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Non wet on QFP package during reflow

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I have a customer complaining of random leads on a QFP package not reflowing during their reflow process. Pictures shows that there was sufficient solder paste on the PCB pad and there was no coplanarity issues. They reported a high fallout (as high as 30% on certain batches of material). Among actions taken so far are changing the solder paste which have reduced the fallout to 0.5% However there have been spikes off and on and customer are demanding for 0% fallout.

I am currently trying to simulate the problem using Ceramic Reflow Test Method, stated in the J-STD-002B. The spec. states that parts needs to be preconditioned (steam aged ) for 8 hrs on tin lead finishing products. However all my parts have failed this test. I am currently using ROL0 class type paste but the spec recommends using a ROL1 class paste. I will be testing this soon.

My questions are :

1. Is anyone familiar with ceramic plate test method?
2. Can anyone give some input on the problem on the problem that the customer is seeing (total non wet on random leads) I can provide photos of the failure mode if someone can shed some experience on this problem

Any help right now would be great.

Thanks,

Vickneswaran
Semiconductor manufacturer - Petaling Jaya, Selangor, Malaysia

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