12 Solvent-Removable Coatings for Electronic Applications
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J. J. LICARI and B. L. WEIGAND Rockwell International Corporation, Electronic Devices Division, P.O. Box 4192, Anaheim, CA 92803
A l a r g e variety o f m a t e r i a l s and processes are c u r r e n t l y employed in the manufacture o f h y b r i d microcircuits. Because of this, it is often difficult t o completely remove t h e last t r a c e s of p r o c e s s i n g m a t e r i a l s through normal c l e a n i n g prior t o hermetic sealing. A single electrically-conductive particle, even in t h e micron s i z e range, can cause an electrical short and result i n a missile o r spacecraft failure. In s p i t e o f the precautions taken by t h e e l e c t r o n i c s i n d u s t r y , it is extremely difficult to completely remove all particles, some being electrostatically attached t o t h e surface o f t h e circuit. One approach t o r e s o l v ing this problem — c o a t i n g t h e e n t i r e circuit w i t h a thin l a y e r of organic polymeric c o a t i n g in order t o immobilize the particles — has been evaluated and has been s u c c e s s f u l . On a limited b a s i s , coatings o f h i g h purity such as some semiconductor grade silicones and p a r a p o l y x y l y l e n e s have been used and are g e n e r a l l y compatible w i t h the a c t i v e and passive devices and w i r e bonds o f a high density circuit. However, these h i g h molecular weight polymer coatings once a p p l i e d and cured are d i f f i c u l t t o remove f o r c i r c u i t rework. Because o f t h e h i g h cost o f m i l i t a r y and space grade m i c r o c i r c u i t s (ranging from s e v e r a l hundred t o s e v e r a l thousand d o l l a r s ) , rework i s important. Any p a r t i c l e i m m o b i l i z i n g c o a t i n g should t h e r e f o r e be e a s i l y removed t o permit t h e r e p a i r s . The h i g h molecular weight and h i g h l y c r o s s l i n k e d polymers are i n h e r e n t l y d i f f i c u l t t o remove. They are i n s o l u b l e i n t h e normal organic s o l v e n t s . Removal by a b r a d i n g , s c r a p i n g , or c u t t i n g i s operator dependent and often r e s u l t s i n f u r t h e r damage t o adjacent devices and i n t e r c o n n e c t i o n s . The o b j e c t i v e o f t h i s work was t h e r e f o r e t o evaluate and s e l e c t a c l a s s o f organic coatings t h a t c o u l d e a s i l y be removed by a "hands-off" process by d i s s o l v i n g them i n organic s o l v e n t s t h a t are commonly used i n a manufacturing environment. T h i s work was supported by t h e A i r Force M a t e r i a l s L a b o r a t o r y , W r i g h t - P a t t e r s o n A i r Force Base, Ohio, Contract No. F 3 3 6 l 5 - 7 7 - C - 5 l k L . 0-8412-0567-l/80/47-132-127$05.00/0 © 1980 American Chemical Society May; Resins for Aerospace ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
RESINS F O R A E R O S P A C E
128
CH, I
CI !
Si
C -
0
I CH~ (a)
Linear Polydimethyl s i loxane
(c)
Figure J.
Table I .
(b)
Polychlorotrifluoroethylene
Block Co-polymer o f α - m e t h y l s t y r e n e and Polydimethyl s i l o x a n e General molecular structures for solvent-soluble coatings
D e s c r i p t i o n o f S i 1icone Coatings S e l e c t e d f o r
Evaluation
Percent Solids (as r e c ' d )
Viscosity (Centistokes)
Polydimethylsiloxane I
27.1
49
Polydimethyls i l o x a n e II
16.0
570
C l e a r , t a c k y , f l o w s under p r e s s u r e a t room tempera ture.
25.5
506
C l e a r , h a r d , tough c o a t i n g , melting point 285-289°C.
C o a t i n g Type
B l o c k Co-polymer of α-methylstyrene & dimethyl s i l o x a n e
Film Description C l e a r , waxy, does not melt o r f l o w below 1 5 0 ° C , flows under p r e s s u r e a t 8 5 ° C .
May; Resins for Aerospace ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
12.
LiCARi A N D wiEGAND
S e l e c t i o n o f Candidate
Solvent-Removable
129
Coatings
Coatings
The f i v e k e y r e q u i r e m e n t s t h a t t h e c o a t i n g h a d t o meet w e r e : (1) Easy a p p l i c a t i o n by spray, d i p o r f l o w c o a t i n g i n t h i n l a y e r s , 0.5 t o 2 m i l s . (2) H i g h d e g r e e o f p u r i t y c o n t a i n i n g l o w l e v e l s o f s o d i u m and c h l o r i d e i o n s a n d no o t h e r c o r r o s i v e c o n s t i t u e n t s s u c h a s amines. (3) Easy removal by solvent d i s s o l u t i o n . (k) T h e r m a l l y s t a b l e t o 150°C t o w i t h s t a n d b u r n - i n t e m p e r atures used f o r h y b r i d c i r c u i t s . (5) No s t r e s s e s o n f i n e - w i r e bonds a n d no d e l e t e r i o u s e f f e c t s on a c t i v e d e v i c e s . C o a t i n g s were s e l e c t e d f r o m a c l a s s o f l o w - t o - i n t e r m e d i a t e molecular weight u n c r o s s l i n k e d s i l i c o n e s and halocarbons which were s o l u b l e i n F r e o n T F , x y l e n e , o r t o l u e n e — c l e a n i n g solvents widely used i n t h e e l e c t r o n i c s i n d u s t r y . Specifically, t h r e e s i l i c o n e s f o r m u l a t e d b y Dow C o r n i n g , f o u r h a l o c a r b o n waxes from Halocarbon C o r p o r a t i o n and m i x t u r e s o f t h e s i l i c o n e s and h a l o c a r b o n s were e v a l u a t e d . The g e n e r a l m o l e c u l a r s t r u c t u r e s and c h a r a c t e r i s t i c s o f t h e s e c o a t i n g s a r e g i v e n i n F i g u r e 1 a n d T a b l e s I and I I . E a r l y i n t h e program, t h r e e o f t h e f o u r h a l o c a r b o n waxes were e l i m i n a t e d f r o m f u r t h e r c o n s i d e r a t i o n . Haloc a r b o n s 6-00 a n d 12-00 were t o o l o w i n m e l t i n g p o i n t t o w i t h s t a n d t h e c i r c u i t b u r n - i n t e m p e r a t u r e s o f 125 t o 150°C. H a l o c a r b o n 19-00 was i n s o l u b l e i n t o l u e n e a n d d i f f i c u l t l y s o l u b l e i n o t h e r s o l v e n t s so t h a t a c o a t i n g s o l u t i o n c o u l d n o t b e f o r m u l a t e d . Two o f t h e s i l i c o n e s , t h e l i n e a r p o l y d i m e t h y l s i l o x a n e t y p e s ( I and I I ) were a l s o e l i m i n a t e d a f t e r p r e l i m i n a r y s t u d i e s showed t h a t r e s i d u e s r e m a i n e d o n t h e c i r c u i t a f t e r F r e o n TF e x t r a c t i o n , t h a t one o f t h e s i l i c o n e s r e m a i n e d t a c k y t h r o u g h o u t p r o c e s s i n g , and t h a t t h e o t h e r s i l i c o n e h a d a h i g h r a t e o f o u t g a s s i n g .
Table I I .
Halocarbon Corp. S e r i a l No.
D e s c r i p t i o n o f H a l o c a r b o n Waxes Selected f o r Evaluation Melting Point or Pour P o i n t
Solubility
6-00
60°C
Freon TF, Toluene
12-00
120°C
Freon TF, Toluene
15-00
lkk°C
Toluene
19-00
210-220°C
Insoluble
May; Resins for Aerospace ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
May; Resins for Aerospace ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
IJ I \ppm/by wt
C h l o r i d e (ppm/by wt)
Titration
Sodium (ppm/by wt) Potassium (ppm/by wt)
Atomic A b s o r p t i o n
Calcium S Loss on I g n i t i o n (wt %)
StrontiumJ
Silver Titanium Ij Nickel
Iron Boron Aluminum Copper
S i l i c o n (wt %) Magnesium \
Emission Spectroscopy
Method/Element
60 209
60.4 18.5
48.86
41.72
44.3 12.1
24 2.2 23
