Advancement in microstructure and mechanical properties of lanthanum-doped tin-silver-copper lead free solders by optimizing the lanthanum doping concentration

Abstract

Purpose – The purpose of this paper is to analyze the effect of lanthanum (La) doping on the microstructure and mechanical properties of tin-silver-copper (SAC) alloys and to find out an optimum La doping concentration upon which the best set of the desirable properties can be possible. SAC tertiary Pb-free solders are thought to be the best substitutes for Pb-based solders but have limitations due to their coarse microstructure. Design/methodology/approach – Three samples with varied La concentrations were synthesized from pure metals. SAC with various concentrations of doped La were studied in detail. Scanning electron microscopy images were studied and were further analyzed by the ImageJ software to measure the average intermetallic compounds (IMCs) size. Optical microscopy was used to study the grains. MTS tensile machine was used determine out the mechanical properties. All the analysis was done before and after thermal aging. Finally, an optimum La doping concentration was found. Findings – By doping a suitable concentration of La in SAC, the average IMCs size as well as grain size was greatly reduced. Yield stress and tensile strength were quite improved as a result. Unlike previous studies, ductility was not lowered. Impact toughness was seen to be significantly improved. Finally, an optimum La doping concentration was found to be 0.3 per cent by weight, as beyond this, ductility drops rapidly. Originality/value – The optimum La doping concentration in SAC resulted in a much refined microstructure and a very good set of the desirable properties, including yield stress, tensile strength, ductility, impact toughness and expectedly creep and fatigue resistances, for the first time.