Abstract
The eutectic SnBi solder alloy is a candidate for Pb-free replacement of the conventional eutectic SnPb solders. This study presents series of results on the binary eutectic SnBi and ternary SnBi-1 wt% Cu solder joints. Composition analysis, solidification behavior, temperature dependent CTEs and wettability of the asfabricated solder alloys are characterized. In addition, microstructure, adhesion strength, fracture surface and contact resistance of the solder joints are also evaluated. The DSC study indicates that the 42Sn-58Bi solder and the SnBi-1Cu solder both melt at 138℃ but solidify at 127℃ and 131℃, respectively, under a heating and cooling rate of 5℃/min. The results of the wetting balance show that the addition of 1 wt% Cu has little effect on the contact angle of the eutectic SnBi solder alloy with various metallization layers. The coarsening rate of the Bi-rich phase in the solder joint under under thermal aging is suppressed by adding 1 wt% copper to the 42Sn-58Bi solder alloy. The Ag3Sn intermetallic compound froms at the solder/PtAg interface, while the Cu6Sn5 intermetallics grows as scallop-like grains into the solder. The adhesion strength of all the solder joint decreases with increasing aging time, which is attributed to the growth of intermetallic compound. Nevertheless, the joints still maintain a sufficient strength after 36 days aging at 120℃. Moreover, the adhesion strength of the SnBi-1Cu solder joint is higher than that of the 42Sn-58Bi joint after aging since intermetallics grow slower in the SnBi-1Cu joint. The adhesion strength of solder joints degrades abruptly after 2000 thermal cycles. In addition, thermal cycleing would result in cracking in the solder joints, which is owing to the mismatch of different thermal expansion coefficients. Portions of the thermal fatigue cracks nucleate at the edge of the solder fillet, and then propagate along the solder/conductor interface. Some cracks are, however, through the Al2O3 substrate. Contact resistance of the solder/Cu joint does not increase after thermal aging since the resistivity of Cu6Sn5 is lower than that of solder. The solder joints of the 42Sn-58Bi/Cu, the SnBi-1Cu/Cu, the 45Sn-58Bi/PtAg, and the SnBi-1Cu/PtAg assemblies maintain their integrity after 100 days aging at 120℃ or 2000 thermal cycles since the increase in contact resistance is rather small (△R＜0.5mΩ).