Or you could scrape or otherwise abrade the pad and see if it takes solder. If it does, the pad is still there. However, you will have removed the evidence which could have given you more of a clue as to how this happened metallurgically.
During the test with the flex circuits I referred to previously, the gray, non-wettable intermetallic was made instantaneously--if it was some kind of oxide, it wasn't a slow growth process! I'd LIKE to know the chemistry of how that happens, but I went deep enough to convince myself that it wasn't indicative of any problem other than the rework technique used.
Naturally, this type of stuff gives us the "willies" because of concern about the reliability of adjacent joints, and the suspicion of some kind of oxide or intermetallic disease which renders the remainder of the boards unusable.
To feed this gnawing worm, I find it is better to just start tearing apart and sectioning adjacent joints, as well as relying on thermal cycling, biased temperature/humidity testing, etc. to gauge reliability.
Wayne
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Wenger, George M. [Contractor]
Sent: Tuesday, April 23, 2013 4:27 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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