In the previous two decades, black-pad-syndrome was a leading cause of intermittent BGA electrical failure. In 2018, this seems to have been replaced by head-in-pillow defect. Both of these conditions generally require microsection analysis for a conclusive diagnosis as non-destructive techniques mostly fail to identify a problem. Here are some examples of head-in-pillow defect analyzed at SEM Lab, Inc.
Head-in-pillow defect at ball-A3
Head-in-pillow defect at ball-B5
This condition is primarily caused by thermal-mechanical warpage of the BGA during the solder reflow process. Some corrective action recommendations can be found in .
 SMT Troubleshooting Guide, “BGA Head-on-Pillow”, www.alpha.cooksonelectronics.com, Issued 10/08, p. 17.