Question: We have been stumped by occasional opens at the corner balls of some BGA components after reflow. We have experimented with many minor adjustments, yet this occasional problem continues. Do you have any suggestions?
Answer: Based upon the warp and twist characteristics of the printed board laminate materials and the BGA, this could be a major starting point for investigating this condition. It has been seen, over the years, that BGA components do warp and many times creates shorts in the center of the device itself. Based upon the size of the component, when this happens, the edges of the components are rising up off the surface of the PCB. The same condition can occur if the PCB itself warps and twist during the soldering process.
I would recommend reviewing the thermal profile and the warp and twist characteristics of both the board and the components. Secondly, as a minor issue, take a look at the paste deposition process to make sure the paste is sufficient on the outer pads of the component.