Intermetallic compounds growth between Sn–3.5Ag lead-free solder and Cu substrate by dipping method

Abstract

The formation and growth of intermetallic compound (IMC) layer at the interface between Sn–3.5Ag lead-free solder and the Cu substrate during soldering and aging were studied. The soldering was conducted at 250 °C using dipping method with wettability tester. It is expected that the scallop shaped η-phase grains were formed almost instantaneously between the liquid solder and the Cu substrate. In this study, nano-size Ag 3Sn particles were found on the surface of these η-phase grains. They were most likely to be adsorbed by the IMC layer to decrease the higher interfacial energy during soldering. A continuous IMC layer was formed in a short time and the dissolution of the IMC played an important role on the IMC growth. Due to the existence of Cu solubility in Sn–3.5Ag liquid solder as well as the larger liquid solder volume in the present research, the dissolution rate is faster. With the increasing of soldering time, both the growth rate and dissolution rate decreased. During aging at 70, 125 and 170 °C up to 1000 h, the initial scallop morphology of the IMC layer changed to that of a more planar type. Using the Arrhenius model, the activation energy values for the growth of the total IMC and Cu 6Sn 5 IMC were found to be 75.2 and 58.6 kJ/mol, respectively. The activation energies were lower when compared with former results in which the IMC was formed by reflow (spread) method. More grain boundary channels of the thinner initial IMC layer lead to higher values of the layer growth coefficient at lower aging temperature by the dipping method.