We are having a small number (~2%) of our boards pass our manufacturing diagnostics, fail once they reach the customer, fail when they get back to us and then start working again after handling them for a short time. We have eliminated a lot of items and the big suspicion right now is our BGAs (two 256-pin plastic packages).
We are getting ready to send some boards out to be cross- sectioned so that we can see what is going on under the BGAs. We are also thinking about just ripping a couple of BGAs off of a board. Is there anything we can tell from that? (e.g. on a good attachement should all of the balls rip PCB material with them, or will some have the solder joint fail?) Are there any other quick checks we can do to see if there are problems with our BGAs?