Just thinking this through and this is what I speculate/hypothesise:
So far as I know you can't plate an alloy, so if they have plated then they
must have co-plated the Sn Ag and Cu in the appropriate proportions for
them to melt/dissolve on soldering, That might do something to the soldering
process, possibly non homogenous/grainy joints, soldering process would be a
little slower and need more time for homogeneity, this would lead to a
reduction in tombstoning though. I doubt it would be that significant
though. IF you can plate an alloy, or functional alloy, the same applies, a
95.XX pure tin coat would not be that different to normal 99.X%
Most probably they have hot tinned them in SAC and used "plating" as a sort
of generic coating term, not a specific. In that case there is likely to be
more solder than on a plated part.
The alloy would still dissolve into the molten Sn/Pb (just as pure tin MP
232C does) as normal. Being hot dipped the solder joint is already made so
wetting would be OK, and joint formation correspondingly quicker. Add in:
The MP of SAC at 217C could be well below peak profile, so now we have a
raised Surface tension mix (from the added high Sn content of SAC) in a fast
joint formation: a recipe for the tombstoning observed by Geert.
Regards
Mike Fenner
Indium Corporation
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David Fischer
Sent: Monday, June 23, 2008 3:41 AM
To: [log in to unmask]
Subject: Re: [TN] SAC Plated Passives with SnPb Solder Paste
These are precision resistors definitely plated with SAC305 - ... it's in
the suppliers spec and in their part numbers, and we have talked with them
about it.
Good feedback so far, thanks everyone. If there is more feedback, I'd
appreciate it.
Regards,
David Fischer
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David Fischer
Sent: Friday, June 20, 2008 6:18 PM
To: [log in to unmask]
Subject: [TN] SAC Plated Passives with SnPb Solder Paste
Good Day,
I am aware of the nuances of mixing SAC solder ball BGA's with SnPb solder
paste, but are there any concerns, cautions or steps to take when soldering
SAC plated passives (chip resistors, chip caps) to a PCB using SnPb solder
paste?
Thanks,
David Fischer
Manufacturing Engineering Manager
ATEOps NPI
Teradyne, Inc
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