I have seen similar on some pads that have fractured, I think it is an intermetallic, I know that cleaning and refluxing does not allow wetting, however I have found that a light burnishing with abrasive will break up whatever barrier material is on the surface, and allow solder to reflow, I find it is on both component pad or lead and land. I too am wondering if there could be some form of oxidization that could have taken place within the boundary layers. I do know that it can be very difficult to get a good CLEAN surface to reflow.
Recent interaction had a test instrument that had been dropped and a component fractured off of the board, I had to abrasively clean the board and lead to get it to reflow. I find it occurs sometimes when I am taking off a component and don't quite heat the lead enough and end up fracturing the joint during the plastic state of the solder.
Pat
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Wenger, George M. [Contractor]
Sent: Tuesday, April 23, 2013 1:27 PM
To: [log in to unmask]
Subject: Re: [TN] QFN Pad Corrosion?
Hi Steve,
It is difficult to tell from the photograph. It does look like the pad is missing. However, you could find out very quickly what if the pad is missing or if what you are looking at is a nickel interface or an oxidized copper-tin intermetallic by examining the pad with XRF which is quick\, easy, and non-destructive.
Regards,
George
George M. Wenger
Failure Signature & Characterization Lab LLC
609 Cokesbury Road, High Bridge, NJ 08829
(908) 638-8771 Home (732) 309-8964 Mobile
E-mail: [log in to unmask]
[log in to unmask]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
Sent: Tuesday, April 23, 2013 1:12 PM
To: [log in to unmask]
Subject: [TN] QFN Pad Corrosion?
Hi All,
I have another question about the QFN16 that I was talking about last week. I was sent a photo of the bottom of one of the devices after it had been removed from a failed board. It is here:
http://stevezeva.homestead.com/QFN16_Corrosion.jpg
It's pointing out a pad that seemed to have a lot of corrosion on it and I was told that solder would not wet to it. I was asked what might cause this?
My first thoughts are that the appearance might have been caused by the rework when it was removed, and that a un-solderable intermetallic has formed on pad but I can't say that for sure. The part is finished with matte tin when it is supplied. When a part is tin finished, they have a barrier nickel plate beneath the tin right?
I don't have the part in front of me, I only got sent the photo. I was asked what my thoughts were, so I thought I'd ask my TechNet friends too... :o)
Thanks,
Steve Gregory
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