Comments on Fenner re Conductive Adhesives

1) Conductivity: adhesive conductivity is always a balance between 
mechanical vs. electrical properties. C-adhesives range from about an 
order of magnitude higher to lower vol. resistivity than solder. Also, 
the typical adhesive joint is about half the height as for solder which 
cuts the difference in half. Our large survey of adhesive evaluators & 
users indicated that only 2% needed higher conductivity; there are much 
more critical deficiencies.

2) Strength: component packages that form butt joints, like chip 
resistors, give about the same shear value as for solder, sometimes 
even higher. The "J" lead gives only about 50% strength at best, for reasons 
well understood and reported in detail. Bright dip solder finish, due to 
smoothness, may drop the adhes. strength to 10%. In our opinion, lower 
mechanical strength is the most serious deficiency for adhesives and the 
hardest problem to solve. Celestica (IBM Canada is studying the problem with 
a major program).

3) We can say, with strong documentation from independent sources, that 
the junction stability of silver-filled conductive adhesive (at least our 
favorite one), is excellent with off-the-shelf solder-coated SMDs. Many have
found that Sn/Pb is the ideal finish for conductive adhesives, even better 
than gold. How can this be? Adhesives that rely on particle-penetration for 
stable junctions, work best with soft metals like solder. Epoxies also bond 
better to metal oxides that pure metal. 

4) We can not agree that conductive adhesives, "do not have a high 
reliability" and offer the following. Poly-Flex Circuits (USA) has 
shipped 10's of millions of flex circuit assemblies using conductive 
adhesive bonded SOLDER-COATED SMDs. A major customer is AT&T who has used 
adhesives in their business telephones for about 13 years. The LEDs on 
many Hewlett-Packard printers are bonded with conductive adhesives. A 
California medical company has assembled millions of blood oxygen sensors 
with conductive adhesive. Since junction failure could be life 
threatening, the use of conductive adhesive here is a strong testimonial 
to reliability.

We do agree that conductive adhesives are competitive only where the 
provide an advantage over solder, such as low temperature assembly. 
Although we love adhesives, you may expect us to happily manufacture tons and 
tons of solder for years and years to come. 

Polymer Valley/Solder City