>Please understand, I have trouble understanding most stuff. However,
what happened to silver
>migration and all the attendant issues? I mean
it's so widely published, no one comes near
>the stuff without some type
polish.
In the "thick-film"
world, we use silver for circuit traces- typically, but ironically because there
is more potential for a problem, with lower cost commercial circuits- bond pads
on such circuits are gold. For military, in hermetically sealed packages,
we use all gold. I learned this morning, from a TechNetter, that most
semiconductors are packaged onto lead frames which have been plated with silver
for wirebonding (don't know if this is gold or aluminum- I suspect aluminum
because it's cheaper). As far as I know, most ceramic chip capacitors use
silver thickfilm for both plates and end terminations- which are often but not
always covered with nickel/solder.
So I think silver is probably ok if
it is covered in some way to prevent exposure to moisture. I have started
looking in MIL-SPEC's to see if they prohibit it for use as a wire bond pad, but
so far I haven't found anything.
I'm not sure what the effect of
tarnish would be on wire bonding (it's probably easily removed?). I have a call
into Alpha Metals- they evidently have an immersion silver bath for PCB's which
is gold ball bondable- I"ll find out!
Resp.
Phil Hersey, Carson City
Nevada USA