David
Maybe I didn't understand your issue right:
- The only test you did was CM-x-ray right? If yes, I believe that microcracking can not be excluded as a failure mode. Only a microsection can give a secure answer.
- If it isn't microcracking in the balls look out for delamination. Kind of a pop corn effect just less violent and not noticed during production. If the mould delaminates from the substrate the bonds are lifted from the pads giving the press effect you mentioned. To be seen with ultrasound inspection or microsections.

Best regards

Guenter

EMPA
Swiss Federal Institute for Materials Testing and Research
Centre for Reliability
Guenter Grossmann,  Senior Engineer

8600 Duebendorf
Switzerland

Phone: xx41 1 823 4279
Fax :     xx41 1823 4054
mail:     [log in to unmask]

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