UV Solder Masks as Insulators for Printed Circuit Boards

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Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 24, 2018 | https://pubs.acs.org Publication Date: March 15, 1984 | doi: 10.1021/bk-1984-0242.ch028

U V Solder Masks as Insulators for Printed Circuit Boards NEIL S. FOX Dynachem Corporation, Santa Ana, CA 92680 The effect of temperature and humidity on the insulation resistance of photopolymerized and thermally cured solder masks was determined. Dry film and screen ink solder masks were applied to a test pattern. The insulation resistance of the coated test pattern decreased with exposure to elevated temperature and humidity. Elevated temperature alone, however, had only a small negative affect on the insulation resistance. Solder masks (solder resists) are protective polymer coatings used on printed circuit boards (PCB's) to prevent solder bridging between conductors during soldering (1-3). The selective applica tion of the solder mask limits the amount of conductor area in contact with the molten solder, minimizing solder usage and slow ing the rate of conductor metal contamination in the solder pot. The ability of a solder mask to protect conductors from physical and chemical deterioration and to insulate adjacent circuitry is a major consideration of PCB manufacturers. The Institute for Interconnecting and Packaging Electronic Circuits (IPC) has defined the requirements for the qualification and per formance of solder masks in the Standard Specification IPC-SM-840 (A). The specification defines classes (1, 2, and 3) to reflect progressive increases in sophistication, functional performance, and testing methods. These classes try to provide PCB manufac turers with assurances of reliability. For example, Class 1 requirements provide the reliability needed by commercial boards used in radios, televisions, and small appliances. Class 2 is for computers and Class 3 for military and life-dependent products. Ultraviolet light (UV) curable screen ink solder masks and photopolymerizable dry film solder masks have been available commercially since 1973 (5). These materials offer the PCB manufacturer many processing and production advantages relative to the conventional solvent-evaporative thermally cured solder masks. These products based on a new emerging technology have helped 0097-6156/84/0242-0367506.00/0 © 1984 American Chemical Society Davidson; Polymers in Electronics ACS Symposium Series; American Chemical Society: Washington, DC, 1984.


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f o s t e r the e x p l o s i v e growth o f the p r i n t e d c i r c u i t i n d u s t r y . Along w i t h t h i s growth i s g r e a t e r concern f o r p r i n t e d c i r c u i t board r e l i a b i l i t y . C u r r e n t l y , t h e IPC-SM-840 i s u n d e r g o i n g a r e v i s i o n w i t h major changes i n the i n s u l a t i o n r e s i s t a n c e t e s t i n g r e q u i r e m e n t s f o r t h e t h r e e c l a s s e s ( 6 ) . Some PCB m a n u f a c t u r e r s c a r e f u l l y t e s t t o t h e i r own i n t e r n a l i n s u l a t i o n r e s i s t a n c e s p e c i f i c a t i o n s and c o n d u c t r e s e a r c h t o i s o l a t e t h e f a c t o r s a f f e c t i n g t h e i n s u l a t i o n r e s i s t a n c e o f t h e i r f i n i s h e d PCB*s (7)· The i n s u l a t i o n r e s i s t a n c e of a c o a t e d t e s t p a t t e r n o f t e n i s measured a t e l e v a t e d t e m p e r a t u r e and h u m i d i t y t o a p p r o x i m a t e h o s t i l e e n v i r o n m e n t s t h e c o m m e r c i a l c i r c u i t a s s e m b l y m i g h t be e x p o s e d t o d u r i n g i t s e x p e c t e d l i f e s p a n and t o a r t i f i c i a l l y a c c e l e r a t e t h e a g i n g p r o c e s s . The IPC-SM-840 m o i s t u r e r e s i s t a n c e t e s t c o n d i t i o n s a r e shown i n T a b l e I .

Table I .

IPC-SM-840 M o i s t u r e R e s i s t a n c e T e s t C o n d i t i o n s

Class 1 2 3

Temperature 35°C 50°C 85°C

Humidity 9 5 % RH 9 5 % RH 9 5 % RH

Time 4 days 4 days 7 days

Many s t u d i e s h a v e b e e n c o n c e r n e d w i t h e l e c t r o m i g r a t i o n on PCB's ( 8 - 1 1 ) . M e t a l l i c ( d e n d r i t i c ) g r o w t h o f m e t a l f r o m one c o n d u c t o r t o a n o t h e r c a n c a u s e microamp c u r r e n t l e a k a g e o r e l e c t r i c a l shorts. Since h i g h i n s u l a t i o n r e s i s t a n c e lowers the r a t e of e l e c t r o m i g r a t i o n , t h e i n s u l a t i o n r e s i s t a n c e o f a s o l d e r mask and the f a c t o r s which i n f l u e n c e i t are of great i n t e r e s t . This s t u d y was p e r f o r m e d t o i s o l a t e t h e a f f e c t s o f t e m p e r a t u r e and h u m i d i t y on the i n s u l a t i o n r e s i s t a n c e o f p h o t o p o l y m e r i z a b l e s o l d e r masks r e l a t i v e t o c o n v e n t i o n a l s o l d e r masks. Chemistry The t y p i c a l UV c u r a b l e s o l d e r mask f o r m u l a t i o n c o n s i s t s o f t h e following. P h o t o i n i t i a t i o n S y s t e m . I n most c a s e s t h e c u r i n g i s i n i t i a t e d by f r e e r a d i c a l s g e n e r a t e d by UV l i g h t and p h o t o i n i t i a t o r s . The f r e e r a d i c a l s c a n be f o r m e d f r o m p h o t o c l e a v a g e o r h y d r o g e n a b s t r a c t i o n mechanisms. B e n z o i n e t h e r s , b e n z i l k e t a l s , and a c e t o p h e n o n e d e r i v a t i v e s are t y p i c a l p h o t o i n i t i a t o r s . P h o t o i n i t i a t o r s + hv Free R a d i c a l s + C r o s s l i n k e r s — • Monomers and O l i g o m e r s .

• Free R a d i c a l s Three D i m e n s i o n a l Polymer M a t r i x

T h e s e m a t e r i a l s a r e p o l y m e r i z e d by

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

free

28.

FOX

U V Solder Masks: Printed Circuit Board Insulators

369

r a d i c a l s and i m p a r t much o f t h e u l t i m a t e p r o p e r t i e s t o t h e s o l d e r mask. E x a m p l e s a r e a c r y l a t e d e p o x y r e s i n s , a c r y l a t e d u r e t h a n e s , and m u l t i - f u n c t i o n a l monomers. A t y p i c a l a c r y l a t e monomer i s t r i m e t h y l o l p r o p a n e t r i a c r y l a t e (TMPTA). CH CH C 3

2

(CH OOCCH=CH )3 2

2


Functional F i l l e r s . These r e l a t i v e l y i n e r t m a t e r i a l s a r e d i s p e r s e d i n the f o r m u l a t i o n to develop the d e s i r e d r h e o l o g y body n e e d e d f o r t h e a p p l i c a t i o n o f t h e s o l d e r mask.

and

Colorants. Pigments o r dyes are used to ease the i n s p e c t i o n of t h e PCB a f t e r s o l d e r mask a p p l i c a t i o n . The c o l o r a n t c o n t e n t i s l i m i t e d by c o m p e t i t i o n w i t h t h e p h o t o i n i t i a t o r s f o r t h e UV l i g h t . Too h i g h a l e v e l o f c o l o r a n t c a n r e s u l t i n a l o s s o f c u r e s p e e d . Additives. T h e s e s p e c i a l t y m a t e r i a l s c a n be f l o w m o d i f i e r s , adhesion promoters, s t a b i l i z e r s , p l a s t i c i z e r s , e t c . Experimental The IPC M u l t i - P u r p o s e T e s t P a t t e r n , IPC-B-25 i s shown i n F i g u r e 1. T e s t b o a r d s w e r e f a b r i c a t e d i n - h o u s e c o n t a i n i n g t h e IPC-B-25 p a t t e r n on b o t h s i d e s . The c e n t e r comb p a t t e r n w i t h 12.5 m i l c o p p e r c o n d u c t o r s and 12.5 m i l s p a c e s was u s e d t o measure t h e i n s u l a t i o n r e s i s t a n c e . A t o t a l of e i g h t v a l u e s were measured f o r e a c h b o a r d and t h e a v e r a g e r e p o r t e d . A G e n e r a l R a d i o M o d e l 1864 Megohmmeter was u s e d t o measure t h e i n s u l a t i o n r e s i s t a n c e u s i n g 100 v o l t s DC and a 60 s e c o n d e l e c t r i f i c a t i o n t i m e . A s a t u r a t e d s a l t s o l u t i o n h u m i d i t y chamber w i t h a g o l d t a b m o u n t i n g f i x t u r e was u s e d t o p r o d u c e 95% r e l a t i v e h u m i d i t y as s p e c i f i e d i n IPC-SM-840. A c o n t r o l l e d t e m p e r a t u r e B l u e M o v e n was modified to a l l o w e x t e r n a l monitoring of i n s u l a t i o n r e s i s t a n c e . The t e s t b o a r d s w e r e i n s p e c t e d v i s u a l l y , c l e a n e d , and b a k e d d r y b e f o r e a p p l i c a t i o n o f s o l d e r mask. The v a r i o u s s o l d e r masks w e r e a p p l i e d and c u r e d a c c o r d i n g t o methods s p e c i f i e d by t h e s o l d e r mask s u p p l i e r . The s o l d e r masks t e s t e d a r e d e s c r i b e d i n Table I I . A t y p i c a l i n s u l a t i o n r e s i s t a n c e determination i s perf o r m e d by p l a c i n g f i v e t e s t b o a r d s i n t h e t e s t f i x t u r e and f o l l o w i n g t h e measurement p r o c e d u r e . The t e s t f i x t u r e i s s e a l e d i n t h e h u m i d i t y chamber and p l a c e d i n t h e o v e n . The s p e c i f i e d c o n d i t i o n s ( T a b l e I ) a r e c o n t r o l l e d by t h e o v e n t e m p e r a t u r e .



370

Code SM-1 SM-2 SM-3 SM-4 SM-5 SM-6 SM-7 SM-8


T a b l e I I . D e s c r i p t i o n o f S o l d e r Masks Description O n e - p a r t UV c u r e d a c r y l i c f o r t i n / l e a d c i r c u i t r y . O n e - p a r t UV c u r e d a c r y l i c f o r c o p p e r c i r c u i t r y and h o t air leveling. O n e - p a r t UV c u r e d a c r y l i c f o r c o p p e r c i r c u i t r y . Two-part epoxy, a l l purpose. One-part alkyd/amine, f o r copper c i r c u i t r y . D r y f i l m s o l d e r mask, a l l p u r p o s e aqueous d e v e l o p a b l e . D r y f i l m s o l d e r mask, a l l p u r p o s e aqueous d e v e l o p a b l e . D r y f i l m s o l d e r mask, a l l p u r p o s e aqueous d e v e l o p a b l e .

R e s u l t s and

Discussion

G e n e r a l l y , i t i s b e l i e v e d t h a t many p l a s t i c s c l a s s e d a s i n s u l a t o r s become p a r t i a l c o n d u c t o r s when " h o t and w e t " ( 1 2 ) . The UV and t h e r m a l l y c u r e d s o l d e r masks i n T a b l e I I I show a d r a m a t i c l o s s o f i n s u l a t i o n r e s i s t a n c e a f t e r s e v e n d a y s a t 85°C and 9 5 % r e l a t i v e h u m i d i t y (RH). A l l t h e t e s t b o a r d s r e c o v e r e d most o f t h e i r i n i t i a l i n s u l a t i o n r e s i s t a n c e a f t e r 24 h o u r s a t a m b i e n t c o n d i tions. T a b l e I V shows t h a t a n a d d i t i o n a l e x p o s u r e t o 85°C a t ambient h u m i d i t y produces an i n c r e a s e i n i n s u l a t i o n r e s i s t a n c e .


28.

Table I I I .


371

UV Solder Masks: Printed Circuit Board Insulators

FOX

Materials SM-2 SM-3 SM-4 SM-5 SM-6 SM-7 SM-8

I n s u l a t i o n R e s i s t a n c e (Ohms), IPC C l a s s I I I C o n d i t i o n s , 85°C a n d 9 5 % RH Seven Initial One Day 8.9 χ 3.0 χ 107 1.4 χ 1013 1.5 χ 6.6 χ 107 1.3 χ 1013 4.8 χ 2.3 χ 1θ8 6.8 χ 1012 7.8 χ 8.1 χ 1 0 1.5 χ 1013 1.7 χ 2.4 χ 106 6.8 χ 1012 4.2 χ 7.4 χ 1012 9.0 χ 10? 4.5 χ 2.6 χ 1 0 8.0 χ 1θ!2

Table IV. Materials SM-4 SM-5 SM-6 SM-7 SM-8

8

7

Days 106 107 107 10 106 10? 10 8

6

F o u r Day D r y Oven T e s t , 85°C F o u r Days Initial 6.15 χ l O H 4.4 X 1010 4.9 χ 1012 2.9 X 1012 9.6 χ 10l° 1.5 X îoio 1.6 χ 1012 4.7 X i o n 2.3 χ l O H 3.9 X i o n

T a b l e V i l l u s t r a t e s t h e e f f e c t o f c u r e a n d s o l d e r i n g on s o l d e r masks t e s t e d a c c o r d i n g t o C l a s s 2 c o n d i t i o n s , 50°C, and 9 5 % RH. S u b j e c t i n g t h e t e s t b o a r d s t o h o t s o l d e r (510°F) i n c r e a s e s the i n s u l a t i o n r e s i s t a n c e .

T a b l e V.

I n s u l a t i o n R e s i s t a n c e (Ohms), IPC-SM-840 C l a s s 2, 50°C and 9 5 % RH Day F o u r Material Day One Initial 1.5 χ 1013 5.9 χ 1 0 H Blank 1.3 X 1012 1.3 χ 1013 Blank 1.2 X 1012 5.7 χ 1 0 l l 1.1 χ 1013 6.9 χ 108 SM-1 (10 FPM) 1.1 X 109 6.4 χ 1012 1.4 χ 109 1.0 X 109 SM-1 (20 FPM) 1.7 χ 1013 1.9 χ 1010 SM-1 (20 FPM, 1.8 X îoio Soldered) 9.9 χ 1 0 1.0 χ 1013 SM-2 (10 FPM) 2.5 X 10* 3.9 χ 109 1.0 χ 1013 SM-2 (20 FPM) 3.3 X 109 >10l3 1.3 χ 1010 SM-2 (20 FPM, 1.1 X îoio Soldered) 5.4 χ 1012 1.4 χ l O l O SM-3 (20 FPM) 2.2 X l o i o 8




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Conclusions The loss of insulation resistance of a solder mask determined at elevated temperature and humidity is dependent mostly on the moisture content of the air. Elevated temperature alone does not cause a dramatic reduction of insulation resistance. The solder ing process increases the insulation resistance of the solder masks examined. It is postulated that hot solder seals the surface of the solder mask, rendering a better moisture barrier. This study suggests higher insulation resistance should be obtained from solder masks with higher hydrophobic properties. Acknowledgments The author thanks Curtis Lustig and Bill Winkler for running the electrical tests. Literature Cited 1. Cole, H. "Printed Circuits Handbook", 2nd ed, C. F. Coombs, Jr., Ed., McGraw-Hill Book Company, New York, N.Y., 1979, Chapter 14. 2. Fefferman, G. B. "Using Ultraviolet Radiation Curable Resins for Printed Circuit Coatings", 2nd International Conference on Radiation Curing, Cincinnati, Ohio, May 1975. 3. Tautscher, C. J. "Protective Circuit Coatings", Novatech Research Corporation, Redmond, Washington, 1981. 4. IPC Standard Specification IPC-SM-840, "Qualification and Performance of Permanent Polymer Coating (Solder Mask) for Printed Boards", Evanston, Illinois, 1977. 5. Axon, F., Cleek, R., Custer, W., Lipson, M., and Mestadagh, D. Circuit World 1978, 4, 24. 6. "IPC Standard Specification IPC-SM-840A", PROPOSAL, September 1982. 7. Tautscher, C. J. and Barmuta, M. J. Printed Circuit Fabrica tion 1982, 5 (10), p 34. 8. Smith, G. A. The IPC Technical Review, May 1980, 227, pp 1115. 9. Jennings, C. W. in "How to Avoid Metallic Growth Problems on Electronic Hardware", IPC-TR-476, Institute of Printed Circuits, Evanston, Illinois, 1977 p 25. 10. Jennings, C. W. Proc. Printed Circuit World Convention II, 1981, 1, p 47. 11. Mitchell, J. P., Welsher, T. L. Proc. Printed Circuit World Convention II, 1981, 1, p 80. 12. Mathes, Κ. N. "Encyclopedia of Polymer Science and Techno logy", Interscience, New York, New York, 1966, 5, pp 531-533. RECEIVED October 12,

1983


UV Solder Masks as Insulators for Printed Circuit Boards

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