Hi Chris - unless we have revised the Periodic table and element
properties, you do not have a case of subliming or vaporizing or etc the
elements involved. However, you did provide a new clue - the horrible
smell! There is a high likelihood that you have decomposed or evaporated
some constituent of the organics on the unit. Bad smelling things are
often sulfur based and sulfur deposited on a printed wiring assembly is
not a good thing. Have you conducted an SEM EDX of the "bridging"
material?
Dave
From: Chris Mahanna <[log in to unmask]>
To: <[log in to unmask]>
Date: 12/19/2013 11:41 AM
Subject: Re: [TN] High Temperature Solder Migration/Creep
Sent by: TechNet <[log in to unmask]>
Hi Joyce
Yes, it looks very much like vapor deposition.
However, when I look at the vapor pressures of Sn, Ag, and Pb versus
temperature, it doesn't look likely.
I also googled Pb sublimation and the crowd seems to believe that pure Pb
doesn't sublime.
Help a simple EE understand what other factors may be at play. Reactive
chemistry? Physics I forgot...?
These were in very well sealed systems. There was a horrible smell when
we opened them that we thought was from the lacquer of a large coil
winding.
BTW - it gets hot when you drill to the center of the earth :)
Chris
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Joyce Koo
Sent: Wednesday, December 18, 2013 6:24 AM
To: [log in to unmask]
Subject: Re: [TN] High Temperature Solder Migration/Creep
With testing temp 200 to 250 and bias ý48 volt,you possiblly close to
melting temp and got some Pb vapor Re deposit. The whole test and
selection of alloy sounds iffy to me. Is it someone's science project try
to accelerate tests to "ultra highly" state? Or you really have
requirement operated in that temp range? (you do not need to answer my
questions. Really!).
Product that are built around function alone have not been designed at
all, but merely engineered. -prof. Ashby
Original Message
From: Mike Fenner
Sent: Wednesday, December 18, 2013 5:45 AM
To: [log in to unmask]
Reply To: [log in to unmask]
Subject: Re: [TN] High Temperature Solder Migration/Creep
I've never heard of migration associated with 10/88/2 or similar alloys,
but that's not to say it's never happened. That alloy starts to melt
around 270C giving soldering temps of 300 plus. At that sort of
temperature everything organic is pretty friable and likely to come
unstuck/delaminated/thermally degraded into ???. Including conventional
rosin/resin fluxes which will probably be caramelising. Certainly, if
you're doing it, post solder cleaning is difficult and likely to be
compromised. Therefore I think the cause is more likely to originate in
those areas rather than directly from the solder. An area to go on to your
investigate list anyway. It would be useful to know what the assembly
process and material types are.
Regards
Mike
BS&P
M: +44 [0] 7810 526 317
T: +44 [0] 1865 522 663
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Chris Mahanna
Sent: Tuesday, December 17, 2013 10:15 PM
To: [log in to unmask]
Subject: [TN] High Temperature Solder Migration/Creep
We have an FA project that appears to be migration of 10Sn/88Pb/2Ag solder
across a 0.024 inch antipad to ground plane on the surface of the
polyimide board.
It is not dendritic in appearance. Looks more like Ag creep corrosion.
Failure occurred under dry heat testing at our lab.
Bias is 48 volts
The test temperatures are 200-250C over 12 weeks Ran at atmosphere, no
added moisture The failure coincides with the loss of solder mask
adhesion, under which the metal can be seen...sort of, as mask is just
about charcoal now.
There is also some evidence of flux residue, which would likely be very
corrosive, but don't we need moisture?
Literature searches get swamped by Ag migration/creep. Anyone got an
idea?
Keywords? Tests for differential diagnosis.
Chris
Chris Mahanna
Robisan Lab
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