Hi Gregg! There have been only 2 of industry reports/papers which blamed
lead carbonate as the cause of solderability loss that I am aware of (if I
can find them in my files I'll let you know) so the appearance of lead
carbonate on pwbs seems to be the exception and not the rule. I can add one
additional data point - we have done several thousand SERA measurements
over the last 7 years on tin/lead finished pwbs which had seen a wide
variety of conditioning treatments (steam, humidity, etc) and have only
detected the occurrence of lead oxide/carbonate/etc species twice. Both
instances were traced to specific pwb fabrication plating issues. Is there
lead somewhere on the ENIG board? How clean was the humidity chamber?

Dave Hillman
Rockwell Collins
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Gregg Klawson <[log in to unmask]>@IPC.ORG> on 01/31/2001 08:32:52 AM

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Subject:  [TN] Lead carbonate growth during humdity testing


We have just completed a 10-day cycled system-level humidity test of
several board assemblies.  The boards are mixed TH/SM predominately SM,
ENIG finished.  Bias was applied for about 45 minutes once every two days
during the test.

At the conclusion of the test microscopic examination (7x-40x-200x) of all
the assemblies revealed shiny metallic appearing flakes around the surface
mount pads, especially around and between the leads on some PQFP
devices.  One board had a significant amount of this flake-like material
piled up between some of the PQFP pads/solder joints.  SEM-EDS analysis of
the flakes showed strong Pb, C and O - lead carbonate.  The analyst stated
that this looks like lead carbonate crystal growth.  The flakes readily
dissolve in acetic acid.

I have trouble believing that this crystalline lead carbonate growth over
ten days is "typical" (carbonate formation from atmospheric CO2), we have
not seen type of corrosion before.  We had a cleaning issue with these
boards, so many of them had to be run through our in-line cleaner up to 10
times.  Armakleen 2000 (carbonate based) is used in our in-line
cleaner.  We checked the cleaned boards using an Ohmegameter for any
evidence of cleaner residues and found none.  So I'm at a loss to explain
why we had some growth on all boards and significant growth on one
board.  One engineer suggested we "over cleaned" the boards and we laughed
at the time but now I'm not so sure.

The other concern is whether lead carbonate formation over time will cause
a reliability problem.   I've been told of failures due to this under
extreme environments.  It appears that the carbonate itself is not
conductive and will not cause an electrical short but I'm guessing some of
the reaction products with moisture may produce some conductivity.

Thanks again for your comments.

Best regards,
Gregg Klawson
General Dynamics Communication Systems
Taunton, Massachusetts, USA
mailto:[log in to unmask]

The views expressed are the author's and do not necessarily reflect the
official position of General Dynamics or any of its subsidiaries.

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