A comparison study of SAC405 and SAC387 lead-free solder ball alloy characteristic and solder joint system on Ni/Au finish

Abstract

In this study, two Sn-Ag-Cu alloys, namely SAC405 & SAC387 were compared in terms of the following characteristics: solder ball surface morphology, solder ball surface cleanliness through Auger elemental depth profile analysis, ball shear strength (at time zero, after test, after multiple reflow and after high temperature storage), IMC morphology and thickness, solderability test and missing ball after functional testing. In terms of surface morphology, SAC405 was found to have coarser dendrites on the ball surface that form notches and channels which may trap contamination and prevent the dispense of flux. The finer dendrites on the SAC387 ball surface prevents contamination and flux entrapment, thus prevents voids in the IMC and provides better mechanical strength. Auger elemental depth profile analysis showed higher carbon and oxide level on SAC405 as well as the presence of phosphorus. This may affect IMC formation and affect solder joint performance. With SAC387, lower carbon and oxide level and no phosphorus are found on the surface of the ball. The sharp cliff in the profile of SAC387 suggests that the surface of the solder ball was smoother. In term of ball shear tests, SAC387 had shown significantly higher ball shear strength and Cpk in all the read points. In terms of IMC morphology and thickness, SAC387 has lower IMC growth than SAC405 in most of the read points. In terms of missing ball after functional testing, characterization result with ∼130units test sample size showed that SAC405 has 0.78% missing ball yield loss at final test whereas SAC387 has zero yield loss. In conclusion, SAC387 solder alloy provides more reliable lead-free solder joint than SAC405.