Microstructure Evolution and Shear Strength of Tin-Indium-xCu/Cu Joints

Artifact-Free Microstructures in the Interfacial Reaction between Eutectic In-48Sn and Cu Using Ion Milling

Eutectic In-48Sn was considered a promising candidate for low-temperature solder due to its low melting point and excellent mechanical properties. Both Cu₂(In,Sn) and Cu(In,Sn)₂ formation were observed at the In-48Sn/Cu interface after 160 °C soldering. However, traditional mechanical polishing produces many defects at the In-48Sn/Cu interface, which may affect the accuracy of interfacial reaction investigations. In this study, cryogenic broad Ar⁺ beam ion milling was used to investigate the interfacial reaction between In-48Sn and Cu during soldering. The phase Cu₆(Sn,In)₅ was confirmed as the only intermetallic compound formed during 150 °C soldering, while Cu(In,Sn)₂ formation was proven to be caused by room-temperature aging after soldering. Both the Cu₆(Sn,In)₅ and Cu(In,Sn)₂ phases were confirmed by EPMA quantitative analysis and TEM selected area electron diffraction. The microstructure evolution and growth mechanism of Cu₆(Sn,In)₅ during soldering were proposed. In addition, the Young's modulus and hardness of Cu₆(Sn,In)₅ were determined to be 119.04 ± 3.94 GPa and 6.28 ± 0.13 GPa, respectively, suggesting that the doping of In in Cu₆(Sn,In)₅ has almost no effect on Young's modulus and hardness.

A New Low-Temperature Solder Assembly Technique to Replace Eutectic Sn-Bi Solder Assembly

We successfully achieved low-temperature assembly by reflowing the 13.5Sn-37.5Bi-45In-4Pb quaternary eutectic solder paste and the SAC 305 solder ball together at 140 °C for 5 min. The wetting angle of the mixed solder joint is 17.55°. The overall atomic percent of Pb in the mixed solder joint is less than 1%, which can be further reduced or eliminated. Moreover, after aging at 80 °C for 25 days, we observed no obvious decrease in shear strength of the fully mixed solder joint, which is the most advantage of this assembly technique over Sn58Bi solder assembly. The Bi phase segregation at the interface is slowed down compared with Sn-Bi solder joint. This low-temperature assembly is promising to be applied in advanced packaging technology to replace the eutectic Sn-Bi solder.