One culprit could be package warping.

Figure 1 shows the Amkor TMV package after microsectioning the board assembly during failure analysis. This particular view shows open joints between the top package and the solder spheres on the bottom package.

This part was soldered using a dip paste. There is no evidence of the dip paste on or around the solder spheres, however, so we would assume that the package reached reflow temperature, and the paste alloy combined with the terminations.

Through measurement of the spheres and the comparison with the original ball size, it should be possible to establish where the paste went. As there is no significant difference in the ball sizes, the opens are probably due to warping, provided the other end or center of the part is soldered.

Figure 1. A TMV part with solder opens, likely caused by warping.

Dr. Chris Hunt is with the National Physical Laboratory Industry and Innovation division (npl.co.uk); chris.hunt@npl.co.uk. His column appears monthly.