Hi Steve-
I see this often.
Solder has some very weird characteristics sometimes, especially during the rework process, and especially when it is reworked with a large thermal gradient.
The first place I analyzed this was with some resistors that came off of some very thin flex. Gray intermetallic residue left over, and it wouldn't wet. This happened to be on the flex pad, not the resistor, but you can easily have it the other way 'round. It took me quite a while to replicate what the mechanics were of how to create this stuff, but I finally got it: I attached a resistor to the pads, then I put bending stress on the flex in a way where I was trying to peel the flex from the resistor. Then I took a very hot soldering iron and put the tip directly in contact with the flex underneath one of the resistor pads (the iron on the opposite side of the thin flex -2mil flex- from the resistor). When the solder melted, the flex peeled off and this non-wetting gray intermetallic resulted.
Since then I've seen this a handful of times, and always associated with a combination of heat and physical stress. I've never tried to see exactly what intermetallic is going on--probably copper and tin.
Wayne Thayer
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert Kondner
Sent: Tuesday, April 23, 2013 1:57 PM
To: [log in to unmask]
Subject: Re: [TN] QFN Pad Corrosion?
Hi,
Probably real simple: It was assembled using a Water Soluble flux.
You really cannot clean under a QFN so a no-clean would be called for.
Bob K.
-----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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