Identification and Evolution of Intermetallic Compounds Formed at the Interface between In-48Sn and Cu during Liquid Soldering Reactions

It is difficult to confirm the existence of intermetallic compounds (IMCs) between SnIn and Cu, as their atomic numbers are very close, making it challenging to differentiate them through experimental tests. In order to determine IMCs and understand their growth mechanism, this study employed phase identification, morphology observation, and growth kinetics analyses on IMCs formed between In-48Sn solder and polycrystalline Cu substrate during liquid soldering. The experiments were conducted within a temperature range of 160 similar to 250 C-degrees for up to 90 min. The obtained results indicated that IMCs formed at the interface depended strongly on the soldering temperature. During long-time soldering below 200 C-degrees, one main IMC species, Cu-2(In,Sn), was found at the In-48Sn solder/Cu interface, which showed two different morphologies: a coarse-grained layer at the solder side and a fine-grained layer within the Cu component. When the soldering temperature was increased to 200 C-degrees, Cu-6(In,Sn)(5) was the only intermetallic compound (IMC) that formed at the point where the In-48Sn/Cu eutectic interface existed. At 250 C-degrees, with an increase in the soldering time, there was a formation of Cu-9(In,Sn)(4) between Cu and Cu-6(In,Sn)(5). The growth kinetics analyses indicated that the fast grain boundary/molten-channel diffusion of Cu into solder and their reaction with solder controlled the growth of the interfacial IMCs, with the activation energy of 24.56 kJ/mol when the sample was liquid-state-aged above 200 C-degrees.