Abstract
Mechanical alloying (MA) process is considered as an alternative approach to produce solder materials. In this study, the effect of Cu concentration in the ternary Sn-3.5Ag-xCu (x=0.2, 0.7, and 1 wt.%) solder by mechanical alloying (MA) was investigated. (Cu, Sn) solid solution was precipitated as Cu6Sn5 IMC which was distributed non-uniformly through the microstructure. Cu6Sn5 IMCs present in the SnAgCu solder with high Cu composition cause the as-milled MA particle to fracture to a smaller size. Distinction in morphology of as-milled MA powders with different Cu content was revealed. When the Cu concentration was low (x=0.2), MA particle aggregated to a spherical ingot with the large particle size. For higher Cu concentration (x=0.7 and x=1), MA particle turned to flakes with smaller particle size. The distinction of the milling mechanism of Sn-3.5Ag-xCu (x=0.2, 0.7, and 1) solder by MA process was discussed. An effective approach was developed to reduce the particle size of the SnAgCu solder down to less than 100 μm by doping the Cu6Sn5 nano particle during MA process. In addition, the DSC results also ensured the compatibility to apply the solder material for the annealing process. Besides, electroless Ni-P (EN) under bump metallization (UBM) has been widely used in electronic interconnections due to the good diffusion barrier between Cu and solder. Solder joints after annealing at 240˚C for 15 min were employed to investigate the evolution of interfacial reaction between electroless Ni-P/Cu UBM and SnAgCu solder with Cu concentration from 0.2 wt.% to 1.0 wt.%. After deliberately quantitative analysis with an electron probe microanalyzer, the effect of Cu content on the formation of IMCs at SnAgCu solder/electroless Ni-P interface would be investigated. With the aid of microstructure evolution, quantitative analysis and elemental distribution by x-ray color mapping, the presence of Ni-Sn-P phase and P-rich layer could be revealed. SnAgCu composite solder reinforced with Cu6Sn5 nano dispersoids was investigated for the related interfacial reactions. The growth of the IMCs formed at composite solder/EN interface was retarded due to the stable distribution of Cu6Sn5 dispersoids in the solder after annealing. In addition, the contact angle of MA solder paste was less than 25˚ and exhibited good wettability.