Stress Analysis of the Silicon Chip-Plastic Encapsulant Interface

C h a p t e r 45

Stress Analysis of the Silicon Chip-Plastic Encapsulant Interface S. Oizumi, N . Imamura, H . Tabata, and H. Suzuki Electrotechnical Research Laboratory, Nitto Electric Industrial Company, L t d . ,

Downloaded by FUDAN UNIV on March 15, 2017 | http://pubs.acs.org Publication Date: August 26, 1987 | doi: 10.1021/bk-1987-0346.ch045

Shimohozumi-cho, Ibaraki-shi, Osaka 567, Japan

I t i s critical to increase the adhesion strength of the interface between the S i - c h i p and the molding compound. During temperature cycle t e s t i n g , delamination s t a r t s from the chip edge. Once the delamination begins, it is facilitated toward the chip center. As the delamination spreads along the interface, the package cracking at the c h i p ' s edge increases because of high t e n s i l e stress and high maximum shear s t r e s s . The highest p r o b a b i l i t y that package cracking will occur e x i s t s with complete delamination. Molding compounds which e x h i b i t a higher thermal expansion c o e f f i c i e n t and a greater f l e x u r a l modulus display a higher tendency toward delamination due to the high shear stress at the chip edge.

P l a s t i c m o l d i n g e n c a p s u l a n t s f o r IC and L S I c o n t r i b u t e t o f a i l u r e s caused by package c r a c k s , d e f o r m a t i o n o f the aluminum p a t t e r n s , passivation layer cracks, etc., during temperature cycle t e s t s . (1,2,3,4) These failures are due to differences between the thermal expansion c o e f f i c i e n t (TEC) o f t h e m o l d i n g compound and t h a t of the S i - c h i p . S o l u t i o n s f o r the above f a i l u r e s a r e not simple. Lowering s t r e s s generated w i t h i n t h e m o l d i n g compound i t s e l f i s one a p p r o a c h . (5,6) S i l i c o n e m o d i f i e d epoxy r e s i n s have b e e n d e v e l o p e d w i t h t h i s o b j e c t i v e w h i c h d i s p l a y a l o w e r TEC a n d f l e x u r a l modulus. R e d e s i g n i n g the s t r u c t u r e of the e n t i r e package i s another method. (7) I n t h i s s t u d y , the mechanism o f f a i l u r e was i n v e s t i g a t e d e x p e r i m e n t a l l y a n d m a t h e m a t i c a l l y . We e s t i m a t e d the r e l a t i v e s t r e s s , which i s c a l l e d e x p e r i m e n t a l s t r e s s i n t h i s s t u d y , f r o m TEC a n d f l e x u r a l m o d u l u s d a t a f o r t h e m o l d i n g c o m p o u n d . I n a d d i t i o n , we a n a l y z e d t h e s t r e s s a t t h e i n t e r f a c e b e t w e e n t h e molding encapsulant and t h e S i - c h i p during temperature cycling tests. The l o c a l s t r e s s e s , w h i c h c a u s e d e l a m i n a t i o n b e t w e e n the molding encapsulant and S i - c h i p , were c a l c u l a t e d by the Finite Element Method (FEM). The e x i s t a n c e o f t h e d e l a m i n a t i o n was c o n f i r m e d u s i n g S c a n n i n g A c o u s t i c Tomography ( S A T ) , a n o n - d e s t r u c t i v e evaluation technique used t o d e t e c t v o i d s and d e t a c h e d surfaces w i t h i n a packaged d e v i c e .

0097-6156/87/0346-0537$06.00/0 © 1987 A m e r i c a n C h e m i c a l Society

Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

POLYMERS FOR HIGH T E C H N O L O G Y

538


BACKGROUND D u r i n g t e m p e r t u r e c y c l e t e s t i n g f r o m - 8 0 °C t o 2 0 0 ° C , l i q u i d t o l i q u i d , we s o m e t i m e s o b s e r v e d p a c k a g e c r a c k i n g , d e f o r m a t i o n o f t h e aluminum pattern, and p a s s i v a t i o n layer cracking. Figure 1 d i s p l a y s SEM p h o t o g r a p h s t h a t i l l u s t r a t e t h e s e p h e n o m e n a . Example A i l l u s t r a t e s p a c k a g e c r a c k i n g s t a r t i n g from t h e c h i p edge moving u p w a r d a t a p p r o x i m a t e l y a 120 a n g l e . Example Β shows d e f o r m a t i o n of an aluminum p a t t e r n . The d i r e c t i o n o f t h e d e f o r m a t i o n i s t o w a r d the c h i p center and the degree of deformation i s highest at the outside of the chip. Example C i s a p a s s i v a t i o n l a y e r c r a c k a t t h e bonding pad on t h e c h i p . The o c c u r r a n c e o f t h e s e f a i l u r e s i s o f t e n used as an i n d i c a t i o n o f the low s t r e s s c h a r a c t e r i s t i c s o f a mold ing compound. A correlation has been established that these failures increase as the number of temperature test cycles increases. EXPERIMENTAL STRESS The

relative

stress

the

following

Equation

i s calculated

as experimental

S -

KÎ E(T)-a(T)dT T ,

JTi

S: K:

Experimental Constant

OC(T):

Thermal

E(T):

F l e x u r a l Modulus o f Molding

Τ:

stress

by using

(1).

(1)

Stress

Expansion Coefficient

of Molding

Compound

Compound

Temperature

The T E C o f t h e S i - c h i p i s n e g l i g a b l e b e c a u s e i t i s o n e o r d e r o f magnitude l o w e r t h a n t h a t o f t h e m o l d i n g compound. The s t r e s s values from Equation ( 1 ) a r e adequate f o r use i n comparing the stress levels generated between the molding compounds a n d t h e Si-chip. F i g u r e 2 s h o w s t h e t e m p e r a t u r e dépendance o f t h e f l e x u r a l m o d u l u s a n d t h e TEC o f a m o l d i n g compound, w h i c h must be c o n s i d e r e d when calculating the experimental stress. Flexural modulus decreases slightly with increasing temperature and decreases rapidly from around 160 C , which is the glass transition temperature. In contrast, the thermal expansion coefficient behavior i s apparently opposite t h a t o f the f l e x u r a l modulus, and i n c r e a s e s r a p i d l y f r o m a p p r o x i m a t e l y 160 C . We c a n c a l c u l a t e t h e e x p e r i m e n t a l s t r e s s u s i n g E q u a t i o n (1) b y m e a s u r i n g t h e t e m p e r a t u r e dépendance o f t h e s e m o l d i n g compound p r o p e r t i e s . Figure 3 exhibits t h e e x p e r i m e n t a l s t r e s s f o r t h r e e t y p e s o f m o l d i n g compounds. Compound A i s a c o n v e n t i o n a l m o l d i n g compound. Compounds Β a n d C a r e low s t r e s s m o l d i n g compounds. T h i s d a t a was g e n e r a t e d f r o m - 5 0 C t o 175 C b y u s i n g E q u a t i o n ( 1 ) . NOTE: The s t r e s s o f t h e m o l d i n g compound t o t h e S i - c h i p i s assumed t o be s t r e s s - f r e e a t t h e m o l d i n g temperature 175 C . This figure i l l u s t r a t e s that the experimental s t r e s s increases as temperature decreases. Constant Κ i s 1 i n t h i s calculation. Compound C a p p a r e n t l y generates less than either


Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987. Figure

1: Package

Failure


5

3

1

S

ι

δ

δ

r

Η >

m

g

Ν G

ο



540

Temp.CC) Figure

2:

Temperature Dependency of Thermal Expansion C o e f f i c i e n t

Figure

3:

Experimental

Flexural Modulus o f Compound

Stress


and

45.

Silicon

OIZUMI ET A L .

Chip-Plastic

Encapsulant

541

Interface


Compound A o r B . A c c o r d i n g l y , Compound A h a s a h i g h e r p r o b a b i l i t y of i n d u c i n g package f a i l u r e s d u r i n g temperature c y c l e t e s t i n g . The e x p e r i m e n t a l s t r e s s i s an i n d i c a t i o n o f a m o l d i n g compound's low stress characteristics. Furthermore, i t provides a satisfactory a p p r o x i m a t i o n o f t h e s t r e s s l e v e l s t h a t a m o l d i n g compound w o u l d impart to the S i - c h i p . We d e f i n e t h i s e x p e r i m e n t a l s t r e s s t o b e the t o t a l or " b u l k y " s t r e s s o f a m o l d i n g compound t o a s i l i c o n chip. Next, we f o c u s e d on l o c a l s t r e s s i n an e n t i r e package to determine which s t r e s s caused the package f a i l u r e . To m e e t t h i s o b j e c t i v e , t h e F i n i t e E l e m e n t M e t h o d was c h o s e n . However, i t s use r e q u i r e d s e v e r a l a s s u m p t i o n s t o be d e f i n e d p r i o r t o c a l c u l a t i o n . ASSUMPTIONS 1.

The t e m p e r a t u r e d e p e n d e n c e o f t h e m a t e r i a l p r o p e r t i e s o f t h e m o l d i n g compound s h o u l d be c o n s i d e r e d t o c a l c u l a t e t h e s t r e s s accurately. The a v e r a g e TEC a n d a v e r a g e F l e x u r a l M o d u l u s w e r e c a l c u l a t e d using Equation (2). These v a l u e s are c o n s i d e r e d t o be a d e q u a t e for the comparison of the l o c a l s t r e s s of the m o l d i n g compound ( T a b l e I).

a -

Γ*αν\ύτ/ΔΤ J t

'

(2)

Ε -

CC : E: C((T): E(T) : Τ: ΔΤ:

E (T) d τ /ΔΤ

Average Thermal Expansion C o e f f i c i e n t Average F l e x u r a l Modulus Thermal Expansion Coefficient F l e x u r a l Modulus Temperature T - T (Thermal Load) 1

2

TABLE I :

MATERIAL

2. 3. 4.

MATERIAL PROPERTIES 2 Ε(kg/mm )

(1/°C)

V

Compound A

2.38

χ

10~

5

1290

0.25

Compound Β

2.04

χ

10"

5

1110

0.25

Compound C

1.81

χ

10~

5

964

0.25

Si-Chip

2.60

χ

10"

6

13000

0.28

Lead

7.00

χ

10~

6

20800

0.29

Frame

Analysis i s 2-dimensional. A n a l y s i s i s plane stress c o n d i t i o n . Element i s q u a d r i l a t e r a l .


542


5.

Number o f

6.

Thermal be

e l e m e n t s i s 2 1 0 7 ; number o f n o d a l p o i n t s i s 2 2 0 0 . F i n i t e Element Model Figure 4 l o a d i s f r o m - 8 0 C t o 175 C . The s t r e s s i s a s s u m e d ο

stress

free

at

the molding

temperature,

to

175 C .


R E S U L T S AND D I S C U S S I O N Figure 5 i l l u s t r a t e s the shear s t r e s s d i s t r i b u t i o n over a c h i p for t h e t h r e e compounds whose e x p e r i m e n t a l s t r e s s v a l u e s a r e shown i n F i g u r e 3. F o r a l l t h r e e c o m p o u n d s , t h e maximum s h e a r s t r e s s o c c u r s a t the c h i p edge, and t h i s s t r e s s d e c r e a s e s r a p i d l y toward the c h i p center. I t i s b e l i e v e d t h a t t h i s maximum v a l u e o f t h e s h e a r s t r e s s causes delamination at the interface between the Si-chip and molding ecapsulant. A s a r e s u l t , i t c a n be c o n c l u d e d t h a t Compound A h a s a h i g h e r p o s s i b i l i t y o f d e l a m i n a t i o n r e l a t i v e t o Compounds Β and C. The o c c u r r a n c e o f d e l a m i n a t i o n was c o n f i r m e d u s i n g S A T , a n o n - d e s t r u c t i v e e v a l u a t i o n t e c h n i q u e u s i n g sound waves t o g e n e r a t e o p t i c a l images. F i g u r e 6 shows t h e d e l a m i n a t i o n advancement d u r i n g temperature cycle t e s t i n g . P r i o r t o t e m p e r a t u r e c y c l i n g , no d e l a m i n a t i o n i s a p p a r e n t between the S i - c h i p and m o l d i n g encapsulant interface. A f t e r 400 c y c l e s , d e l a m i n a t i o n o r i g i n a t e s at the four c o r n e r s of the s i l i c o n c h i p . I n c r e a s i n g t o 800 c y c l e s » d e l a m i n a t i o n advances half-way across the c h i p s u r f a c e . At 1,000 c y c l e s , t h e r e c e a s e s t o be a n y a d h e s i o n . Figure 7 graphically compares experimental stress versus p r i n i c p a l s t r e s s at the c h i p ' s edge. The l a r g e r t h e experimental stress, the l a r g e r the compressive stress, tensile stress, and maximum s h e a r s t r e s s . Compound A e x h i b i t s t h e l a r g e s t experimental stress value and, therefore, the highest p o s s i b i l i t y of package cracking. Based on the fact that delamination advances during temperature c y c l e t e s t i n g , h y p o t h e t i c a l c h a n g e s w e r e made i n t h e l e n g t h o f d e l a m i n a t i o n d u r i n g FEM c a l c u l a t i o n s f o r Compound A , s l i p was* a l l o w e d b e t w e e n e l e m e n t s . F i g u r e 8 shows t h e r e s u l t s o f t h i s e v a l u a t i o n f o r d e l a m i n a t i o n advancement. In a d d i t i o n to the c h i p ' s e d g e , we s e e that there are a l s o peaks of shear s t r e s s at the hypothetical delamination points. These peaks of shear s t r e s s are positioned higher than the shear s t r e s s for complete adhesion. From t h i s d a t a i t h a s b e e n c o n c l u d e d t h a t t h e s e p e a k s facilitate t h e s p r e a d i n g o f d e l a m i n a t i o n from t h e edge o f t h e c h i p t o w a r d s t h e chip's center. F i g u r e 9 shows t h e p r i n c i p a l s t r e s s e s a t t h e c h i p edge w i t h the d e l a m i n a t i o n advancement. We u n d e r s t a n d t h a t t e n s i l e stress and maximum shear stress at the c h i p edge i n c r e a s e s as the delamination advances. When c o m p l e t e d e l a m i n a t i o n o c c u r s , t e n s i l e s t r e s s and s h e a r s t r e s s have reached t h e i r h i g h e s t v a l u e s . I t can b e c o n c l u d e d t h e n , t h a t t h e maximum p o s s i b i l i t y o f p a c k a g e c r a c k i n g o c c u r s when t h e r e i s c o m p l e t e d e l a m i n a t i o n . This package c r a c k i n g phenomena must a l s o be related to aluminum p a t t e r n d e f o r m a t i o n and p a s s i v a t i o n l a y e r c r a c k i n g . CONCLUSIONS 1.

Lowering

the

thermal

expansion

coefficient

and

flexural


mod-

OIZUMI ET A L .

Silicon

Chip-Plastic

Encapsulant

Interface


45.


543



544

Figure

ô: D e l a m i n a t i o n Test

Advancement

During

Temperature

ο Compound A Δ Compound Β • Compound C

10

-rrna^^—ο

0

{Tension I ^Compression I

10

1

30Ô

1

400

2

50Ô"

Experimental Stress (kg/cm)

Figure

7: P r i n c i p a l Stress

f o r 3 Compounds


Cycle

OIZUMI ET A L .

Silicon

Chip-Plastic

Encapsulant

Interface

10,


9F

Distance from the Chip Center (mm) Figure

8: Shear

Stress

D i s t r i b u t i o n with a Delamination

o Complete Adhesion - Δ Delamination 0.2mm 0 Delamination 0.4mm • Delamination 0.95mm . • Complete Delamination ^ ^ ~ — r

• Τ max

r\ /

5

Δ

V 0 1

Figure

0

•

·

05 1

ΪΌ 1

1L5

Length of Delamination (mm)

9: P r i n c i p a l Stress

with a Delamination



546

2.

u l u s o f a m o l d i n g compound i s one method t o p r e v e n t p a c k a g e c r a c k i n g at the c h i p edge. I n c r e a s i n g the a d h e s i o n s t r e n g t h o f a m o l d i n g e n c a p s u l a n t and Si-chip interface i s a n o t h e r method t o p r e v e n t the package c r a c k i n g at the c h i p edge.

ACKNOWLEDGMENTS The a u t h o r s w o u l d l i k e t o t h a n k D r . T . M o r i u c h i a n d M r . K . I k o o f N i t t o E l e c t r i c I n d u s t r i a l C o . , L t d . , a n d M r . K . Kuwada o f N i t t o D e n k o A m e r i c a , I n c . f o r t h e i r h e l p f u l comments a n d e n c o u r a g e m e n t s .


LITERATURE CITED 1. 2.

3.

4. 5. 6. 7.

R.E. Thomas, Stress-Induced Deformation of Aluminum Metalization in Plastic Molded Semiconductor Devices, Proc. 35th ECC, 1985, pp. 37-45. M. Isagawa, Y. Iwasaki, and T. Sutoh, Deformation of A1 Metalization in Plastic Encapsulated Semiconductor Devices Caused by Thermal Shock, 18th Annual Proceedings, Reliability Physics, 1980, pp. 171-177. S. Okikawa, M. Sakimoto, M. Tanaka, T. Sato, T. Toya, and Y. Hara, Stress Analysis of Passivation Film Crack for Plastic Molded LSI Caused by Thermal Stress, Proc. International Society for Testing and Failure Analysis, Oct. 1983, Los Angeles, CA. K. Miyake et a l . , Thermal Stress Analysis of Plastic Encapsulated Integrated Circuits by FEM, IECE Proc., Japan, 1984, p. 2833. K. Kuwata, K. Iko, and H. Tabata, Low Stress Resin Encapsulant for Semiconductor Devices, Proc. 35th ECC, 1985, pp. 18-22. H. Suzuki, T. Moriuchi, and M. Aizawa, Low Mold Stress Epoxy Molding Compounds for Semiconductor Encapsulation, Nitto Electric Industrial Co., Ltd., 1979. Steven Groothuis, Walter Schroen, and Masood Murtuza, Computer Aided Stress Modeling for Optimizing Plastic Package Reliability, 23rd Annual Proceedings, Reliability Physics, 1985, pp. 184-191.

RECEIVED

April 8, 1987


Stress Analysis of the Silicon Chip-Plastic Encapsulant Interface

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