This is a somewhat classic case of solder joint fatigue failure, but with the added factor of gold embrittlement. The solder alloy is SN63, the package is a J-lead PMIC, and it is soldered to an alumina ceramic substrate.

solder fatigue 1

The elemental spectrum of the bulk solder joint suggested it contained ~ 3 wt% of gold, which is considered a threshold for gold embrittlement.

solder fatigue 2

The thermal fatigue fracture showed classic characteristics such as grain boundary separation and propagation through the bulk solder joint.

solder fatigue 3

The AuSn4 intermetallic compound is clearly visible in the solder joint microstructure at about 12% by area in the section. This harder phase in the Sn-Pb matrix likely accelerated the thermal fatigue failure.

The analysis results suggest that the thermal fatigue damage was likely due to the combination of (1) CTE mismatch between the J-lead package and the alumina substrate and (2) Au-Sn IMC in the solder joint microstructure at the threshold for gold embrittlement.

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