I was building some boards that were similar to bityExpress last week. The boards had SMT components on both sides and the back-side build went perfectly. When I ran the top sides (which included a similar FPGA), everything looked good until I inspected the cards closely. The solder on all of the SMT components flowed well and everything centered up nicely except the FPGA BGA. At first I assumed that I had placed the parts badly, but when I spoke with my friendly neighborhood failure analysis expert he zeroed in on a different cause: a too-rapid cooling profile.
His theory is that if the BGA and PCB are allowed to cool at different rates, the stresses applied to the cooling solder can cause the part to twist before the solder solidifies. Since my current profile involves opening the toaster oven door 90 seconds after all of the solder joints have flowed, I think that his theory is plausible. I should have my bityExpress-C boards in this weekend so we'll see if I have better luck with the BGA soldering with a slower cooling profile.
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