BEHAVIOUR AND STRENGTHENING EFFECTS OF SB IN A LOW-BI SN-CU SOLDER ALLOY

摘要

As the conditions under which Pb-free solder joints have to function have become more severe, solder alloy formulators have been exploring ways of achieving a strengthening of the βSn matrix of solders that is sustainable in long term service. With the effectiveness of dispersion strengthening deteriorating as a result of inevitable particle coarsening, attention has turned to solid solution strengthening. Bi has been identified as an effective solid solution strengthener but the strong temperature sensitivity of its solubility in beta-Sn means that when the level of addition exceeds about 2wt%, changes in the distribution and morphology of the Bi precipitated from a saturated solid solution can mean that the performance of the alloy is variable in applications where it is exposed to thermal cycling. Sb has been identified as another effective solid solution strengthener that can also provide dispersion strengthening when at a level high enough that the SbSn intermetallic compound precipitates. In this paper the authors report some of the results of a study on the behaviour of Sb in Ag-free solder alloys, Sn-0.7Cu, and commercial alloys Sn-0.7Cu-0.05Ni alloy ("SN100C") and Sn-0.7Cu-0.05Ni-l.5Bi ("SN100CV") and the effect of the Sb addition on the performance of the alloy in isothermal shear fatigue. For comparison purposes there are also some observations on the behaviour and effect of an Sb additions on the default Pb-free solder Sn-3.0Ag-0.5Cu ("SAC305").