I have a feeling the bad hashing engines are related to manufacturing defects in the actual ASIC chips themselves, not in the mounting to the PCB. I don't know much about ICs and reflow though so that's sort of a guess.
But it's certainly an interesting idea... maybe we'll see people trying it once more miners achieve positive ROI.
Ya I'm gonna agree with you here. I'm no EE, nor do I have much experience in reflow repair, but it was my impression that reflow was only necessary if A) The original chip placement was done incorrectly, or B) time and wear/tear caused cracks in the solder, and needed to be repaired.
Neither one of those would account for a non-working or partially-working engine in a BFL chip.