Thermal cycling test in Sn-Bi and Sn-Bi-Cu solder joints

The eutectic SnBi solder alloy is a candidate for Pb-free replacement of th e conventional eutectic SnPb solders. This study presents series of results on the binary eutectic SnBi and ternary SnBi-1 wt % Cu a solder joints. Co mpositional analysis and wettability of the as-fabricated solder alloys are reported. In addition, microstructure, adhesion strength, fracture surface and contact resistance of the solder joints are also evaluated. The result s of the wetting balance show that the addition of 1 wt % Cu has little eff ect on the contact angle of the eutectic SnBi solder alloy with various met allization layers. The adhesion strength of solder joints degrades abruptly after 2000 thermal cycles. In addition, thermal cycling would result in cr acking in the solder joints, which is due to the mismatch in thermal expans ion coefficients. Portions of the thermal fatigue cracks nucleate at the ed ge of the solder fillet, and then propagate along the solder/conductor inte rface. Some cracks are, however, through the Al2O3 substrate. The contact r esistance of the solder/Cu joint does not increase after thermal cycling si nce the resistivity of Cu6Sn5 is lower than that of the solder. The solder joints of 42Sn-58Bi/Cu, SnBi-1Cu/Cu, 42Sn-58Bi/PtAg, and SnBi-1Cu/PtAg asse mblies maintain their integrity after 2000 thermal cycles since the increas e in contact resistance is rather small (DeltaR <0.5 m Omega). (C) 2000 Klu wer Academic Publishers.