Polymers in Electronics - American Chemical Society

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Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 10, 2015 | http://pubs.acs.org Publication Date: March 15, 1984 | doi: 10.1021/bk-1984-0242.ch029

UV-Curable Conformal Coatings C. R. MORGAN, D. R. KYLE, and R. W. BUSH W. R. Grace & Co., Columbia, MD 21044 This paper discusses a new type of conformal coating that is solventless, one-component and cured by a combination of UV and heat. Completed printed circuits often require total encapsulation by conformai coatings to insulate the electronic components and to provide protection from moisture, dust, solvents, and other con taminants present in the environment. The current most common conformai coatings are one- or two-component systems whose main constituents are epoxies, acrylics, urethanes or silicones. While these conformai coatings can provide excellent protection for the finished circuit board assembly, they have one or more undesirable features such as high solvent content, need for mixing, or long cure times. For example, the handling times range from 10 min to 5 hr, and the time to reach optimum properties ranges from 25 hr to 1 wk, depending on the system and curing conditions. To overcome these disadvantages, we have developed a new type of conformai coating that is solventless, one-component, and cured by exposure to UV followed by heating. The UV exposure rapidly sets the coating, thereby allowing for immediate handling and pre venting any runoff during the heating step. The heating step com pletes the cure of the parts of the coating that are not exposed to the UV (e.g., under the components). This paper discusses the chemistry, curing, and properties of these dual UV/thermally cur able conformai coatings. Chemistry These dual UV/thermally curable conformai coatings are based on the ability to rapidly transform a liquid composition into a cross-linked solid by the free-radical polymerization of a polyfunctional acrylate in the presence of a low concentration of a polyfunctional thiol ( 1). Under these conditions, the polythiol functions partly by adding across the acrylic double bond and partly as a chain transfer agent in the acrylate homopolymeriza0097-6156/ 84/0242-0373S06.00/0 © 1984 American Chemical Society

In Polymers in Electronics; Davidson, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

374

POLYMERS IN ELECTRONICS

T

n

e

d

u

a

l

t i o n ( 2 , 3)· UV/thermal c u r a b i l i t y i s obtained by adding b o t h a p h o t o i n i t i a t o r and a t h e r m a l i n i t i a t o r ( 4 , ^5, 6 ) . The most e f f i c i e n t p h o t o i n i t i a t o r s f o r t h e s e t h i o l / a c r y l a t e s y s t e m s a r e ones t h a t c l e a v e t o f r e e r a d i c a l s o n e x p o s u r e t o UV (7) · An example o f t h i s t y p e o f p h o t o i n i t i a t o r i s a l p h a , a l p h a dimethoxy-alpha-phenylacetophenone which i s reported t o photoc l e a v e as i n E q u a t i o n 1 t o form a h i g h l y r e a c t i v e methyl r a d i c a l (8) .


0 OCH

0

3

PhC-CPh — — — >

A

CH

OCH

PhC- + PhCOCH

0

jj

3

>

PhCOCH„ + CH · *3 ~"3

(1)

3

W i t h t h i s t y p e o f p h o t o i n i t i a t o r and w i t h t h e p r o p e r UV s y s t e m , c u r i n g o f t h e s e t h i o l / a c r y l a t e c o n f o r m a i c o a t i n g s c a n b e accompl i s h e d i n 10 s e c o r l e s s . The t h e r m a l i n i t i a t o r i s b e n z o p i n a c o l ( 4 ) , w h i c h i s s t a b l e a t room t e m p e r a t u r e ( s h e l f - l i f e o f t h e u n c u r e d l i q u i d c o n f o r m a i c o a t i n g i s more t h a n 6 months a t room t e m p e r a t u r e ) , b u t w h i c h u n d e r goes a h o m o l y t i c c l e a v a g e upon h e a t i n g t o f o r m d i p h e n y l h y d r o x y methyl r a d i c a l s (Equation 2 ) , which are believed t o i n i t i a t e p o l y m e r i z a t i o n b y h y d r o g e n atom t r a n s f e r t o t h e d o u b l e bond o f t h e a c r y l a t e ( E q u a t i o n 3) ( 9 ) . OH OH I I Ph C—CPh 2

A

>

2

OH I 2 Ph C

(2)

2

OH P h C - + CH =CHR 2

2

>

Ph CO 2

+

CH^HR

(3)

DSC a n a l y s e s ( P e r k i n - E l m e r DSC-2C) o f t h i o l / a c r y l a t e / b e n z o p i n a c o l s y s t e m s show t h a t p o l y m e r i z a t i o n c a n be i n i t i a t e d a t t e m p e r a t u r e s as l o w a s 85°C. However, t h e r e a c t i o n i s t o o s l o w a t 85°C f o r t h e c o n f o r m a i c o a t i n g a p p l i c a t i o n and t e m p e r a t u r e s o f 100°C o r h i g h e r a r e r e q u i r e d t o g e t p r a c t i c a l h e a t c u r e t i m e s o f 5 t o 30 m i n . The u n c u r e d c o n f o r m a i c o a t i n g s a r e c l e a r , t r a n s p a r e n t l i q u i d s w i t h v i s c o s i t i e s o f 700 t o 1400 cp a t 25°C. A t y p i c a l c o m p o s i t i o n w h i c h h a s a v i s c o s i t y o f 820 cp a t 25°C c o n s i s t s o f 43 wt % o f a h i g h v i s c o s i t y u r e t h a n e a c r y l a t e o l i g o m e r , 49 wt % o f a l o w v i s c o s i t y m o n o a c r y l a t e d i l u e n t , 5 wt % o f a p o l y f u n c t i o n a l t h i o l , 2 wt % o f b e n z o p i n a c o l a n d 1 wt % o f a l p h a , a l p h a - d i m e t h o x y - a l p h a phenylacetophenone. A t r a c e amount o f a f l u o r e s c e n t d y e may b e added, i f d e s i r e d , t o f a c i l i t a t e i n s p e c t i o n o f t h e f i n i s h e d c o a t ing.


29.

MORGAN ET AL.


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and C u r i n g

P r i n t e d c i r c u i t b o a r d s , c l e a n e d by u l t r a s o n i c vapor d e g r e a s i n g w i t h a F r e o n s o l v e n t , were c o a t e d b y d i p p i n g them i n t o t h e c o n f o r m a l c o a t i n g c o m p o s i t i o n d e s c r i b e d a b o v e . The d i p c o a t i n g c y c l e was a s f o l l o w s : i m m e r s i o n r a t e - 1 i n / m i n ; s t a n d i n g t i m e i n comp o s i t i o n - 2 m i n ; w i t h d r a w a l r a t e - 1 i n / m i n ; d r i p t i m e - 0.5 m i n . ( F o r m u l a t i o n s o f t h i s type c a n a l s o be s p r a y - c o a t e d , b u t o n l y d i p c o a t e d boards were used f o r the work d i s c u s s e d i n t h i s paper.) A f t e r d i p p i n g , t h e c o a t i n g was i m m o b i l i z e d b y r o t a t i n g t h e b o a r d f o r 10 s e c a t a b o u t 70 cm f r o m a B e r k e y A d d a l u x 300 W / i n medium p r e s s u r e m e r c u r y lamp. A f t e r t h i s s h o r t e x p o s u r e t o s e t t h e s u r f a c e , t h e c o a t i n g was f u r t h e r UV c u r e d b y e x p o s i n g t h e b o a r d u n d e r t h e A d d a l u x f o r 1 m i n on e a c h s i d e u n d e r a n a t m o s p h e r e o f n i t r o g e n to e l i m i n a t e s u r f a c e t a c k caused by oxygen i n h i b i t i o n . Following t h e UV c u r e , t h e c o a t i n g was h e a t - c u r e d b y p l a c i n g t h e b o a r d i n a 120°C f o r c e d a i r o v e n f o r 30 m i n . F o r c e r t a i n t e s t s t h e c o m p o s i t i o n was drawn down o n 0.23-mm t h i c k aluminum s h e e t s o r g l a s s p l a t e s a n d t h e n c u r e d f o r 1 m i n u n d e r a n a t m o s p h e r e o f n i t r o g e n u n d e r t h e A d d a l u x UV lamp f o l l o w e d by h e a t i n g a t 120°C f o r 30 m i n . The c o a t i n g s o n aluminum w e r e u s e d f o r f l e x i b i l i t y t e s t i n g and t h o s e on g l a s s w e r e removed f o r tests requiring free-standing films. The c u r i n g c o n d i t i o n s d e s c r i b e d above w e r e u s e d t h r o u g h o u t t h i s work t o m a i n t a i n s t a n d a r d c o n d i t i o n s f o r p r e p a r i n g v a r i o u s t e s t samples. I n a c t u a l p r a c t i c e , h e a t c u r i n g t i m e s o f 30 m i n a t 100°C, 20 m i n a t 110°C, o r 10 m i n a t 120°C w i l l p r o b a b l y b e a d e quate, depending on the c o a t i n g t h i c k n e s s , t h e s i z e o f t h e c i r c u i t b o a r d , a n d t h e k i n d s a n d c o n f i g u r a t i o n s o f t h e a t t a c h e d components. The UV c u r i n g t i m e c a n be s h o r t e n e d c o n s i d e r a b l y a n d t h e n i t r o g e n a t m o s p h e r e c a n be e l i m i n a t e d i f lamps o f h i g h e r i n t e n s i t y and/or d i f f e r e n t s p e c t r a l d i s t r i b u t i o n are used. F o r example, i n a s e p a r a t e s e r i e s o f e x p e r i m e n t s , i t was shown t h a t t h i s c o n f o r mai c o a t i n g c a n b e c u r e d t a c k - f r e e i n 2 p a s s e s a t 18 fpm ( e s t i mated e x p o s u r e t i m e o f 4 s e c ) u n d e r a F u s i o n S y s t e m s Type D 300 W/in doped medium p r e s s u r e m e r c u r y lamp l o c a t e d 8 cm above t h e sample. T e s t i n g and P r o p e r t i e s M e c h a n i c a l T e n s i l e P r o p e r t i e s , m e a s u r e d a c c o r d i n g t o ASTM D638 on 3 - m i l t h i c k cured f i l m s o f t h e c o n f o r m a i c o a t i n g , a r e : modulus o f 1750 p s i , t e n s i l e o f 2000 p s i , and e l o n g a t i o n - a t - f a i l u r e o f 285%. F l e x i b i l i t y was d e t e r m i n e d o n 2 - m i l t h i c k f i l m s o f t h e c u r e d c o n f o r m a l c o a t i n g o n 0.23-mm t h i c k a l u m i n u m s h e e t s b y b e n d i n g o v e r a 1/8 i n m a n d r e l a c c o r d i n g t o Fed. S t d . 141, Method 6221. The f i l m showed no c r a c k i n g o r l o s s o f a d h e s i o n a t t h e b e n d . W a t e r V a p o r P e r m e a b i l i t y was m e a s u r e d o n c u r e d

films of the


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c o n f o r m a i c o a t i n g e s s e n t i a l l y a c c o r d i n g t o ASTM E96 e x c e p t r e l a t i v e h u m i d i t i e s o f 0-75% w e r e u s e d i n s t e a d o f 0-50%. The w a t e r v a p o r p e r m e a b i l i t y f o r t h i s c o a t i n g a t 23°C i s 1.72 ( g ) ( m m ) / ( d a y ) (m )(cm Hg). 2

Volume R e s i s t i v i t y was d e t e r m i n e d on c u r e d f i l m s o f t h e c o n f o r m a i c o a t i n g i n a c c o r d a n c e w i t h ASTM D257 and f o u n d t o b e 3.5 χ Ι Ο ^ ohm-cm a t 23°C and 5 0 % r e l a t i v e h u m i d i t y .


1

Solvent Resistance. P r i n t e d c i r c u i t b o a r d s w e r e d i p - c o a t e d and c u r e d a s d e s c r i b e d above and t h e n immersed i n v a r i o u s s o l v e n t s . The c o n f o r m a i c o a t i n g was r e a d i l y removed b y t o l u e n e , a c e t o n e , and m e t h y l e n e c h l o r i d e b u t n o t removed b y w a t e r , i s o p r o p a n o l o r ethylene g l y c o l . Thermal S t r e s s R e s i s t a n c e . Epoxy-glass p r i n t e d c i r c u i t boards, c o n t a i n i n g v a r i o u s e l e c t r o n i c components w e r e d i p - c o a t e d and c u r e d a s d e s c r i b e d a b o v e a n d t h e n c y c l e d f r o m -65°C t o +125°C i n f o r c e d a i r chambers a c c o r d i n g t o t h e f o l l o w i n g s c h e d u l e : 1 h r a t -65°C, 2 m i n a t room t e m p e r a t u r e , 1 h r a t +125°C, 2 m i n a t room t e m p e r a t u r e , 1 h r a t -65°C, e t c . Such b o a r d s s u r v i v e d 20 t o >50 c y c l e s w i t h o u t c r a c k i n g and/or d e l a m i n a t i o n o f t h e c o a t i n g . Hydrolytic Stability. Epoxy-glass c i r c u i t boards w i t h t h e conduc t o r p a t t e r n shown i n F i g u r e 1 w e r e e n c a p s u l a t e d i n a 2 - m i l t h i c k c u r e d c o n f o r m a i c o a t i n g and t e s t e d f o r h y d r o l y t i c s t a b i l i t y b y m a i n t a i n i n g them a t 100°C and 9 5 % r e l a t i v e h u m i d i t y and o b s e r v i n g c h a n g e s i n a p p e a r a n c e and i n s u l a t i o n r e s i s t a n c e w i t h t i m e . After 1,000 h r i n t h i s t e s t , t h i s c o a t i n g showed o n l y a s l i g h t d i s c o l o r a t i o n and e s s e n t i a l l y no change i n i n s u l a t i o n r e s i s t a n c e .

F i g u r e 1. C i r c u i t p a t t e r n f o r h y d r o l y t i c s t a b i l i t y t e s t i n g . C i r c u i t : T i n / l e a d o v e r c o p p e r . C o n d u c t o r w i d t h : 0.7 mm. Space between c o n d u c t o r s : 1.2 mm.


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Summary We have described a new type of conformai coating which is shelfstable, solventless, one-component, and rapidly cured by a combi nation of UV and heat. Once cured, this coating has very good electrical properties, thermal stress resistance, and hydrolytic stability. With the proper choice of solvents, all or a portion of the cured coating can be removed from a circuit board to repair the electronic parts. Acknowledgments The authors thank Leslie Schulz and William Ehmann for their technical assistance, Ronald Andrejak for the DSC curves, and John Arnreich for the electrical and mechanical tensile properties. Literature Cited 1. Kehr, C. L. U.S. Patent 4 008 341, 1977. 2. Walling, C. "Free Radicals in Solution"; John Wiley & Sons, Inc: New York, N.Y., 1957; p. 313 ff. 3. Morgan, C. R.; Ketley, A. D. J. Rad. Curing 1980, 7(2), 10. 4. Morgan, C. R. U.S. Patent 4 020 233, 1977. 5. Morgan, C. R. U.S. Patent 4 288 527, 1981. 6. Morgan, C. R. U.S. Patent 4 352 723, 1982. 7. For a review of photoinitiators for photopolymerization, see Pappas, S. P.; McGinness, V. D. in "UV Curing: Science and Technology", Pappas, S. P., Ed.; Technology Marketing Corpo ration, Stamford, Conn., 1978, p. 1. 8. Sander, M. R.; Osborn, C. L. Tetrahedron Lett. 1974, 415. 9. Braun, D.; Becker, Κ. H. Ind. Eng. Chem. Prod. Res. Develop. 1971, 10(4), 386. RECEIVED September 2, 1983


Polymers in Electronics - American Chemical Society

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