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August 2004

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Subject:
From:
Brian Ellis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Brian Ellis <[log in to unmask]>
Date:
Tue, 3 Aug 2004 16:35:49 +0300
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text/plain
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text/plain (84 lines)
Two thoughts spring to mind:

1. If, as you say, the parts are not adequately cleaned, then it is
possible for metallic salts from any previous process to be dissolved
into the uncured coating.

2. Many silicone-based CCs rely on condensation polymerisation with
crosslinking ensured by an organic acid. If the metallic parts are
oxidised, it is not inconceivable that these acids may react with the
oxides.

Brian

Rich Lasko 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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