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

Area of Science:

• Materials Science
• Metallurgy
• Surface Science

Background:

• Eutectic In-48Sn is a promising low-temperature solder with good mechanical properties.
• Previous studies observed Cu$_{2}$(In,Sn) and Cu(In,Sn)$_{2}$ at the In-48Sn/Cu interface after high-temperature soldering.
• Mechanical polishing can introduce defects, complicating interfacial reaction analysis.

Purpose of the Study:

• To investigate the interfacial reaction between In-48Sn and Cu during soldering using a novel preparation technique.
• To accurately identify intermetallic compounds formed at the solder/copper interface.
• To determine the mechanical properties of the formed intermetallic phases.

Main Methods:

• Cryogenic broad Ar$^{+}$ beam ion milling for interface preparation.
• Soldering of In-48Sn on Cu at 150 °C.
• Electron Probe Microanalysis (EPMA) for quantitative analysis.
• Transmission Electron Microscopy (TEM) with selected area electron diffraction for phase identification.

Main Results:

• Cu$_{6}$(Sn,In)$_{5}$ was identified as the only intermetallic compound formed during 150 °C soldering.
• Cu(In,Sn)$_{2}$ formation was attributed to room-temperature aging after soldering, not the soldering process itself.
• The microstructure evolution and growth mechanism of Cu$_{6}$(Sn,In)$_{5}$ were elucidated.
• Young's modulus (119.04 ± 3.94 GPa) and hardness (6.28 ± 0.13 GPa) of Cu$_{6}$(Sn,In)$_{5}$ were determined, showing minimal impact from In doping.

Conclusions:

• Cryogenic ion milling is effective for preparing defect-free interfaces for accurate interfacial reaction studies.
• The interfacial reaction mechanism at low temperatures is clarified, distinguishing between soldering and aging effects.
• The mechanical properties of the primary solder-formed intermetallic compound, Cu$_{6}$(Sn,In)$_{5}$, are quantified.