of plastic-encapsulated ICpackages, the viscoelastic property of materials such as epoxymolding compound or underfill has usually been neglecteddue to the complexity of viscoelasticity characterization andthe epoxy molding compound is often modeled as an elasticmaterial. At the same time, the warpage using the elastic

property of epoxy molding compound will overestimate thestress and energy release rates during finite element analysis[1, 2]. However, the warpage is actually underestimated basedon the elastic property of epoxy molding compound accordingto some former studies. It seems that the ignorer of chemicalcure shrinkage due to the crosslinking of epoxy resin is themain reason for the underestimation of warpage [3, 4] ratherthan the effect of viscoelasticity property. Thus it may bepossible that FEA using temperature-dependent elasticproperty, temperature-dependent thermal expansioncoefficient and accurate chemical cure shrinkage can providean accurate prediction of warpage.

Some former studies have already been carried out on theeffect of chemical cure shrinkage [3, 4]. However, themeasurement of chemical cure shrinkage in a moldingcompound and the effect of chemical cure shrinkage onwarpage have not been clarified. The extent of warpage isgoverned by total shrinkage (chemical cure shrinkage +thermal shrinkage) of the epoxy molding compound thatencapsulates the IC chip. In particular, the chemical cureshrinkage exerts great influence on warpage and has almostthe same effect on warpage compared with the thermalshrinkage. This paper addresses the warpage problem fromthe viewpoint of experiment measurement and numericalsimulation. We have developed an evaluation method basedon the measurement of the warpage of bimaterial model andPBGA (plastic ball grid array). The results show that FEAsimulations without chemical cure shrinkage show goodagreement with experimental measurements of packagewarpage over a range of temperatures. It is necessary to takeinto account molding compound chemical cure shrinkage inaddition to thermal shrinkage, for the accurate prediction ofwarpage for plastic encapsulated packages.

Bimaterial Model for Warpage TestCure shrinkage refers to volumetric reduction measured at

the same temperature of the EMC before and after curing dueto the chemical crosslinking reaction. For pure epoxy resins,the volume change between the cured and the uncured state ofthe resin is usually a reduction in volume (chemicalshrinkage) of 1 to 6% [5] which should be smaller for epoxymolding compound due to the high percentage of silica (70/O-900o/).

It is well known that epoxy molding compound hasdifferent thermal expansion coefficients below and aboveglass transition temperature ( Tg ). Below Tg, polymer is

glassy and molecules have little relative mobility. Above Tg,

the polymer becomes rubbery and polymer chains gainenough mobility to slide by each other. Thus the thermalexpansion coefficients (CTE) are different below and above

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Tg, which are determined from the slope of expansion

curves. As shown in Fig. 1, the total shrinkage for a moldedpart includes two parts that chemical cure shrinkage occurs ina metal mold at molding temperature and thermal shrinkageoccurs during cooling:8total =L°I(Tg-T'5°2)+at(T75°C -Tg)]+ 8chemical cure (1)

ThusChemical cure shrinkageshrinkage

Total shrinkage -Thermal(2)

..T

TherinalSlulitkage

Cheinical Clwe

Sl inkige

beam of unit width heated from temperature To to T inabsence of external forces by Timoshenko is [7]:

2

d 3L2 LI+tb craXNT)(ta hb ) 3(1 ta ) (Ii( t

(3)

where ta and tb is the thickness of the two layers of the

bimaterial plate, Ea and Eb are the Young's modulus, andcra and acb are the thermal expansion coefficients for thematerials of these layers, respectively.

319 ( + ta [(rib-ra XT-TO ) + 8chemical cure]

4(ta + +t ) +a2 ta Ea ta2 + tb Eb

L tb tb Eb t tb2

ta Ea )2

(4)

V.,

25°C TgTempelatine ( C)

1 (5rC

Fig. 1 Chemical cure shrinkage and thermal shrinkage duringtransfer molding.

It is usually difficult to model fully the chemical cure

shrinkage in an epoxy molding compound because of largechanges in viscoelastic properties during the curing process

and the thermal shrinkage after curing [6]. Therefore, indirectexperimental techniques are needed to develop usefulinformation about cure shrinkage.

The test piece used for warpage is shown in Fig. 2. Thetop layer is made of copper which is conventionally used inQFN. The bimaterial specimen is selected for the experimentand the size of specimen is 50mmXlOmmXO.25mm forcopper layer and 50mmXlOmmX2.5mm for moldingcompound layer. The test piece is molded in a transfermolding system under the following molding conditions:molding temperature is 175°C, cure time is 2 minutes, andpost cure is carried out at 175°C over 4 hours. The warpages

within the temperature range of 25°C to 260°C are measuredusing shadow moire.

Copper- 0.25iran

EMC: 2.5mm

10m 5Orumi

Fig. 2 Dimension of bimaterial specimen.

The warpage in a bimaterial specimen can be simulatedusing the linear bimetallic beam model or using finite elementanalysis. Timoshenko [7] was the first to study the stresses inlayered structures and used elementary beam theory to obtainthe curvature of a bimetallic beam due to uniform temperaturechange. The general expression for the warpage of curvature

For the consideration of chemical cure shrinkage, thegeneral expression for the warpage of curvature beam of unitwidth heated from temperature To to T is expressed using

equation (4) where 8chemical cure is the chemical cure

shrinkage.

Discussion 3 Cure Shrinkage Analysis using BimaterialModel

N

Tenperttwe (IC)

Fig. 3 Young's modulus of epoxy molding compound.

The values of storage modulus, which can be taken as theYoung's modulus of EMC, are used in the calculation ofbimaterial beam warpage under different temperatures.Temperature-dependent Young's moduli of EMC (see Fig. 3)are used for Timoshenko's Bi-metal beam equation and finiteelement analysis.

Figure 5 shows warpage measured by shadow moire (seeFig. 7), warpage calculated using Bi-metal theory (equation 3)and warpage calculated using FEA, where only thermalshrinkage is considered. From the results, it can be seen thatwarpage calculated using Bi-metal theory matches well withthe warpage calculated using FEA which means that quiteaccurate prediction can be made based on the Bi-metal theory.However, the warpages using Bi-metal theory or FEA are

quite far from the warpage measured by shadow moire.Especially the warpages using Bi-metal theory or FEA are

zero at 175°C, but the experiment measured warpage is non-

zero. The result confirms the existence of cure shrinkage

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during the cross-linking of epoxide groups from epoxy resinwith hydroxide groups from hardener. This result shows thatBi-metal theory and FEA simulations without chemical cureshrinkage fail to accurately predict the package warpage.

On the other hand, the warpages using Bi-metal theory(equation 4) and FEA have been done with the considerationof the chemical cure shrinkage. From Fig. 6, it can be seenthat the calculated warpage using Bi-metal theory and FEAmatch well with the experiment measured values. Thechemical cure shrinkage value 0.21% is in the same scalecompared with other studies [3, 4]. It can be concluded thatchemical cure shrinkage is quite necessary to take intoaccount as well as thermal shrinkage for the accurateprediction of warpage for EMC encapsulated packages.

050 100 150 200 250 300Te (pe-t- (IC)

Fig. 4 Thermal expansion coefficients of epoxy moldingcompound.

Temperature-dependent thermal expansion coefficients ofEMC (see Fig. 4) are used for Timoshenko's Bi-metal beamequation and finite element analysis.

* Expeiiment-shadowmoireo Bi-metal Theory-without cure shrinkagea FEA-without cure shrinkage

Fig. 8 FEA meshes of one quarter ofPBGA.

i l.OE-04-.9

e O.OE+00O

-1 .OE-04

R 50

Tempertur (IC)

Fig. 5 Comparison of warpage based on shadow moire, Bi-metal theory and FEA (without cure shrinkage).

* Experi-t-sh moireo Bi-metal Theoy-with c-urehsrlkage,- FEA-with cure shrinkage

3.OE-04 8 s _,-

2.OE-04. A g

-

a 6LOE-04-

O.OE+00O

-1OF0 50 100 150 200 250 300

Fig. 6 Comparison of warpage based on shadow moire, Bi-metal theory and FEA ( chemica1 cure 021 ).

Discussion 4 Effect of Cure Shrinkage on PBGA WarpagePBGA (plastic ball grid array) is a package has been

acknowledged and popular for years due to many advantagesover conventional packages. The PBGA structure differsgreatly from conventional package structures, especially onthe laminated BT substrate. Epoxy molding compound coverssilicon chip and the major part of the substrate.

FEA modeling on PBGA package (see Fig. 8) isconducted in three dimension analysis using ANSYS. Bothexperiment using shadow moire and finite element analysisusing ANSYS have been done within the temperature rangeof 25° to 260°. The package is in a concave warpage aftercooling down to room temperature from molding temperatureand a convex warpage happens after going to lead-free solderreflow temperature 260°C. This is mainly due to the reasonthat the deformation ofEMC is larger than the deformation ofBT substrate. The temperature-dependent elastic property(Fig. 3), temperature-dependent thermal expansion coefficient(Fig. 4) and chemical cure shrinkage (Fig. 6) have been usedduring finite element analysis for the prediction of warpage.

* Exper,iment-shadow, moire. FEA-w,ithout cre shfinkage^1 FEA-with cre hfinkage

I. 0 0 a.A

WilFu Jtl--314 mi tuls1 TUmp-rature(°C)Fig. 7 Example of warpage measurement using shadow moire Fig. 9 Effect of chemical cure shrinkage on FEA simulation of(1250C). PBGA warpage.

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Fig. 10 Example of PBGA warpage measurement usingshadow moire (25°C).

Fig. 11 Example ofPBGA warpage based on FEA.

Figure 9 shows warpage measured by shadow moire (seeFig. 10) and warpage calculated using FEA. From the results,it can be seen the warpages using FEA without theconsideration of chemical cure shrinkage are quite far fromthe warpage measured by shadow moire. The result confirmsthe existence of cure shrinkage again with the results frombimaterial model (see Fig. 5). On the other hand, thewarpages using FEA with the consideration of the chemicalcure shrinkage agree well with the experiment measuredvalues. It can be concluded that FEA using temperature-dependent elastic property, temperature-dependent thermalexpansion coefficient and chemical cure shrinkage canprovide an accurate prediction of warpage. With theknowledge of chemical cure shrinkage, one series of epoxymolding compound have been formulated to reduce PBGAwarpage in Fig. 12. The example of warpage measurement forPBGA using shadow moire has been shown in Fig. 10 and theexample ofPBGA warpage simulation has been shown in Fig.11.

ConclusionsIn this study, Bimaterial model has been selected for the

characterization of chemical cure shrinkage. The results showthat can be seen that warpage calculated using Bi-metaltheory matches well with the warpage calculated using FEAand the warpages based on numerical calculations withoutchemical cure shrinkage are far away from experimentmeasured values. On the other hand, with the consideration ofthe chemical cure shrinkage, the calculated warpage using Bi-metal theory and FEA match well with the experimentmeasured values. It can be concluded that chemical cureshrinkage is quite necessary to take into account as well asthermal shrinkage for the accurate prediction of warpage forEMC encapsulated packages.

At the same time, the warpages of PBGA using FEA withthe consideration of the chemical cure shrinkage agree wellwith the experiment measured values based on chemical cureshrinkage from Bimaterial model. The result double confirmsthe existence of cure shrinkage. It can be concluded that FEAusing temperature-dependent elastic property, temperature-dependent thermal expansion coefficient and chemical cureshrinkage can provide an accurate prediction of warpage.

AcknowledgmentsThe authors would like to acknowledge my colleagues in

R&D lab (Chen Yee Lai, Che Wei and Bhaskaran SSagadeven) for the experiment support.

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