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QFN - FCT failed after reflow

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Wayne

#47256

QFN - FCT failed after reflow | 6 February, 2007

The QFN is found to have contact problem during FCT. The paste that I am currently using is SnPb 63/37 water soluble paste. Is it because of water soluble paste? (unable to clean after reflow process as QFN is entrapped by paste/flux) Should we use no clean solder paste on this?

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JAX

#47275

QFN - FCT failed after reflow | 7 February, 2007

Water soluble residues probably won't cause instant failures. The failures are most likely caused by excess paste being deposited on the center pad of the QFN. The excess solder keeps the part from sitting down in reflow and increases the possibility for poor joint formation on the rest of the leads.

You might want to verify that you have adequate cleaning. Although I don't think it caused this problem, Leftover residue can cause near and long term failures.

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#47286

QFN - FCT failed after reflow | 7 February, 2007

Had the very same thing happen the first time we used the small QFNs. The aperture for the thermal pad in the center needs to be cut in a grid form, either squares (with around a 10 mil spacing between openings) or circles, like a BGA pattern. Too much, and when that solder resolidifies, it raises the QFN up high enough to prevent contact and joint creation on the perimeter.

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