The Design And Calibration Of A Semiconductor Strain Gauge Array

A semiconductor s t r a i n gauge a r r a y , c o n s i s t i n g o f d i g i t a l l y addres sab le r e s i s t o r ed t o measure t h e mechanical t e d du r ing i n t e g r a t e d c i r c u i t This paper d i s c u s s e s s t r a i n t h e convers ion o f measured p r i n c i p a l stresses on t h e An automated test s e t u p or t h e convenient de te rmit a m e c o e f f i c i e n t s and t h e o f r e s i s t a n c e . For t h i s nding j i g was des igned and cons t ruc t ed so t h a t it could be c o n t r o l l e d from wi th in an environmental chamber. The s t r a i n gauge w i l l be used t o map t h e p r i n c i p a l stress f i e l d s c h a r a c t e r i s t i c o f i n t e g r a t e d c i r c u i t assembly and f a t i g u e l i f e t e s t procedures. I n t r o d u c t i o n During t h e assembly o f i n t e g r a t e d c i r c u i t s , mechanical stresses a r e imparted on to t h e d i e . These stresses r e s u l t from t h e mismatch i n thermal expansion between t h e va r ious assembly m a t e r i a l s , i.e., t h e s i l i c a f i l l e d epoxy molding compound, t h e copper leadframe and t h e s i l i c o n i n t e g r a t e d c i r c u i t . Undes i rab le consequences o f t h e s e mechanical stresses inc lude pass iva t ion , encaps u l a n t and d i e c racking . The aim o f t h e p r e s e n t work is t o provide a t o o l which can map t h e p l ana r stress t e n s o r over t h e s u r f a c e o f t h e i n t e g r a t e d c i r c u i t . Due t o t h e q u a n t i t y o f measurements and c a l c u l a t i o n s r equ i r ed fo r stress mapping, an automated d a t a a c q u i s i t i o n and a n a l y s i s system was implemented. This system was designed around an IRM AT pe r sona l computer, a Data Trans l a t ion DT2801-5716 d a t a a c q u i s i t i o n board, a high s t a b i l i t y Ke i th l ey 220 programmable c u r r e n t sou rce and t h e ASYST programming language. Add i t iona l ly , a RANSCO environmental chamber was used i n t h e p i e z o r e s i s t a n c e c a l i b r a t i o n experiments. The TP86 S t r a i n Gauge The b a s i c stress senso r is an a r s e n i c i m p l an ted resistor rosette c o n s i s t i n g o f 4 r e s i s t o r s o r i e n t e d a t Oo, 45O, 90° and 1 3 5 O , f i g . 1. The compact size o f t h e rosette s t r u c t u r e (220x220p2) is necessa ry i n o r d e r t o make l o c a l i z e d p r i n c i p a l stress c a l c u l a t i o n s . In o r d e r t o i n v e s t i g a t e t h e i n f l u e n c e o f d i e s i z e upon stress, d i e dimensions of 2 . 5 ~ 2 . 5mm2 4 . 5 ~ 4 . 5mm2 6 . 5 ~ 6 . 5mm2, 2. 5x4.5mm2 and 4.5x6.5mm2) a r e a v a i l a b l e . The t o t a l number o f r e s i s t o r s chosen f o r a p a r t i c u l a r d i e s i z e is implemented i n "powers o f 2" due t o mul t ip l ex ing requi rements the 2 . 5 ~ 2 . 5mm2 and 2 . 5 ~ 4 . 5m2 d i c e i n c o r p o r a t e 128 t o t a l resistors; t h e 4.5x4.5mm2 and 4 . 5 ~ 6 . 5mm2 d i c e i n c o r p o r a t e 256 r e s i s t o r s ; and t h e 6.5x6.5m2 d i e i n c o r p o r a t e 512 r e s i s t o r s . 2.5mm2, s t r a i n gauge d i e is shown i n f i g . 2. The arrangement of t h e 128 r e s i s t o r , 2 . 5 ~ The sed t o addres s i n d i v i d u a l resisned i n t h e l a r g e r e c t a n g u l a r f e a t u r e s on t h e t o p and r i g h t edges o f t h e d i e . S ince t h e h ighes t stress l e v e l s a r e expected a t t h e d i e co rne r s , t h e s t r a i n gauge arrangement i s map t h i s r eg ion wi th t h e h ighes t e t te d e n s i t y on t h e Process ing o f t h e senso r invo lves t h e unusual combination of MOS t echnology, f o r t h e s e l e c t i o n l o g i c , and (111) o r i e n t a t i o n s i l i c o n , f o r piezoFig. 1 The b a s i c s t r a i n gauge rosette ( 2 2 0 ~ 220p2). Also shown a r e t h e primed and unprimed c o o r d i n a t e systems used f o r c a l c u l a t i o n s i n t h i s paper. F ig . 2 The 2.5x2.5mr2, 128 r e s i s t o r , TP86 s t r a i n gauge. CH2560-1/88/0000-0185$01.00@1988 IEEE 18