Hi Bill - Just to give you a short background, Rockwell Collins utilizes
all of the immersion finishes -  immersion tin, immersion silver and ENIG -
in our avionics designs depending on the use environment conditions. So we
have had the "opportunity" to see the good, the bad, and the ugly for each
of the finishes. Immersion silver has traditionally had two disadvantages:
a) silver is a very reactive element so many folks have corrosion concerns;
b) silver is a very reactive element so surface changes are very apparent.
The IPC committees have done some extensive work demonstrating that a
correctly applied immersion silver surface finish is equal to or better
than tin/lead in many (but not all) use environments. There are some use
environments that will cause problems - direct/constant exposure to a
marine environment would be one example - but overall ImAg is pretty
robust. Silver oxides is as conductive as elemental silver (which is a very
cool characteristic in the metal's world) and are very solderable. Silver
chloride and silver sulfide are the two surface species that can cause
issues. My packaging issue comment relates to those two species - if you
don't package ImAg pwbs using some type of "anti-tarnishing" paper or don't
strictly control your pwb storage conditions then you risk the issue of
having the tarnish issues. A little tarnish on the surface isn't a killer -
most flux systems deal with it quite well and good solder joints are
possible. But if you get a heavy black tarnish then the pwb solderability
is an issue.  Don Cullen and the MacDermid folks have published a couple of
studies demonstrating the impact of ImAg tarnish on solderability. And
there is an IPC committee currently working on a PWB Handling/Storage
guidelines document to aid the industry in this area. Hope this helps- good
luck on your investigation.

Dave Hillman
Rockwell Collins
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To Dave:  What specific packaging issues are there for Immersion  Silver???
My company may be heading in this direction!!!!!

Thank in advance for your inputs!!!!!

Bill Black
Q.A.Mgr.
Medco

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