The Maximum Fluidity Length of Solidifying Sn-Cu-Ag-Ni Solder Alloys

During wave soldering, it is important that a solder is able to flow easily to fill joints and to drain away to leave tidy fillets. The maximum fluidity length (L _f) is a simple measure of the flow behavior of solidifying alloys, defined as the distance a cooling and solidifying alloy can flow in a constant cross-section before the developing microstructure arrests flow. This paper explores the influence of alloy composition on L _f in Sn-rich Sn-Cu-Ag-Ni alloys with compositions relevant to wave soldering. Significant differences in L _f are measured among candidate lead-free solder alloys, which are discussed with respect to the phase diagrams and the mode of solidification.