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April 2013

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From:
Robert Kondner <[log in to unmask]>
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Date:
Tue, 23 Apr 2013 13:56:57 -0400
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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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