Authored By:
Sinan Su, Minghong Jian, Francy John Akkara, Dr. Sa’d Hamasha, Dr. Jeff Suhling, Dr. Pradeep Lall
Auburn University
AL, USA
Summary
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 high-reliability 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-1, 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.
Conclusions
This study investigated the effect of surface finish and solder alloy on ball shear and cyclic fatigue testing. The results are summarized as follow:
1) Without considering the effect of surface finish, shear strength increases with the increasing shear strain rates. Fatigue life decreases with the increase of stress amplitudes.
2) Brittle failure was observed for ENIG surface finish at all shear strain rate, for both Innolot and CycloMax solder joints. Solder joints with OSP and ImAg surface finish are prone to ductile failure during shear testing.
3) Solder joints with high Ag and Bi content demonstrated better reliability in terms of shear and fatigue testing. Ag and Bi enhanced the strength of IMC particles in the solder, thus harden the solder bulk and increase the fatigue resistance.
Initially Published in the SMTA Proceedings
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