Microstructural development in a rapidly cooled eutectic Sn–3.5% Ag solder reinforced with copper powder

Abstract

Experiments using eutectic Sn–3.5% Ag solder paste were conducted with the objective of examining the conjoint influence of copper particles addition and rapid cooling on microstructural development. The composite solder mixture was made by thoroughly mixing a pre-weighed amount of copper particles with a commercial Sn–3.5% Ag solder paste. The experiments were quite similar to the heating and cooling cycle of an industrial reflow soldering process. Heating of the samples was conducted in a furnace whose temperature was carefully controlled. The cooling process was conducted on a chilled aluminum block through which coolant was circulated at 0.5 °C. When the solder temperature reached 250 °C, the circulating system would turn on automatically and the sample, which is still molten, is forced to cool rapidly. Temperature records of the solder samples revealed that addition of copper particles to the eutectic Sn–3.5% Ag did not appreciably affect the heating and melting properties when compared to the unreinforced Sn–3.5% Ag counterpart. However, copper particles did change the solidification temperature of the composite solder. Detailed observations for varying amounts of copper particle addition revealed that copper particles less than 1.0 wt.% lowered the solidification temperature of the composite solder. For copper particles greater than 1.0 wt.%, the solidification temperature increased a few degrees Celsius, indicating that some of the copper particles did not completely dissolve in the Sn-dominant solder during the melting process. Results reveal that as-solidified microstructures of the eutectic Sn–3.5% Ag solder contain columnar type dendrites of the Sn-rich phase and a eutectic mixture of the Sn 3Ag and Sn-rich phase located between the dendrite columns. The addition of copper particles to the eutectic Sn–3.5% Ag solder does refine the morphology of the primary phase, which is attributed to the presence and distribution of the Cu 6Sn 5 intermetallic in the solder matrix.