When joints look perfect but nonetheless are failing, check the plating adhesion.
Exhausted flux can cause incomplete reflow.
Figures 1 and 2 show two images of the same problem: one x-ray, the other optical. In both cases it’s the result of sulphur corrosion on the surface of copper and silver surfaces.
Both surfaces remain exposed after a soldering operation, and in the presence of sulphur, it is possible to witness corrosion. In time it can result in an open connection, as the figures show. Figure 1, taken on a Dage x-ray system, shows different degrees of corrosion on the tracking just below the solder joint fillet. In failure investigations it’s important to see and record as much as possible without change or damage to the sample. On one joint there was an open connection on this QFP device.
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.