Thermodynamics and Kinetics of Oxidation of Pure Indium Solders Harry Schoeller, Jongman Kim, Seungbae Park, and Junghyun Cho Mechanical Engineering, SUNY at Binghamton, Vestal Pkwy East, Binghamton, NY, 13902-6000 Abstract MicroElectroMechanical System (MEMS) devices often require low-temperature, fluxless soldering techniques due to their high temperature sensitivity and performance requirements of the components. While seeking the development of a soldering technology using pure indium, the major focus of this study is to assess the thermodynamics and kinetics of indium oxidation at various solder reflow environments that will ultimately provide a processing window for solder reflow and surface oxide cleaning. With a glove box employed to generate reducing environments, oxygen, moisture, and hydrogen contents were varied to examine their effects on oxidation and reduction behavior of indium. We also explored oxidation mechanisms at different regimes of temperature and time. In particular, electron transport from indium to indium oxide is shown to be the rate controlling mechanism under specific oxidizing conditions. For accurate thickness measurements, a spectroscopic ellipsometer was employed. In addition, the effect of indium oxidation on solder joint reliability was observed via wetting angle and interfacial shear strength measurements. Introduction Recent trends within the microelectronics industry away from the use of lead based solder because of environmental concerns has lead to much new research in the lead-free domain [1]. A lead-free solder of particular interest in the low temperature regime is pure indium solder, due to its low melting point (156ºC) and unique mechanical properties [2]. Ductility and fatigue resistance of indium at low temperatures makes it an ideal candidate for compression seals in many packaging applications [3]. However oxidation of indium, especially in fluxless applications reduces the reliability of the seal [4]. It is for this reason a systematic study of the oxidation and reduction of indium is needed. Information about the oxidation behavior in the temperature and time domain will help manufacturers develop reflow profiles, which reduce if not eliminate any oxides present thereby enhancing reliability. Apart from time and temperature there are many factors that can influence the oxidation behavior of metals, such as purity, surface roughness, microstructure, and environment [5]. In this paper, much emphasis was placed on the influence of the reflow environment, particularly O 2 , H 2 O, H 2 concentrations. Manipulation of these parameters along with temperature can change the reflow environment from oxidizing to reducing [6]. Complimenting this thermodynamic study, an investigation into the kinetics of indium oxidation is presented. In air, the oxidation kinetics seems to follow a logarithmic relationship up to the temperature of 220ºC. This observation along with information about the initial activation energy of oxidation points to Uhlig’s electron transport as the rate controlling mechanism for oxide growth [7]. Mater. Res. Soc. Symp. Proc. Vol. 968 © 2007 Materials Research Society 0968-V03-07