Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.

QFN Rework

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QFN Rework | 18 July, 2008

I have a QFN which we are adding after process. (it was not available the day we ran the 4 prototypes.) We have removed most of the solder on all the pads, My plan was that a LITTLE solder on the center pad would wet and center the device (using a gel flux) with our BGA hot air rework station. Then we would touch each pin with a tiny iron to make the pad connections. Well we had it nicely lined up on the gel paste, then reflow was good, but it shifted (nice and square) ONE SET OF PADS offset. Rev two of this attempt will be to put less gell flux on, and a tiny dab of paste the pads, hoping to have the PADS do the alignment. Any better suggestions out there? I was actually expecting my 1st approach to be just fine. Peter

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QFN Rework | 21 July, 2008

We use a ministencil with a QFN pattern that is very similar to the orginal stencil. Here's a link to one of the better white papers on QFN rework:

QFN with less than a 2 thou standoff are less reliable than those with higher standoff. Bumping to 4 thou shows a substantial improvement in reliability where die to package size ratio is greater than 70%.

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Metcal soldering rework