30 Thermal Expansion Coefficients of Leadless Chip Carrier Compatible Printed Wiring Boards
Downloaded by FUDAN UNIV on February 28, 2017 | http://pubs.acs.org Publication Date: March 15, 1984 | doi: 10.1021/bk-1984-0242.ch030
Z. N. SANJANA, R. S. RAGHAVA, and J. R. MARCHETTI Research & Development Center, Westinghouse Electric Corporation, Pittsburgh, PA 15235 Packaging densities and reduction of circuit lengths for very high speed integrated circuits have led to the development of leadless hermetic chip carriers. The chip carrier, made of a ceramic material, is soldered directly to the printed wiring board and the flexibility associated with leads is no longer present to accommodate differential thermal expan sions. Thus it becomes necessary to match the thermal expansion characteristics of the wiring board in the X-Y plane to that of the chip carrier which is made of a ceramic material having a thermal expansion coefficient of 6 x 10 /ºC. Several materials combinations were examined and it was determined that fabrics made from fibers with expansions significantly lower than that of conventional Ε-glass have to be used to make boards that have the desired thermal expansion character istics. Aramid reinforced laminates appear to be particularly useful in this application and their thermal expansion characteristics were studied using several experimental techniques and using a computer model to study the effect of resin content, resin modulus and resin Tg on these materials. Data on a quartz fabric reinforced epoxy laminate is also presented. -6
Present day integrated circuits use dual-in-line packages having leads which are soldered to the printed wiring board (PWB). The presence of such leads provides a degree of flexibility between the packages and the PWB, therefore, a considerable amount of thermal mismatch can be tolerated during thermal cycling of the assembled board. Requirements of increased density of packaging and very high speed processing of information (generally known as VHSIC) has 0097-6156/ 84/ 0242-0379S06.00/ 0 © 1984 American Chemical Society Davidson; Polymers in Electronics ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
380
P O L Y M E R S IN E L E C T R O N I C S
led to the development of a package g e n e r a l l y known as l e a d l e s s chip c a r r i e r , LCC, or hermetic chip c a r r i e r , HCC. The LCC has pads that are d i r e c t l y soldered onto the PWB thus reducing c i r c u i t l e n g t h and i n c r e a s i n g component d e n s i t y . U n f o r t u n a t e l y , present day PWBs which are f l a t sheet laminated compositions made from Ε-glass f a b r i c impregnated w i t h epoxy or p o l y i m i d e r e s i n are not t h e r m a l l y compatible w i t h the c h i p c a r r i e r m a t e r i a l which i s a ceramic having l i n e a r thermal c o e f f i c i e n t of expansion (TCE) of 6 χ 10" /°C. The TCE of g l a s s r e i n f o r c e d PWBs i n the X-Y p l a n e , i . e . , i n the plane of the laminate are of the order of 12-18 χ 10-6/ C. During thermal c y c l i n g there occurs a s t r a i n on the s o l d e r j o i n t . Solder being i n e l a s t i c undergoes p l a s t i c f l o w and a f t e r a number of c y c l e s i t cracks due to thermal f a t i g u e ( l , 2 ) . Thus, i t i s necessary to develop LCC compatible PWBs. The com p a t i b i l i t y i s p r i m a r i l y a case of matching the TCE of the LCC and the PWB. Obviously advantageous would be a compatible board which could be processed s i m i l a r to c o n v e n t i o n a l PWBs, i . e . , an o r g a n i c m a t r i x , f a b r i c r e i n f o r c e d sheet m a t e r i a l w i t h copper bonded to one of both s i d e s . K e v l a r , or aramid as i t i s g e n e r i c a l l y known, i s a h i g h l y o r i e n t e d polymer - poly(p-phenylene terephthalamide). K e v l a r f i b e r has the i n t e r e s t i n g p r o p e r t y of having a l i n e a r TCE i n i t s a x i a l d i r e c t i o n of -2 χ 10" /°C. The Ε-glass used i n normal PWBs has a p o s i t i v e l i n e a r TCE of +5 χ 10~^/ C. I t has been found that K e v l a r r e i n f o r c e d epoxy or p o l y i m i d e laminates y i e l d room tempera ture TCEs i n the X-Y plane of around 4 to 8 χ 1 0 ~ / C ( 3 ) and s o l d e r j o i n t c r a c k i n g i s g r e a t l y reduced or e l i m i n a t e d ( 2 ) . Quartz i s another reinforcement, a v a i l a b l e i n f a b r i c form, which can be used to c o n t r o l the p l a n a r TCEs of a laminate PWB. Quartz has a TCE of +0.54 χ 10-°/°C i n the a x i a l d i r e c t i o n . The thermal expansion c h a r a c t e r i s t i c s of a laminate used i n PWBs f o r c a r r y i n g LCCs have to be obtained over a range of tem peratures that are expected to be observed by the assembly. Such a range, a t l e a s t f o r m i l i t a r y a p p l i c a t i o n s , i s -60°C to +150°C. We have found that i n t h i s range the thermal expansion of K e v l a r laminates i n the X and Y d i r e c t i o n are not n e c e s s a r i l y l i n e a r . In f a c t , depending on the T of the r e s i n used, the expansion curve as a f u n c t i o n of temperature can a c t u a l l y show a change of slope from p o s i t i v e to n e g a t i v e . We have a l s o found that the method used to o b t a i n TCEs may a f f e c t the v a l u e s obtained. 6
Downloaded by FUDAN UNIV on February 28, 2017 | http://pubs.acs.org Publication Date: March 15, 1984 | doi: 10.1021/bk-1984-0242.ch030
e
6
e
6
e
g
Experimental The m a t r i x m a t e r i a l f o r the laminates f o r t h i s i n v e s t i g a t i o n was a t y p i c a l flame r e t a r d a n t epoxy r e s i n system used i n PWB technology. The epoxy r e s i n was a brominated d i g l y c i d y l ether of bisphenol-A cured w i t h dicyandiamide and c a t a l y z e d by b e n z y l dimethylamine. The weight r a t i o s of epoxy, c u r a t i v e and c a t a l y s t used were 100/3/0.25.
Davidson; Polymers in Electronics ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
Downloaded by FUDAN UNIV on February 28, 2017 | http://pubs.acs.org Publication Date: March 15, 1984 | doi: 10.1021/bk-1984-0242.ch030
30.
SANJANA ETAL.
Thermal Expansion Coefficients: Printed Boards
381
The f a b r i c s u s e d w e r e K e v l a r and q u a r t z . D a t a on two d i s s i m i l a r s t y l e s of K e v l a r f a b r i c are presented. Both were o b t a i n e d f r o m C l a r k - S c h w e b e l C o r p . w i t h an e p o x y c o m p a t i b l e s u r f a c e f i n i s h and a r e as f o l l o w s : CS348 - 8H S a t i n Weave, 50 χ 50 c o u n t , .008" t h i c k CS352 - P l a i n Weave, 17 χ 17 c o u n t , .004" t h i c k . Q u a r t z f a b r i c was o b t a i n e d w i t h an e p o x y c o m p a t i b l e f i n i s h f r o m J . P. S t e v e n s and i s as f o l l o w s : A s t r o q u a r t z S y t l e 503 - P l a i n Weave, 50 χ 40 c o u n t , .005" t h i c k . P r e p r e g was p r e p a r e d by h a n d - d i p p i n g t h e f a b r i c i n t o t h e r e s i n m i x d i s s o l v e d i n an a p p r o p r i a t e s o l v e n t ( m e t h y l c e l l o s o l v e / acetone). D r y i n g and B - s t a g i n g was done i n a h o t - a i r c i r c u l a t i n g o v e n a t 150°C f o r a b o u t 6 m i n u t e s . The p r e p r e g was c u t and s t a c k e d t o p r o v i d e a n o m i n a l 1/8" t h i c k n e s s l a m i n a t e . The s t a c k was l a m i n a t e d a t 500 p s i f o r 1 h o u r a t 180°C. L a m i n a t e s w e r e c o o l e d u n d e r p r e s s u r e and no p o s t - b a k e o r s t r e s s r e l i e f was a p p l i e d t o t h e l a m i n a t e s . W h i l e d a t a was c o l l e c t e d on l a m i n a t e s o f d i f f e r i n g r e s i n c o n t e n t s , most o f t h e d a t a p r e s e n t e d h e r e i s on l a m i n a t e s c o n t a i n i n g 50% by w e i g h t r e s i n c o n t e n t . The TCEs w e r e m e a s u r e d on s a m p l e s m a c h i n e d f r o m t h e 1/8" t h i c k l a m i n a t e s by u s i n g two t e c h n i q u e s . In a h o r i z o n t a l quartz t u b e d i l a t o m e t e r , s a m p l e s 2" l o n g and 1/4" w i d e w e r e m e a s u r e d i n t h e 2" d i r e c t i o n . The measurements w e r e made i n a c c o r d a n c e w i t h p r o c e d u r e s d e f i n e d i n ASTM E-228. S i n c e t h e d i l a t o m e t e r i s h o r i z o n t a l , measurement o f s h r i n k a g e , i f any, r e q u i r e s t h a t t h e p r o b e be k e p t i n c o n t a c t w i t h t h e s p e c i m e n . T h i s i s done by means o f a l i g h t s p r i n g w h i c h a p p l i e s a l o a d o f a b o u t 3 o z s on t h e s a m p l e a t room t e m p e r a t u r e . Measurement o f e x p a n s i o n c a n a l s o be made w i t h o u t the s p r i n g l o a d . A P e r k i n E l m e r TMS-2 t h e r m a l a n a l y s i s s e t - u p was u s e d t o measure TCEs on 1/4" χ 1/4" s a m p l e s m a c h i n e d f r o m t h e l a m i n a t e . I n t h i s t e c h n i q u e , commonly known as TMA, a v e r t i c a l p r o b e ( f l a t , 0.140" d i a m e t e r ) i s p l a c e d on t h e s a m p l e and t h e r m a l e x p a n s i o n m e a s u r e d as t h e s a m p l e i s h e a t e d o r c o o l e d . The v e r t i c a l p r o b e i s b a l a n c e d i n o i l s u s p e n s i o n and a c e r t a i n w e i g h t i s added t o t h e p r o b e t o a r r i v e a t a n e t z e r o w e i g h t on t h e s a m p l e . A d d i t i o n a l w e i g h t may be p l a c e d on t h e p r o b e t o i m p r o v e t h e s i g n a l / n o i s e r a t i o , b u t t h i s a f f e c t s t h e r e s u l t s as does s p r i n g l o a d i n g i n t h e h o r i z o n t a l d i l a t o m e t e r . S t r a i n gauge t e c h n i q u e s w e r e a l s o e x a m i n e d by us and w e r e f o u n d t o be l e s s u s e f u l . ( 4 ) A minimum o f two c y c l e s were r u n t o m e a s u r e t h e t h e r m a l e x p a n s i o n and s e c o n d c y c l e v a l u e s w e r e u s e d . Mathematical Modeling. The r e s u l t s o b t a i n e d by v a r i o u s d i l a t o m e t r i c t e c h n i q u e s w e r e compared t o t h e o r e t i c a l p r e d i c t i o n s o f t h e r m a l expansion u s i n g a s i m p l e mathematical model. Thermal p r o p e r t i e s o f K e v l a r f a b r i c / e p o x y l a m i n a w e r e s i m u l a t e d by c o n s i d e r i n g t h e f a b r i c t o be made o f two c o n s e c u t i v e 0° and 90° l a m i n a o f
Davidson; Polymers in Electronics ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
382
POLYMERS IN ELECTRONICS
unidirectional Kevlar/epoxy composite. Influence of fiber cross over present i n a fabric on thermal properties was neglected. Linear TCE for a unidirectional composite i n the fiber direction (αχι) and transverse to i t (0122) have been calculated as suggested by Schapery(5), who used complementary and potential energy principles i n their derivations. The following equations give values of α^χ and o>22 f ° unidirectional lamina: r a
E-cx-v + Ε α ν t f î m m m α„, = — ;— 11 Ε
