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September 2013

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Subject:
From:
Gerry Gagnon <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Gerry Gagnon <[log in to unmask]>
Date:
Fri, 27 Sep 2013 08:31:08 -0400
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Glenn,
 
I agree with your assessment in light of the data we have received.
 
For more on item 2, yes, the first thing folks expect is thin silver, which does occur from time to time and the culprit is copper oxide as you state.
 
However, I have seen wild discolorations in areas with silver thickness well within spec, but these were circumstances where the boards received very high amounts of heat in localized areas, particulary along the rails of panelized boards where the rails were silver. When determining reflow parameters, most folks, myself included, do not tend to thermocouple the rails, so they may not know how hot it gets there. I recommend soldermasking the rails.
 
Brs,
 
Gerry Gagnon

 
> Date: Fri, 27 Sep 2013 01:11:56 -0500
> From: [log in to unmask]
> Subject: Re: [TN] Immersion Silver Board Finish Tarnishing
> To: [log in to unmask]
> 
> I see two possibilities:
> (1) the test probes are coated and not making reliable contact.
> (2) If a shading other than silver is observed, it may be that the silver was too thin and the copper migrated to the surface and was oxidized by time or by the processing temperature, if a process was attempted.  I will suspect copper oxide and I would examine some unprocessed bare boards under a microscope looking for signs of visible copper. In three days, even at pre-baking temperatures, you should not have copper oxides on the surface. 
> 
> The silver is a very thin surface protectant (approx. 0.10um) and migrates into the solder when processed. You solder to the copper.
> Glen
 		 	   		  

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