Effect of Mold Surface Roughness on the Interfacial Heat Transfer Coefficient During Solidification of Solder Alloys

Solder joints are strongly dependent on how well the solder alloy can wet the substrate. One of the parameters which can be used to characterize the wettability of solder alloys on a substrate is the heat transfer coefficient at the interface alloy/substrate, hi. This study focus on the effect of the surface roughness of the substrate on the interfacial heat transfer coefficient during solidification of solder alloys. A comparative study is carried out with two lead-free solders alternatives and the traditional Sn-Pb solder (Sn 0.7wt%Cu, Sn 3.5 wt%Ag and Sn 38wt%Pb, respectively). These alloys were directionally solidified using a solidification apparatus having a water cooled bottom made of low carbon steel with two different surface finishing: machined and polished. The experimental thermal data collected by thermocouples positioned along the casting length were used as input information into an Inverse Heat Transfer Code implemented in this work in order to determine the hi variation in time. A power–law function given by (where a and m are constants which depend on the alloy composition, substrate and melt superheat and t is the time) which is based on both theoretical and experimental analyses is proposed. The transient hi profile has a typical drastic reduction from a high initial value due to the development of an air gap, followed by a recovery to an essentially constant value. The literature generally reports a decrease in hi with increasing surface roughness. However, in the present work an opposite behavior has been detected, which is explained based on contact interactions between alloy and substrate that are subjected to thermal contraction and thermal expansion during the soldering process, respectively.