Abstract
In the practice of package system, solder plays a crucial role in the assembly and interconnection of silicon die. In this study, mechanical alloying (MA) process was used to produce the lead free solder pastes of Sn-3.5Ag and Sn-3.5Ag-4Bi.To understand the milling mechanism of Sn-3.5Ag and Sn-3.5Ag-4Bi powders during mechanical alloying, the particle morphology of MA powders milled for various time was observed and discussed. In the Sn-Ag system, the ductile powders were first flattened to thin slices, and then fractured to small particles caused by the work-hardening. With further milling, the fractured particles would cold weld to larger alloy ingots. However, in the Sn-Ag-Bi system, even 4wt% addition of Bi powder made the alloying and welding limited. Instead, “agglomeration” phenomenon was found. Another method of two-stage milling was also used to further understand the effect of addition of brittle components, in which the Sn-Ag-Bi powders were fractured to even smaller particles by the trapped Bi. It was found that the crystal orientation was affected by the particle morphology after various milling time. The intensity ratio of (101)/(200) Sn was calculated and plotted to correlate with the observed powder morphology discussed earlier.Due to the induced high energy by repeated fracturing and welding, the grinding media play an important role during MA process. Ceramic container was used to provide stronger impact force, which could induce the phase transformation, than the Teflon container. In addition, it’s found that 1cm balls could fracture Bi particles and promote it dissolved into Sn matrix. On the contrary, the mixing effect was much predominant when using 3mm balls. MA powders after milling with 3mm balls showed a small endothermic peak from the DSC profile at 138□C, which was the eutectic temperature of Sn-Bi. The melting points of MA powders in ceramic container were measured to be 221?C and 203?C, respectively, for Sn-3.5Ag and Sn-3.5Ag-4Bi from the DSC curves. The reduced melting point ensured the complete melting during reflow with a peak temperature of 240□C. The formation of Ag3Sn was also observed from the X-ray diffraction peaks, indicating successful alloying by mechanical alloying. The solder pastes could thus be produced by adding flux into the MA powders. The wetting property of the solder joint was also evaluated. The as-prepared solder pastes on electroless Ni-P/Cu/Si exhibited sufficient metallurgical bonding with contact angles less than 20□.