Fatigue and shear properties of high reliable solder joints for harsh applications

The reliability of an electronic product in harsh applications is typically limited by the fatigue failure of one of the interconnected solder joints. Mechanical properties of common lead-free solder joints have been studied for large bulk solder samples. However, the microstructure of solder joints in a realistic application has been proved completely different than that of bulk solder samples, which can lead to different mechanical properties. SnAgCu (SAC) based solder alloys are widely used in the lead -free applications, however, solder alloys with only Sn, Ag and Cu demonstrate low reliability in harsh environments. In this study, individual highreliability SnAgCu based solder joints (Innolot, MaxRel, Ecolloy) were tested to investigate their shear and fatigue properties. The experiment includes shear of individual solder joints under 4 strain rates: 1, 0.1, 0.01, 0.001 sec, and fatigue of individual solder joints under various stress amplitudes. For the shear testing, shear strengths of solder joints for each strain rate were systematically recorded and compared. For the fatigue testing, the effect of stress amplitude on the fatigue life and work dissipation was studied as well. Moreover, the effect of surface finish (OSP, ImAg, and ENIG) on the solder joints reliability was also investigated. The experiment results showed that shear strength tends to increase with the increase of shear strain rate. Solder joints with high Bi and Ag contents demonstrate superior shear and fatigue properties. However, excessive Bi may lead to the embrittlement of solder joints and change of failure modes during mechanical testing.