BGA coming loose?
Have you had BGAs with some or all of the solder balls connecting poorly or not connecting at all? If you use an ENIG (Electroless Nickel Immersion Gold) board finish, this may be due to the infamous "black pad" syndrome.
This is more common on larger BGAs due to the greater impact of thermal expansion or board flexing on the larger parts. In troubleshooting a board failure or reliability problem, you may have found that a few of the solder balls on the edges or corners of the BGA were not mechanically connected to the PCB.
On an ENIG finished board, the gold is really more of a surface preservative. When properly soldered, the gold will dissolve and the solder will adhere directly to the nickel layer. Sometimes, however, the nickel layer has small amounts of contaminates that prevent good adhesion. Take a look at this article from empf.org (A publication of the National Electronics Manufacturing Center of Excellence) for more detail on the problem.
Other things, such as an incorrect oven profile, poor solder mask registration or mixing lead and lead-free can cause BGA problems but if your BGA is having problems as described above and you are using an ENIG surface, talk to your board fab house. It is likely a process issue in their shop.
Black pads? We don't need no stinking black pads