Re: American PCB fabs defaulting to lead-free


"Joerg" <notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote in message
news:3il0i.1382$LR5.441@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Jeff L wrote:

"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!


Thanks for sharing your experience, Jeff. Maybe HASL would then be an
option for us, as long as we wet everything. The latter could be a
problem in thru-hole areas such as DIN connectors.

Proper through hole has solder wetting to the top. Problem areas could be
untented vias, mounting pads and holes, etc. Basically if its not a pad,
don't leave it exposed to become plated with solder in the HASL process.

OSP (bare copper with an organic protector) might be another option and it's
cheap, but comes with it's own problems.


ENIG looked a bit scary to me from the beginning so I politely declined
when they offered. The nickel-copper is generally good though for
mechanical stuff, I have used that a lot for shield mounts and the like
in order to avoid dissimilar metals touching (nickel plating the shield
studs a well).

Thermocouples would spell disaster for this circuit as it relies on
microvolt level DC paths to be stable.

--
Regards, Joerg

http://www.analogconsultants.com


.

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