I going to make a wild guess....
I suspect your "discoloration" may be incomplete etching.   This can occur when you get immersion Tin on Copper, during photoresist stripping.

There are specific additives which SHOULD be in photoresist strippers to prevent this, but many vendors are unaware of this.

Rudy Sedlak
RD Chemical Company



--- On Fri, 8/6/10, Steve Gregory <[log in to unmask]> wrote:

From: Steve Gregory <[log in to unmask]>
Subject: [TN] RE: Discoloration under the soldermask
To: [log in to unmask]
Date: Friday, August 6, 2010, 7:09 AM

Hi Amol!

I have your pictures posted:

http://stevezeva.homestead.com/IMG_8358.JPG

http://stevezeva.homestead.com/IMG_8359.JPG

http://stevezeva.homestead.com/IMG_8360.JPG

Steve

-----Original Message-----
From: Amol Kane [mailto:[log in to unmask]] 
Sent: Friday, August 06, 2010 9:35 AM
To: Steve Gregory; TechNet E-Mail Forum
Subject: Discoloration under the soldermask

Steve, would you be so kind to put them up on your site?....the
discoloration is underneath the soldermask. do any of you guys/gals know
what may cause this to happen?. this is across multiple date codes. I am
looking for a possible root cause so that I can decide whether or not to
reject these boards and issue a corrective action to the vendor.

Thanks!

Amol


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