We are running an assembly with a 504 pin BGA and have had rejects for opens. 
There will be one open per board, and the 8 board rejects have had the open in 
the same location of the BGA. 

The boards are 6L and foil construction. Warpage for bare boards is about .6%.

The cross sections show that the pad and prepreg(not core) have been torn up 
from the PCB and moved .005" in the X(or Y) direction. If it were more than one 
pad I would think there were component mismatches, but the other pads look OK. 
The copper pads are secure to the torn prepreg, but it seems that the prepreg is
fractured around the fiber bundles. The fractures appear on the 1st layer of 
glass below the base foil.

The solder joints look smooth and round. There is no evidence of excess solder 
or any problems on adjacent pads.

The PCB's are supplied with HASL finish and the BGA's are solder bumps. This is 
a Type I board with active components on both sides. The PLGA material is BT.

Sorry for the poor explanation, but does anyone have any theories on this?

Steve Joy
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