Abstract
Mechanical alloying (MA) process was employed as an alternative method to produce SnAgNi solder pastes in this study. The properties of solder powders were investigated by doping various Ni concentration into Sn-3.5Ag-xNi alloys (x=0.1, 0.5, 1.0, 1.5, and 2.0 wt. %). When the Ni concentration was low (x=0.1, 0.5 wt. %), MA particles agglomerated to a flat ingot with the particle size larger than 100μm. For higher Ni concentration (x= 1.0, 1.5, and 2.0 wt. %), MA particles turned to fragments and the particle size was below 100μm. The results of XRD and SEM revealed the formation of alloys dispersed in solder powders, which led to the decrease of particle size after flattening, cold welding and fracturing. It appeared that the particle size of solders was dependent on the Ni concentration. To reduce the particle size of SnAgNi alloys with low Ni concentration, Ni3Sn4 nanoparticles were further doped into Sn and Ag powders to derive the SnAgNi composite solder. For the Ni3Sn4-doped solder, the particle size was smaller than that of the Ni-doped solder. The distinction of milling mechanism for both Ni3Sn4-doped solder and Ni-doped solder by MA process was probed and discussed. Besides, the DSC results ensured the feasibility to apply the solder material for the reflow process. SnAgNi solder joints with Ni concentration from 0.1 to 2.0 wt.% after 3 times reflow at 240oC were employed to investigate the evolution of interfacial reaction between SnAgNi solders and electroplated Cu. For the Ni-doped solders, the Cu6Sn5 phase with little Ni was formed after deliberately quantitative analysis with field emission electron probe microanalyzer. The addition of Ni substantially increased the amount of intermetallic compound at the SnAgNi solders/Cu interface and also enhanced the dissolution of Ni in (Cu,Ni)6Sn5. By doping nano-sized Ni3Sn4 particles into Sn-Ag solder, the morphology of (Cu,Ni)6Sn5 IMC became pebble-shape. The thickness of (Cu,Ni)6Sn5 IMC was much larger in Ni3Sn4-doped solder than that in Ni-doped solder after multiple reflow times. Hence, the reaction at the interfaces of Ni3Sn4-doped solder/Cu was more rapid than that at Ni-doped solder/Cu. In addition, wettability test revealed that the wetting angles of Ni3Sn4-doped solder with low Ni concentration (0.1 and 0.5wt. %) were smaller than that of Ni-doped solder between solders and Cu substrate. The wetting angles of SnAgNi solders were also comparable with commercial Sn-3.5Ag and Sn-3.0Ag-0.5Cu solders on either Cu substrate or electroplated Ni metallization. Favorable wettability of the as-derived solder in this study was clearly demonstrated.