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From:
Wayne Thayer <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Wayne Thayer <[log in to unmask]>
Date:
Thu, 19 Dec 2013 18:52:54 +0000
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I believe Chris also said this was continuous for 12 weeks!

A lot can happen in that time, and he's already indicated the solder mask has broken down.  When you heat polymers up that high for an extended period, you may get some interesting chemistry emanating from them.  Maybe some kind of liquid acid condensed on the solder, dissolved some of the solder, dripped onto the board, and finally dried up or instantly evaporated when the sealed container was opened.

I know Chris is smart, but he is being cagey about what exact materials are being used here.  I'm sure he has great reasons for why he's doing this.

Wayne

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bob Landman
Sent: Thursday, December 19, 2013 1:28 PM
To: [log in to unmask]
Subject: Re: [TN] High Temperature Solder Migration/Creep

Chris,

Testing at 200-250C - really? 

Pb doesn't sublime and it's vapor pressure is very low until you get to 3164F

http://www.lbl.gov/ehs/ih/pdf/safeSolderingFinal.pdf

Based on standard soldering iron temperatures of 620°F-700°F and the melting point of lead (621°F), with a vapor pressure of 0.0 mm Hg and a boiling point of 3,164°F, it is unlikely that lead fume will be generated during electronic soldering, unless the solder is heated to extreme temperatures 

Lead-free solder may contain tin, copper, silver, and sometimes bismuth, indium, zinc, antimony, and other metals in varying amounts. The lead-free replacements for conventional Sn60/Pb40 solder have higher melting points, requiring re-engineering of most components and materials used in electronic assemblies.

I would suspect a constituent of the soldermask as I can't imagine it's designed to tolerate 200-250C for 12 weeks. Of course it can withstand the soldering temp cycles which are considerably shorter in duration.

Bob Landman
H&L Instruments

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Chris Mahanna
Sent: Thursday, December 19, 2013 12:40 PM
To: [log in to unmask]
Subject: Re: [TN] High Temperature Solder Migration/Creep

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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