Reg: Voids in solder bumps

Hi Everybody Is anyone aware of the possible causes that may lead to voids in the solder bumps after assembly. We assembled some dice on thin substrates and after assembly, void formation in the solder bumps were observed. We have already tried baking the laminate and subjecting it to reflow prior to assembly, but that does not seem to help. Does anyone has any insights on impurities in gold plating on the substrate affecting void formation. Any suggestions would be highly appreciated. Regards Manish

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