R
Raveninghorde
Having done a lot of reading, I think the industry has got away with
it, due to a number of factors:
1) The whiskers need an elevated temp to grow well, say +60C
2) The whiskers need a reduced pressure to grow well (aircraft and
satellite apps)
3) The whiskers won't grow on component leads unless *bright* (smooth)
tin plated, which most aren't (but some definitely ARE)
4) Ref 3) what really helps is if the surface is under compression
(concave curve e.g. the *inside* of a transistor case)
5) SAC solder doesn't seem to have a big problem with whiskers because
the surface finish is very dull
6) IC lead pitch miniaturisation stopped c. 0.65mm pitch (went to BGA
for high lead count packages)
7) Very few products are going to fail within say a 2 year warranty
8) IF SOMETHING FAILS IT GETS CHUCKED AWAY AND NOBODY LOOKS FOR
WHISKERS!
9) The vast majority of electronic mfg is consumer stuff which gets
chucked away after a few years at most
So there you are...
I *would* be concerned about elevated temps and TSOP packages, in
industrial products which are expected to work for years. Conformal
coatings can be used there, or just use leaded solder and nobody will
notice
The conformal coating I mentioned earlier is difficult to store (6
month shelf life and a crazy price) and extremely hazardous if warmed
up.
Does anybody make SMT solder paste with say 2-3% lead? I guess it must
exist because the military demand 2-3% lead.
We have used SAC solder since RoHS came in. All are stuff goes to
industry and military so any failures are chased down to the root
cause. We have not found a single failure due to whiskers. We never
conformal coat.
Possible reasons.
SAC solder.
We have only recently moved to ICs with a lead spacing of less than
0.8mm. Many new parts don't have a large body version so we now have
to use small pitch devices. I have been designing in 0.65mm pitch
parts for about 2 years and 0.5mm pitch for a few months.
Luck?