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 (θ) were determined for the three alloys examined. The dependence of θ 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.