Microstructural evolution during thermomechanical fatigue of 62 Sn-36 Pb-2 Ag and 60 Sn-40 Pb solder joints

Thermomechanical fatigue tests were performed on two near-eutectic Sn-Pb solder alloys, 60 Sn-40 Pb and 62 Sn-36 Pb-2 Ag, to examine the effect silver additions have on solder joints. The cyclic load was found to have consistent trends between the two silver alloys (for given amounts of total strain and strain rates). It was found that a decreasing strain rate increased the life of both alloys equally in thermomechanical fatigue. At slower strain rates, the dislocation substructure recovers faster than it work-hardens, which tends to minimize subsequent recrystallization and heterogeneous coarsening of the solder joint. The microstructure of 62 Sn-36 Pb-2 Ag contained large whisker-like Ag₃Sn precipitates that nucleate and grow out from the Cu₆Sn₅ interfacial intermetallics. At this size, the Ag₃Sn precipitates have little effect on the deformation behavior of the solder. The intermetallics are not detrimental in that they do not prematurely crack, nor are they beneficial because they are too large to stabilize the microstructure. It does not appear, from a microstructural viewpoint, that adding silver to near-eutectic Sn-Pb has any significant effect on improving the thermomechanical fatigue behavior