Hi Rich,
This issue would be similar to some program studies that were accomplished during my stay at China Lake.  
In brief summary, it was the residual surface contamination from flux/plating chemistries that reacted to the copper base material, differently that the plated and subsequently soldered leads. 
The conformal coating had little to do with the issue other than to localize the condition on the component seal surface where the metalization occured between the base material (copper) sources and along the lead frame seal. This is not limited to components but also other areas of the assembly that exhibit  equivalent contamination levels and exposed copper. 
I would recommend looking at the contamination levels under the coating. As I recall these were organic in my case. 


Mel Parrish
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Rich Lasko
Sent: Tuesday, August 03, 2004 8:08 AM
To: [log in to unmask]
Subject: [TN] Chemical Reaction


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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