BGA ball crack at pad/solder ball interface

Dear all,

Recently, we run 2 models with heat sink attachment on top of this BGA(on top of this BGA, there is flip chip attached at the center of the BGA). Hence, the center of the BGA is higher than corner/side.

We used conventional way to attach heat sink to the BGA. Heat sink anchor pullout is a retention spring that press across the BGA package top surface and locks on both sides at the 2 hooks soldered to the board next to the edge of the BGA package. We found crack at the corner and outside rows of BGA ball. The crack appear at the pad/solder ball interface, but, majority are still at solder ball interface.

FYI, this crack problem only happen on these 2 models that using larger heat sink dimension than the BGA dimension. Also, only these 2 models that used this larger heat sink. Other no problem models also used almost the same BGA type but with heat sink dimension almost similar to BGA dimension(Same size).

Question: Is the heat sink dimension bigger than BGA dimension could lead to this uneven forces applied to the BGA ball during heat sink installation process and eventually cause BGA ball crack?? In others words, does heat sink size matter in this case?

Need help..Thanks in advance.

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