The image below shows a row of solder joints on a large BGA near the center of the row.

The corner balls on the BGA were elongated into an “hour glass” shape due to the warpage of the package (e.g. image below). 

The chart below shows the solder joint height versus the distance from the corner ball for three different assemblies (Ax, BX, & CX).

The amount of warpage in BGA assembly is driven by a number a factors including the BGA design, the board design, and the reflow process parameters (e.g. top versus bottom-side temperature and cool down rate).  In the present case, it appeared that the BGA design (i.e. CTE constrained by large die) was the primary factor.

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