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June 2004

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From:
"Wenger, George M." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 16 Jun 2004 10:34:45 -0400
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Rich,

The first thing I would do is to measure the thickness of the immersion silver.  From what you describe it sounds like the silver thinkness is below 2 microinches (my guess would be probably 0.5 to 1.0 microinches). We use immersion silver not as a solderable surface finish but as an indication that the copper below is clean and solderable.  If the features change from a yellow copper color to a silver color during the imemrsion plating process that is usually an indication that the board fabricator didn't screw up their tin or solder strip operation or their sodler mask operation.  Our current spec for immersion silver boards is 2 to 20 microinches.  We picked the value of 2 because below this value you can see the yellow copper color coming though the thin silver.  The picked the 20 microinch maximum becuse we tested silver at 45 microinches thickness and it passes electromigraion and we wanted to have at least a 50% saftey margin below what we tested.  Now having said that we sometimes get immersion silver boards from fabricators that are outside of that range.  Not far out.  For example, I have boards right now that have about 1 microinch and they solder fine and I have boards that are about 25 microinches and they solder fine also.  We use these boards because there is nothing wrong with them but we do issue SCARs to our fabricators and ask them to improve their process controlls to give us boards between 2 and 20.

Like I indicated the nice thing about immersion silver is we don't solder to the silver we solder to the copper and only use the silver as an indicator that the copper is solderable.  

Regards,
George

George M. Wenger (908)-546-4531 
Reliability / FMA Engineer
Base Station and Subsystems Group
Andrew Corporation,  40 Technology Drive, Warren, NJ 07059
[log in to unmask]



-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Richard Kraszewski
Sent: Wednesday, June 16, 2004 10:13 AM
To: [log in to unmask]
Subject: [TN] Yellow Discoloration of Immersion Silver Boards


We are seeing yellow discoloration of Immersion Silver Boards right out
of the package from our board house.  They seem to solder fine, however
this is causing some concern with our customer.  I suspect that it could
be a thin film of  silver bromide or silver carbonate both which are
yellow powders.
 
Any one seen this before?
 
What is it?
 
How does the board house remedy this situation?
 
Any associated concerns or other bits of wisdom?
 
I would truly appreciate any input.
 

Rich Kraszewski 

Advanced Manufacturing Engineer - KEDS

Phone:  260.925.8919

 

 

 

 

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