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August 2004

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Subject:
From:
R Sedlak <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, R Sedlak <[log in to unmask]>
Date:
Tue, 3 Aug 2004 07:11:58 -0700
Content-Type:
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Rich:  I agree with everything said so far, and further remind you that having Copper and Tin exposed and connected promotes corrosion dramatically because of the Galvanic effect.  Covering the lead completely with Tin should minimize or eliminate this issue.

Rudy Sedlak
RD Chemical Company

Rich Lasko <[log in to unmask]> wrote:
Good morning all,



I have a customer who is having problems with a couple of their
assemblies. It appears that the conformal coating (Dow Corning 1-2577)
is reacting with the untinned copper portions of an IC (MC68HC908AZ60A).
The untinned portion is on the sides of the lead right near where the
lead enters the package body. It seems that the copper is leaching out
into the conformal coating and lowers the resistance between the two
pins. (When the two pins are measured they read approx. 80K ohms - 150K
ohms). Without the conformal coating the resistance is much higher (>
1M). This lower resistance is causing some erratic failures from time
to time. When my customer sliced through the conformal coating between
the pins with an exacto knife the resistance returned to a more normal
reading and the unit worked fine. They have sent this unit to a lab and
the lab report stated that they found traces of copper in the conformal
coating. So my questions are as follows:



1.) Is it common to have untinned portions of a SMT IC such as the
one identified above? Or should the lead be completing tinned?

2.) Has anyone else seen this type of problem with copper in the
conformal coating? If so, what did you do about it?

3.) Is there a concern from a solder paste position and how the flux
in the paste reacts with the copper and then is ultimately conformal
coated in a later process? (This question applies to both an aqueous
SMT process and a no-clean SMT process)



Need a little help and insight here.



Thanks!



Rich Lasko




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