Microstructural characterisation of reflowed and isothermally-aged Cu and Ag particulate reinforced Sn-3.5Ag composite solders

Composite solders were prepared by mechanically dispersing 15v% of Cu or Ag particles into the eutectic Sn-3.5Ag solder. The average sizes for the nominally spherical Cu and Ag particles were 6 and 4 microns, respectively. Two different processing methods were used to prepare the composite solders: blending the powdered particles with solder paste, and adding particles to the molten solder at 280 degreesC. The composite solders were characterised by studying the morphology, size and distribution of the reinforcing phase. particular interest and emphasis are given towards the modifications of the reinforcements during the reflow process. Microstructural features and chemical analysis of the composite solders were studied using optical and scanning electron microscopy (SEM), and energy dispersive x-ray (EDX) analysis. The effect of reflow and isothermal ageing on the microstructure as well as the morphological changes in the interfacial IM layer of the composite solders were extensively analysed. A mechanism for IM layer growth is proposed for solid state isothermal ageing.