Oxidation and reduction kinetics of eutectic SnPb, InSn, and AuSn: a knowledge base for fluxless solder bonding applications

For microelectronics and especially for upcoming new packaging technologies in micromechanics and photonics fluxless, reliable and economic soldering technologies are needed. In this article, we consequently focus on the oxidation and reduction kinetics of three commonly used eutectic solder alloys: (1) SnPb; (2) InSn; (3) AuSn. The studies of the oxidation kinetics show that the growth of the native oxide, which covers the solder surfaces from the start of all soldering operations is self-limiting. The rate of oxidation on the molten, metallic solder surfaces is significantly reduced with decreasing O₂ partial-pressure. Using in situ Auger electron spectroscopy (AES) it could be shown for the first time, that H₂ can reduce Sn-oxide as well as In-oxide at moderate heating duration and temperatures. In the second part of this study, the results, obtained by the investigation of oxidation and reduction kinetics, are applied to flip-chip (FC) bonding experiments in vacuum with and without the injection of H₂. Wetting in vacuum is excellent but the self-alignment during flip-chip soldering is restricted. The desired, perfectly self-aligned FC-bonds have been only achieved, using evaporated and reflowed AuSn(80/20) and SnPb(60/40) after the introduction of H₂