Thermal Stress in Epoxy Molding Compounds and Packaged Devices

Chapter 28

Thermal Stress in Epoxy Molding Compounds and Packaged Devices 1

W. F. van den Bogert, M . J. Molter, S. A. Gee, D. J. Belton , and V. R. Akylas

Downloaded by GEORGETOWN UNIV on August 25, 2015 | http://pubs.acs.org Publication Date: September 5, 1989 | doi: 10.1021/bk-1989-0407.ch028

Signetics Division, Philips Research Laboratories, Sunnyvale, CA 94086-3409

Studies on thermomechanical stresses in plastic pack aged integrated circuits after molding and in encap sulation materials, show the resulting stress distri bution on the surface of the die and clarify the fundamental mechanism in the generation of thermo mechanical stresses in molding compounds. Use was made of a piezoresistive strain gauge array to measure the stress distribution on the sur face of the die. A beam bending apparatus was used to study the importance of the thermoviscoelastic properties of the molding compound. The strain gauge allowed for the study of the effects of thermal shock testing. The largest stresses are observed as shear stresses at the corners of the die at the lowest temperature. Three commercially available epoxy -based molding compounds were studied. Two of these materials are standard packaging formulations for smaller devices. Both strain gauge and beam bending experiments showed comparable stress levels with these two materials. The third material is a rubber modified, low stress material. As expected, stress levels in devices packaged with this material, as well as stresses observed in the beam bending apparatus, were considerably lower than those for the other two materials. In integrated circuit packaging, thermosetting polymers are import ant due to cost considerations in both manufacturing and material selection. Plastic packaging (J[) has therefore become the dominant method of protecting integrated circuits from the environment. The manufacturing process involves a transfer molding operation in which an integrated circuit is encapsulated with a silica f i l l e d 1

Current address: Signetics Korea Company, Ltd., Seoul, Korea 0097-6156/89/0407-0344$06.00/0 ο 1989 American Chemical Society

In Polymeric Materials for Electronics Packaging and Interconnection; Lupinski, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


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e p o x i d e ( a p p r o x i m a t e l y 70% f i l l e r , 20% p o l y m e r i c m a t e r i a l , and 10% o t h e r a d d i t i v e s ) a t e l e v a t e d t e m p e r a t u r e s (175°C) and p r e s s u r e s (7 MPa) i n 20 seconds o r l e s s . The d e s i g n o f t h e mold ( i . e . runner system, c a v i t i e s , g a t e s and v e n t s ) , t h e r h e o l o g i c a l c h a r a c t e r i s t i c s of t h e molding compound, and t h e p r o c e s s parameters a r e i m p o r t a n t to ensure a v o i d - f r e e package w i t h minimal d i s t o r t i o n o f t h e w i r e c o n n e c t i o n s between d i e and l e a d frame ( w i r e sweep). Alignment o f f i l l e r p a r t i c l e s and f l o w p a t t e r n s i n t h e c a v i t i e s w i l l l e a d t o a c e r t a i n amount o f a n i s o t r o p y i n t h e molding m a t e r i a l . To ensure some m e c h a n i c a l i n t e g r i t y , a c e r t a i n amount o f c u r e i s n e c e s s a r y , and t h e p a r t s a r e kept i n t h e mold a t 175°C f o r a p p r o x i m a t e l y 90 seconds. S u b s e q u e n t l y t h e y a r e removed from t h e mold and quenched to room t e m p e r a t u r e . D i f f e r e n c e s i n thermomechanical p r o p e r t i e s between t h e s i l i c o n d i e / l e a d frame assembly and t h e molding compound, i n c o m b i n a t i o n w i t h t h e m a n u f a c t u r i n g p r o c e s s , cause r e s i d u a l t h e r m a l s t r e s s e s (2). Furthermore, t h e i n c r e a s i n g complexity o f i n t e g r a t e d c i r c u i t s w i t h an accompanying i n c r e a s e i n d i e s i z e l e a d s t o l a r g e r t h e r m a l s t r e s s e s . At t h e same t i m e r e l i a b i l i t y c o n c e r n s n e c e s s i t a t e s e v e r e t h e r m a l s t r e s s t e s t i n g o f packaged i n t e g r a t e d c i r c u i t s i n o r d e r t o e v a l u a t e package d e s i g n , m a t e r i a l s e l e c t i o n , and m a n u f a c t u r i n g operations. T h i s a c c e l e r a t e d l i f e t e s t i n g may cause metal s h i f t , top t o bottom metal s h o r t s o r package c r a c k i n g (_3). S e l e c t i o n o f molding compounds f o r i n t e g r a t e d c i r c u i t packag ing must be based on t h e measurement o f m a t e r i a l p r o p e r t i e s and an u n d e r s t a n d i n g o f t h e r e l a t i o n s h i p between t h e s e p r o p e r t i e s and thermal s t r e s s e s . To determine t h i s r e l a t i o n s h i p a b i l a y e r beam s t r u c t u r e has been u t i l i z e d . A beam bending a p p a r a t u s a l l o w s f o r the measurement o f t h e c u r v a t u r e , a d i r e c t measure o f t h e s t r e s s i n the b i l a y e r s t r u c t u r e . The t i m e - t e m p e r a t u r e h i s t o r y o f t h e molding process i s important i n determining t h e r e s u l t i n g mechanical p r o p e r t i e s o f t h e molding compound. The t r a n s f e r molding o p e r a t i o n introduces other s t r e s s determining f a c t o r s as w e l l . Stress measurement s h o u l d t h e r e f o r e not be l i m i t e d t o t h e beam bending s t r u c t u r e , but should a l s o i n c l u d e t h e use o f a p i e z o r e s i s t i v e s t r a i n gauge d e v i c e t o a l l o w f o r t h e measurement o f p a c k a g e - r e l a t e d stresses at the d i e l e v e l . Experimental M a t e r i a l s . I n t h i s study t h r e e c o m m e r c i a l l y a v a i l a b l e m o l d i n g com pounds were s t u d i e d . M a t e r i a l s A and Β a r e s t a n d a r d p a c k a g i n g f o r m u l a t i o n s . However, m a t e r i a l Β has a lower c h l o r i d e c o n t e n t t o improve h u m i d i t y r e s i s t a n c e . M a t e r i a l C i s a rubber m o d i f i e d v e r s i o n o f m a t e r i a l Β t h a t was developed t o reduce s t r e s s r e l a t e d problems. A l l m a t e r i a l s a r e based on an epoxy c r e s o l n o v o l a c phenol formaldehyde n o v o l a c r e s i n , a t e r t i a r y amine c u r i n g agent and t h e u s u a l a d d i t i v e s such a s flame r e t a r d a n t s , r e l e a s e a g e n t , s i l i c a f i l l e r , c o u p l i n g agent and pigment. Samples were p r e p a r e d u s i n g in-house molding f a c i l i t i e s from preforms heated t o 85°C p r i o r t o molding a t 175°C w i t h a p r o c e s s t i m e o f 90 seconds and a maximum p r e s s u r e o f 6.9 MPa. A l l samples were post mold cured a t 175°C f o r 4 hours and were s t o r e d i n a desiccator u n t i l testing.


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POLYMERS FOR ELECTRONICS PACKAGING AND INTERCONNECTION

Thermomechanical p r o p e r t i e s . M e c h a n i c a l p r o p e r t i e s were o b t a i n e d t h r o u g h Dynamic M e c h a n i c a l A n a l y s i s ( D y n a s t a t , Imass) i n t h r e e p o i n t bending over a f r e g u e n c y range from 0.1 Hz t o 10Hz and a temperature range from -150°C t o 200°C. Time-temperature super p o s i t i o n (4) y i e l d e d master c u r v e s i n t h e t r a n s i t i o n range, g i v i n g t h e s t o r a g e modulus, E , and t h e l o s s modulus, E", a s f u n c t i o n s o f reduced f r e g u e n c y . The d i s p l a c e m e n t a l o n g t h e f r e g u e n c y a x i s , n e c e s s a r y t o superimpose t h e s t o r a g e and l o s s m o d u l i c u r v e s measured a t d i f f e r e n t t e m p e r a t u r e s , i s e q u a l t o l o g a y , t h e l o g a r i t h m o f t h e s h i f t f a c t o r , ay. The s u p e r p o s i t i o n t h e r e f o r e a l s o y i e l d e d the s h i f t f a c t o r as f u n c t i o n o f temperature. Thermal e x p a n s i o n was determined with Thermo M e c h a n i c a l A n a l y s i s ( P e r k i n Elmer TMA-7) over a temperature range from -150°C t o 180°C a t h e a t i n g r a t e s t h a t were e q u i v a l e n t t o t h o s e used i n t h e beam bending e x p e r i m e n t s .


1

B i l a y e r beam a n a l y s i s . Thermal s t r a i n s cause a b i l a y e r beam s t r u c t u r e t o bend a s a f u n c t i o n o f t e m p e r a t u r e . I f both m a t e r i a l s a r e e l a s t i c t h e r e s u l t i n g c u r v a t u r e i s determined by Young's m o d u l i , t h e r m a l e x p a n s i o n c o e f f i c i e n t s and t e m p e r a t u r e d i f f e r e n c e o n l y ( 5 ) . T h e r m o s e t t i n g polymers behave v i s c o e l a s t i c a l l y , and t h e c u r v a t u r e becomes a f u n c t i o n o f a more c o m p l i c a t e d v i s c o e l a s t i c m a t e r i a l d e s c r i p t i o n , t e m p e r a t u r e dependent t h e r m a l e x p a n s i o n , t e m p e r a t u r e d i f f e r e n c e , and r a t e o f change o f t e m p e r a t u r e . Cal c u l a t i o n o f t h e r e s u l t i n g c u r v a t u r e i s c o m p l i c a t e d and o n l y possible f o r thermorheoloqically simple m a t e r i a l s , materials t o which t i m e - t e m p e r a t u r e s u p e r p o s i t i o n a p p l i e s ( 6 ) . A phenomenologi c a l approach, v a l i d f o r s u f f i c i e n t l y s m a l l r a t e s o f t e m p e r a t u r e change a s e x p e r i e n c e d i n t h e beam bending e x p e r i m e n t s (1-5°C/min), has been d e s c r i b e d e l s e w h e r e (7). I n t h a t approach t h e v i s c o e l a s t i c b e h a v i o r o f t h e m o l d i n g compound i s d e s c r i b e d by t h e t h r e e parameter, s t a n d a r d l i n e a r s o l i d model i n c o m b i n a t i o n w i t h a t e m p e r a t u r e dependent modulus a t lower t e m p e r a t u r e s . The s t a n d a r d l i n e a r s o l i d model c o n s i s t s o f a s p r i n g p a r a l l e l t o a s p r i n g i n s e r i e s w i t h a dashpot ( 8 ) . To model t h e t e m p e r a t u r e dependence o f t h e m e c h a n i c a l p r o p e r t i e s , t h e v i s c o s i t y o f t h e dashpot i n t h e model i s assumed temperature dependent. The s p r i n g con s t a n t s a r e independent o f t e m p e r a t u r e . At lower t e m p e r a t u r e s t h e v i s c o s i t y becomes v e r y h i g h and t h e model reduces t o t h e e q u i v a l e n t o f a s i n g l e s p r i n g , w i t h a s p r i n g c o n s t a n t t h a t i s assumed t o be t e m p e r a t u r e dependent. The parameter v a l u e s a r e based on t h e measurement o f t h e complex modulus a s a f u n c t i o n o f f r e q u e n c y and t e m p e r a t u r e . The temperature dependence o f t h e v i s c o s i t y parameter i s o b t a i n e d from t h e t e m p e r a t u r e dependence o f t h e s h i f t f a c t o r s . The model d e s c r i b e s t h e b a s i c f e a t u r e s , i . e . t h e f i n i t e v a l u e s f o r t h e s t o r a g e modulus a t low and h i g h t e m p e r a t u r e s , o f a c r o s s l i n k e d polymer. However, t h e t r a n s i t i o n between t h e s e two r e g i o n s i s modelled p o o r l y . With t h e measured t h e r m a l e x p a n s i o n d a t a , s t r e s s e s i n t h e m o l d i n g compound can be c a l c u l a t e d from t h e model. These s t r e s s e s a r e p r o p o r t i o n a l t o t h e c u r v a t u r e 1/R o f t h e b i l a y e r structure. A s c h e m a t i c o f t h e beam bending a p p a r a t u s i s shown i n F i g u r e 1. Samples were p r e p a r e d by c o a t i n g a p o l i s h e d s i l i c o n s t r i p (50.0mm χ 6.0mm χ 0.5mm) w i t h a l a y e r (0.3mm) o f one o f t h e molding compounds i n the molding p r e s s .


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S t r a i n gauge d e v i c e . S t r e s s f i e l d s , generated i n d e v i c e s d u r i n g assembly due t o d i f f e r e n c e s i n thermomechanical p r o p e r t i e s , a r e o f g r e a t importance i n i n t e g r a t e d c i r c u i t p a c k a g i n g . A semiconductor s t r a i n gauge a r r a y was s p e c i f i c a l l y designed t o measure t h e s e m e c h a n i c a l s t r e s s f i e l d s (9)· The b a s i c s t r e s s sensor i s a compact r e s i s t o r r o s e t t e c o n s i s t i n g o f f o u r r e s i s t o r s o r i e n t e d a t 45 d e g r e e s on a p l a n a r a r e a o f 220x220 pm , as shown i n F i g u r e 2. The d i e s i z e used i n t h i s study was 4.5x4.5 mm , w i t h a t o t a l o f 64 s e n s o r s , s i t u a t e d p r i m a r i l y i n one o f t h e c o r n e r s . The decoding l o q i c , n e c e s s a r y t o a d d r e s s each i n d i v i d u a l r e s i s t o r , i s c o n t a i n e d on t h e d i e i n t h e o p p o s i t e c o r n e r . The d e v i c e s a r e p r o c e s s e d u s i n g MOS t e c h n o l o g y on (111) o r i e n t e d s i l i c o n f o r piezoresistance i s o t r o p y . A f t e r c a l i b r a t i o n , u s i n g a 4 p o i n t bending j i g , measured r e s i s t a n c e changes can be c o n v e r t e d t o a s t r e s s f i e l d on t h e s u r face of the d i e . For t h i s study i n d i v i d u a l d i e were packaged i n 28 l e a d DIP packages (36.195 mm χ 13.970 mm χ 3.810 mm) u s i n g s t a n d a r d assembly technology. A f t e r d i c i n g , t h e d e v i c e s were a t t a c h e d t o copper l e a d frames w i t h a p o l y i m i d e a d h e s i v e . Based on p r e v i o u s e x p e r i e n c e s t r e s s e s i n t r o d u c e d d u r i n g d i e a t t a c h were n e g l e c t e d . After thermosonic g o l d w i r e b o n d i n g , p a r t s were e n c a p s u l a t e d i n one o f the m o l d i n g compounds, d e f l a s h e d , and t h e l e a d s were trimmed, formed and s o l d e r e d . The f o u r measured r e s i s t o r changes can be c o n v e r t e d t o a s t r e s s f i e l d w i t h r e s p e c t t o a c o o r d i n a t e system p a r a l l e l t o t h e edges o f t h e d i e . Assuming t h a t s t r e s s e s p e r p e n d i c u l a r t o t h e s u r f a c e can be n e g l e c t e d (10), the stress f i e l d i s completely determined by t h e s e r e s i s t o r changes, and i s g i v e n by two normal stresses, o and oy, and a shear s t r e s s , τχγ. I t i s often c o n v e n i e n t t o c o n s i d e r t h e p r i n c i p a l s t r e s s f i e l d on t h e s u r f a c e o f the d i e . T h i s f i e l d i s c h a r a c t e r i z e d by a z e r o shear s t r e s s and i s o b t a i n e d from t h e o r i g i n a l s t r e s s f i e l d by r o t a t i n g t h e c o o r d i n a t e system over an a n g l e Θ g i v e n by: 2


2

x

t a n (2Θ) =

^ 0*X

(1) -

Oy

The p r i n c i p a l s t r e s s e s and o a r e g i v e n r o t a t e d c o o r d i n a t e system and a r e e q u a l t o : 2

Ο χ + oy θ χ - oy 1,2 = — γ ± [ ( — Γ /

σ

2 ) + τ

with

9 n 2 χ γ

l/2 ]

respect

to this

(2)

W i t h both t h e two p r i n c i p a l s t r e s s e s and t h e r o t a t i o n a n g l e known the s t r e s s f i e l d i s c o m p l e t e l y determined. At 45 degrees t o t h e p r i n c i p a l s t r e s s f i e l d o r i e n t a t i o n , one f i n d s t h e d i r e c t i o n s o f maximum shear s t r e s s : tmax =

{

3

)

T h i s maximum i n - p l a n e shear s t r e s s can a l s o be used t o r e p r e s e n t the s t r e s s f i e l d on t h e s u r f a c e o f t h e d i e .

American Chemical Society Library 1155 16th S t ,Interconnection; H.W. In Polymeric Materials for Electronics Packaging and Lupinski, J., et al.; ACS Symposium Series; American ChemicalD.C* Society: Washington, DC, 1989. Washington, 20036


348


F i g u r e 1:

Figure 2 :

The beam bending a p p a r a t u s .

The b a s i c s t r a i n gauge.



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Thermal shock t e s t i n g . One o f a l a r g e number o f methods t o t e s t the r e l i a b i l i t y o f p l a s t i c packaged d e v i c e s i s t h e r m a l shock (THSK) t e s t i n g a c c o r d i n g t o M i l STD-883C, method 1 0 1 1 Λ , C o n d i t i o n C. Samples a r e exposed t o r a p i d l y changing t e m p e r a t u r e s by immersing i n hot (150°C) and c o l d (-65°C) i n e r t l i q u i d w i t h a 5 min. d w e l l t i m e and l e s s then 10 s e c . t r a n s f e r t i m e . T h i s i s an e x t r e m e l y s e v e r e t e s t on t h e m e c h a n i c a l c o m p a t i b i l i t y between package and die. The r a p i d l y changing t e m p e r a t u r e a l s o causes a d d i t i o n a l m e c h a n i c a l s t r e s s due t o temperature g r a d i e n t s i n t h e package. In d e v i c e t e s t i n g , d e v i c e s w i l l undergo a c e r t a i n number o f c y c l e s and be t e s t e d a f t e r exposure. I n t h i s study p a r t s were exposed t o 100, 300 and 500 c y c l e s and s t r e s s e s were measured and compared a f t e r each t e s t . B i l a y e r beam samples c o u l d not be t e s t e d even a f t e r 100 THSK c y c l e s due t o poor a d h e s i o n o f t h e molding compound t o t h e die. R e s u l t s and D i s c u s s i o n The t h e o r e t i c a l s t r e s s l e v e l s f o r t h e t h r e e molding compounds A, B, and C a r e compared i n F i g u r e 3 based on t h e measurement o f t h e i r thermomechanical p r o p e r t i e s and the c a l c u l a t i o n o f t h e c u r v a t u r e f o r t h e b i l a y e r beam s t r u c t u r e . The low s t r e s s m a t e r i a l C has t h e l o w e s t c a l c u l a t e d s t r e s s l e v e l s , w h i l e m a t e r i a l A has s t r e s s l e v e l s t h a t are comparable t o t h o s e o f m a t e r i a l B. As d i s c u s s e d above, t h e s e s t r e s s l e v e l s a r e determined by t h e t h e r m a l e x p a n s i o n d i f f e r e n c e between t h e m o l d i n g compound and s i l i c o n , t h e m o d u l i , and t h e amount o f r e l a x a t i o n t h a t o c c u r s dur i n g the experiment due t o t h e v i s c o e l a s t i c n a t u r e o f t h e molding compound. The g l a s s t r a n s i t i o n t e m p e r a t u r e , Tg, o f a molding compound i s i m p o r t a n t s i n c e t h e b e h a v i o r i n terms o f t h e r m a l expan s i o n , moduli and r e l a x a t i o n t i m e s changes a b r u p t l y i n t h i s tempera ture reqion. By d e c r e a s i n g t h e g l a s s t r a n s i t i o n temperature r e s i d u a l s t r e s s e s can be reduced but i n g e n e r a l such a d e c r e a s e i s u n d e s i r a b l e i n v i e w o f t h e r e q u i r e d performance o f molding com pounds a t h i g h e r t e m p e r a t u r e s . The t h e r m a l e x p a n s i o n o f m a t e r i a l s A and Β i s comparable over the t e m p e r a t u r e range o f i n t e r e s t . However, t h e s t o r a g e modulus v a l u e s f o r m a t e r i a l A a r e s m a l l e r than t h o s e f o r m a t e r i a l Β over the complete range o f e x p e r i m e n t a l f r e g u e n c i e s and t e m p e r a t u r e s . At t h e same t i m e , t h e t r a n s i t i o n range f o r m a t e r i a l A i s found t o be s h i f t e d t o lower reduced f r e q u e n c i e s w i t h r e s p e c t t o t h e same reference temperature. T h i s r e s u l t s i n c o n s i d e r a b l e longer r e l a x a t i o n t i m e s f o r m a t e r i a l A than f o r m a t e r i a l Β a t each t e m p e r a t u r e . The observed s h i f t i s a l s o r e f l e c t e d i n a h i g h e r g l a s s t r a n s i t i o n t e m p e r a t u r e f o r m a t e r i a l A than f o r m a t e r i a l B. The i n f l u e n c e o f t h e s e d i f f e r e n c e s on t h e t h e o r e t i c a l s t r e s s l e v e l s i s p r e d o m i n a n t l y a c o m b i n a t i o n o f h i g h e r t h e r m a l s t r a i n , due t o d i f f e r e n c e s i n t h e g l a s s t r a n s i t i o n t e m p e r a t u r e , w i t h s m a l l e r modulus v a l u e s , r e s u l t ing i n a p p r o x i m a t e l y t h e same s t r e s s l e v e l s f o r m a t e r i a l s A and B. M a t e r i a l C has l e s s t h e r m a l e x p a n s i o n and l o w e r s t o r a g e modulus v a l u e s than e i t h e r o f t h e o t h e r two m a t e r i a l s , w h i l e t h e r e l a x a t i o n b e h a v i o r i s comparable t o t h a t of m a t e r i a l B. T h i s was somewhat e x p e c t e d s i n c e m a t e r i a l C i s a rubber m o d i f i e d v e r s i o n o f m a t e r i a l Β and based on t h e same r e s i n system. The g l a s s t r a n s i t i o n t e m p e r a t u r e i s t h e r e f o r e almost e g u a l t o t h a t o f m a t e r i a l B. This



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c o m b i n a t i o n o f decreased t h e r m a l e x p a n s i o n , s m a l l e r modulus v a l u e s , and s u f f i c i e n t l y h i g h g l a s s t r a n s i t i o n t e m p e r a t u r e , r e s u l t s i n t h e l o w e s t t h e o r e t i c a l s t r e s s l e v e l s f o r m a t e r i a l C. These t h e o r e t i c a l s t r e s s e s can be compared t o t h e s t r e s s e s o b s e r v e d i n beam bending e x p e r i m e n t s as a f u n c t i o n o f t e m p e r a t u r e . The r e s u l t s o f t h e e x p e r i m e n t s f o r t h e d i f f e r e n t m o l d i n g compounds are g i v e n i n F i g u r e 4. These e x p e r i m e n t s show good agreement w i t h the t h e o r e t i c a l s t r e s s l e v e l s . R e s u l t s o f t h e s t r a i n gauge e x p e r i m e n t s a r e g i v e n i n F i g u r e s 5 t h r o u g h 8. F i g u r e 5 shows t h e maximum i n - p l a n e shear s t r e s s d i s t r i b u t i o n f o r a s t r a i n gauge d i e e n c a p s u l a t e d i n m a t e r i a l C, immediately a f t e r packaging. I t i s c l e a r that the stresses are a f u n c t i o n o f p o s i t i o n on t h e d i e s u r f a c e and a r e maximized a t t h e corners. The edge o f t h e d i e o r i e n t e d i n t h e l e n g t h d i r e c t i o n o f t h e DIP package, shows t h e h i g h e s t s t r e s s . The o v e r a l l d i s t r i b u t i o n resembles t h e w e l l known p i l l o w shape p r e d i c t e d by F i n i t e Element a n a l y s i s (11 ). With an i n c r e a s i n g number o f c y c l e s i n THSK t e s t i n g one o b s e r v e s a r e d u c t i o n o f t h e h i g h e s t maximum i n - p l a n e shear s t r e s s a t t h e c o r n e r s . The s t r e s s e s i n t h e c e n t e r o f t h e d i e remain almost c o n s t a n t . With respect t o the o r i e n t a t i o n o f t h e c o o r d i n a t e system o f t h e p r i n c i p a l s t r e s s system on t h e s u r f a c e o f the d i e we have observed a change from a s t r o n g o r i e n t a t i o n toward t h e c e n t e r o f t h e d i e t o no p r e f e r e n c e i n o r i e n t a t i o n a t a l l a f t e r 500 c y c l e s THSK t e s t i n g . Both t h e s e o b s e r v a t i o n s a r e b e l i e v e d t o be r e l a t e d t o changes i n t h e i n t e g r i t y o f t h e i n t e r f a c e between t h e molding compound and t h e s u r f a c e o f t h e d i e . F i g u r e s 6 t h r o u g h 8 show r e s u l t s o f t h e s t r a i n gauge e x p e r i ments on t h e t h r e e d i f f e r e n t molding compounds A, B, and C. Each o f t h e s e f i g u r e s g i v e s t h e p r i n c i p a l s t r e s s e s σι and σ and t h e maximum shear s t r e s s umax a s measured i n t h e c e n t e r and on t h e corner of the d i e . These r e s u l t s were o b t a i n e d a f t e r a v e r a g i n g measurements on a t l e a s t 10 i n d i v i d u a l d i e . The r e s u l t s a r e g i v e n as f u n c t i o n o f t h e number o f c y c l e s i n THSK t e s t i n g . E x c e p t f o r the measurement a f t e r 300 c y c l e s t h e s t r e s s l e v e l s a t a l l p o s i t i o n s a f t e r any number o f c y c l e s a r e s m a l l e r f o r m a t e r i a l C than f o r m a t e r i a l B. The s t r e s s e s f o r m a t e r i a l Β a r e comparable t o t h o s e f o r m a t e r i a l A. The d e v i a n t b e h a v i o r a f t e r 300 c y c l e s o b s e r v e d w i t h m a t e r i a l A shows a l a r g e r e d u c t i o n i n s t r e s s , i n d i c a t i v e o f a l o s s o f a d h e s i o n . However, t h e i n c r e a s e observed a f t e r 500 c y c l e s cannot be e x p l a i n e d i f i n d e e d t h e i n t e g r i t y o f t h e i n t e r f a c e has been compromised. As observed p r e v i o u s l y f o r m a t e r i a l C i n F i g u r e 5, t h e maximum shear s t r e s s i s h i g h e s t a t t h e c o r n e r o f t h e d i e f o r a l l mate rials. The f i r s t measurement a f t e r 100 c y c l e s THSK shows t h e l a r g e s t decrease i n s t r e s s l e v e l . Subsequent measurements a f t e r a l a r g e r number o f c y c l e s show no s i g n i f i c a n t d i f f e r e n c e s i n s t r e s s l e v e l s . A l t h o u g h t h i s maximum i n - p l a n e shear s t r e s s i s not d i r e c t l y r e l a t e d t o t h e s t r e s s e s t h a t cause such e f f e c t s a s m e t a l s h i f t , i t s b e h a v i o r does i n d i c a t e t h a t a f t e r o n l y a l i m i t e d number o f THSK c y c l e s s t r e s s r e l i e f , l e a d i n g t o i n c r e a s e d movement o f m a t e r i a l s w i t h r e s p e c t t o one a n o t h e r , has o c c u r r e d . Damage t o t h e d i e s u r f a c e i s then i n e v i t a b l e . F u r t h e r a n a l y s i s o f t h e p a r t s and s t r e s s measurement a f t e r a s m a l l e r number o f THSK c y c l e s , i s n e c e s s a r y t o g a i n more i n s i g h t i n t h e changes i n s t r e s s l e v e l s and o r i e n t a t i o n 2


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C a l c u l a t e d s t r e s s l e v e l s i n beam bending e x p e r i m e n t .

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Thermal Stress in Epoxy Molding Compounds and Packaged Devices

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