High-temperature non-eutectic indium-tin joints fabricated by a fluxless process

A new alternative solder joint made of a non-eutectic indium-tin (In-Sn) multilayer composite deposited in high vacuum is reported. The unique features of this design are that it is fluxless, oxidation-free, and more importantly the fabricated joint achieves a re-melting temperature significantly higher than the bonding temperature. The In-Sn non-eutectic multilayer structure with a thin gold film evaporated as a cap layer has a predominantly Sn-rich matrix with a composition of 6 wt.% Au, 14 wt.% In and 80 wt.% Sn. The quality of the joints was first evaluated using a combination of X-ray microfocus and scanning acoustic microscopy techniques, and results have shown that the joints are nearly void-free. In addition, analysis by scanning electron microscopy (SEM) equipped with an energy-dispersive X-ray (EDX) spectroscope performed on the joint cross-section clearly indicated a uniform joint thickness of 7.6 μm, while AuIn₂ grains embedded in a heavily Sn-rich matrix were clearly detected. Lastly, we set out to determine the re-melting temperatures of these fabricated joints. Temperature values ranging from 175 to 190 °C were found, higher than the bonding temperature of 150 °C. The results clearly show that the joint composition is heavily Sn-rich, leading to the high re-melting temperature The increase in re-melting temperature opens up the post-processing temperature window of devices in manufacturing processes. No flux is needed during the bonding process, making it particularly useful to packaging devices for which the use of flux is strictly prohibited.