Failure analysis techniques for Cu-Ni-Sn solder joint separations on mounted PBGA devices

There have been several papers published on PBGA Cu-Ni-Sn joint separations in which the phenomena has yet to be fully understood. Failure analysis on an open solder joint indicating the separation occurred at the intermetallic layer is not sufficient to warrant a corrective action or a fix as the root cause is not known. The Cu-Ni-Sn joint separations reported normally occur at the inner ring solder balls or solderballs near the center of the PBGA. The application of finite elemental analysis on modelling the stresses seen by a PCB mounted device, was not able to correlate the position of solder balls with high stresses to the inner ring of solder balls. Experiments also indicated that joint separation may not be only on dependent on high concentration of ternary intermetallic formation at the solder pad interface, as suspected by various parties.