you’ve ever wondered why people tend to avoid BGA, it isn’t only the cost associated with multi-layer PCBs, it’s also because it’s impossible to inspect them after they have been soldered to way you would a QFP chip for solder bridges, etc. The solution in any professional production line is simple … they all get scanned with a specialised XRAY machine. The image above is from a board I had reworked, and wanted to make sure everything was OK underneath since I only had one sample. I thought the results were worth sharing. The solid black boxes underneath are capacitors on the bottom side of the board directly beneath the BGA chip.
How do you Test a BGA? - [Link]