Re: American PCB fabs defaulting to lead-free


"Joerg" <notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote in message
news:aoP%h.1857$zj3.633@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Almost got a black eye here on a prototype. Ordered a bunch of large
boards at Advanced Circuits and it turns out that the free upgrade to
lead-free finish is actually a default that cannot easily be changed.
IOW you can normally not decline this "upgrade". However, they were very
understanding that we really didn't want lead-free and will now do our
protos as production runs. No idea why we go RoHS here in the US now
(this is a company that actually produces in the US).

Anyhow, just wanted to let you guys know about that before a nasty
surprise happens to you.


No surprises - assuming HASL, they work fine using normal leaded solder.
This includes soldering with irons, wave and reflow methods. It does not
change the process hardly at all. The leaded solder just dissolves it,
creating a slightly non eutectic solder (closer to 60/40). It's the same
with tin plated parts which have been used for quite a few years, other then
lead free BGA's. The downside is you have a possibility of tin whiskers
forming in areas that have not been wetted with the leaded solder. The tin
whisker risk is somewhat dependant on the alloy used for the HASL. Tin
dendrites are from currents forming in moisture and can be avoided. White
tin is really bad, as it compresses the surface during plating and causes
rapid tin whisker growth. We've done many thousands of boards with lead free
HASL without much issue.

Gold plating (ENIG) is bad, as it is a flash of electroless gold deposits a
few atoms thick, over nickel which is plated over the copper traces. The
gold is sometimes hard to wet (BGA's for one - they are so bad we pre tin
the pads). The intention of the process is when applying solder it dissolves
the gold, thus wetting the normally difficult to wet nickel. You now have a
solder joint with a little gold in it, which if in high enough concentration
causes the solder to go brittle. The nickel to solder intermetallic layer is
poor and is prone to cracking (a well known repair problem in the repair
industry). I think the copper to nickel bond is ok, but I never really
looked into it in much detail. Other problems that can arise is weird
thermocouples from all of the dissimilar junctions. This would have been a
potential problem with some research I did awhile back where were measuring
temperature from a thermocouple with a resolution of around 1/1000 of a deg
C. The thermocouple was so sensitive you could peg the graph by breathing
heavy several feet away!

--
Regards, Joerg

http://www.analogconsultants.com


.

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