The Microstructure, Thermal Fatigue, and Failure Analysis of Near-Ternary Eutectic Sn-Ag-Cu Solder Joints

The electronic industry is making substantial progress toward a full transition to Pb-free soldering in the near future. At present, the leading candidate Pb-free solders are near-ternary eutectic Sn-Ag-Cu alloys. The electronic industry has begun to study both the processing behaviors and the thermo-mechanical fatigue properties of these alloys in detail in order to understand their applicability in context of current electronic card reliability requirements. In recent publications, the solidification behavior of the near-ternary eutectic Sn-Ag-Cu alloys has been reported in terms of the formation of large Ag3Sn plates and their effects on mechanical properties of Pb-free solder joints. Several methods have been employed to minimize the growth of the large Ag3Sn plates in the Sn-Ag-Cu solder joints by controlling the cooling rate during solidification, reducing Ag and/or Cu content, or adding minor alloying elements which reduce the amount of undercooling required for the nucleation of tin dendrites. In the present study, the results of accelerated thermal cycle fatigue tests are reported with the near-ternary eutectic Sn-Ag-Cu alloys of reduced Ag contents. Changes in microstructure and mechanical properties are also discussed by comparing the solder joints before and after thermal cycling.