We started our first ENIG process recently and to our surprise, we have many failure on the one and only BGA on the board. The BGA is a 31 mils pitch plastic. Sending the PCBA to 1 cycle of heating/cooling @ 70/-5 degree C yield 40% of failure. With such high reject, those that pass testing could be faulty and those that fail test may pass again. We ply the joint apart. Physically about 90+% of joint break-off at the pad. About 30% of the pad is flat (sign of no wetting) or partially soldered. One failed part was send for XPS and 2 joints were analyzed. One of the joint has 2 elements that we think is abnormal: Zn and Si. Also there is no sign of Pb!!! We suspect Si could come from the BGA packaging. Anyone can help to shed some light and explain what we are seeing? Possible solution/recommendation is appreciated. Thanks you. (Note the same BGA package is used on other product on a HAL PCB w/o any failure. So we think part of the problem is in the ENIG PCB. The plating thickness is spec at 4 micron)
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