Solder/substrate interfacial thermal conductance and wetting angles of Bi-Ag solder alloys

Bi-Ag lead-free alloys are considered interesting alternatives to Pb-based traditional solders due to compatible melting point and strength. During soldering, the ability of a liquid alloy to flow or spread over the substrate is crucial for the formation of a metallic bond driven by the physicochemical properties of the liquid solder/solid substrate system. In addition, the wettability is intimately associated with the solder/substrate thermal conductance represented by a heat transfer coefficient, h(i). In this work, three Bi-Ag alloys (hypoeutectic-1.5 wt%Ag, eutectic-2.5 wt%Ag and hypereutectic-4.0 wt%Ag) were directionally solidified under upward unsteady state heat flow conditions. Both time-dependent h(i) profiles and wetting behavior represented by contact angles (theta) were determined for the three alloys examined. The dependence of theta on the alloy Ag content is assessed experimentally. Also, thermal readings collected during directional solidification of the Bi 1.5, 2.5 and 4.0 wt% Ag alloys are used with a view to permitting h(i) versus time (t) profiles to be computed. It is shown that along a first solidification stage (t < 16 s) the h(i) values followed the trend experimentally observed by the contact angles for the three alloys examined, while for t > 16 s the volumetric expansion of the Bi-rich phase is shown to have a dominant role inducing a sudden increase in h(i). For each alloy a couple of time-dependent h(i) expressions is needed to represent the entire solidification progress.