Microstructure and mechanical properties evolution of intermetallics between Cu and Sn-3.5Ag solder doped by Ni-Co additives
The evolution of intermetallic compounds (IMCs) generated between Sn-3.5Ag solder doped by additive couples (namely, 0.2mass%Co and 0.1mass%Ni) and Cu substrate was characterized. After soldering, the additive couples, Co-Ni, were all detected at the intermetallic region. The microstructure of intermetallic was identified as (Cu, Ni, Co)6Sn5 by electron probe microanalysis (EPMA) and x-ray diffraction (XRD). However, the morphology of (Cu, Ni, Co)6Sn5 was converted to columnar like and was not as dense as the typical scallop-like Cu6Sn5. A duplex structure of (Cu, Ni, Co)6Sn5, namely, two distinct regions bearing different concentrations of Ni and Co, was observed. Much higher Ni and Co concentrations were probed in the outer intermetallic region adjacent to the solder matrix, while lower concentration at the inner region was verified. After aging, the intermetallic (Cu, Ni, Co)6Sn5 tended to be dense, while the growth rate was depressed at the early stage. In addition, the Cu3Sn phase was not detected after aging at 110°C, while it appeared at 130°C and 150°C for 504 h. Using the nanoindentation technique, some mechanical properties of (Cu, Ni, Co)6Sn5 were investigated. The lower hardness and Young’s modulus of the outer intermetallic region was revealed. After aging treatment, both the hardness and Young’s modulus values were elevated.