The effects of different cooling rates and reinforcement particle amount on in-situ Cu6Sn5 reinforced composite solder and Ni reinforced composite solder were investigated. The results indicate that the optimal microstructure characterizations, which hold the smallest intermetallic compound (IMC) size and uniform distribution in the solder matrix, of in-situ Cu6Sn5 reinforced composite solder can be achieved under air cooling condition (the cooling rate is 2 oC/s). From the microstructure observation of Ni reinforced composite solder, it is found that big sized IMCs have a significant effect on the growth orientation of ternary eutectic phases. Besides, snow- and leaves-shaped IMCs clusters are found in Sn3.0Ag0.5Cu-0.45Ni solder matrix. When Ni addition increases to 2.0 wt%, dark spots with different sizes and shapes appear uniformly in the solder matrix due to the solid solution effect of Ni in the solder