Fluxless soldering in air and nitrogen

Abstract

A unique process is described wherein solder reflow and joining can be effectively performed without flux in air or in nitrogen. The process involves first converting surface oxides to a new compound which permits reflow and joining to occur. A simple tool has been designed and built for the surface conversion-it is called 'PADS' for Plasma Assisted Dry Soldering. An optimized cycle has been defined and utilized to pre-treat solder components prior to reflow. After the PADS pre-treatment, samples can be stored in air for up to one week, or in nitrogen for up to two weeks, before the reflow/join operation is performed. Characterization of the soldering processes were done quantitatively with wetting balance, balling, spreading, and joining metrics. Excellent results were obtained-for example, balling and wetting indexes up to 100% in air and nitrogen. Dependencies of heating rates and ambient gas were determined. Post reflow temperature/humidity corrosion tests were performed up to 334 hours at 85/spl deg/C/85RH. X-ray emission analysis of the surface films was done at all phases of the process, including varying the starting oxide thickness by oxidation and etching procedures. This process eliminates flux, flux dispensing, flux cleaning, flux cleaning solvents, and disposal of the spent chemicals. It is concluded that the technique is not only viable, but recommended for both fast and slow soldering processes, with very simple tooling, and improved process windows compared to alternative fluxless processes which have been reported. >