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In this study, various amounts of Ni particles were added in situ to Sn–3·5 wt-%Ag lead free solder to form new composite solders. Copper substrates were then dipped into these solders and aged at 150°C for 0, 25, 225, or 1000 h. The microstructure and microhardness of the as solidified solder and the aged solder/copper couples were investigated. Experimental results revealed that the addition of Ni particles increased the microhardness of the composite solder. Ni additions of less than 3 wt-% yielded a microstructure of β-Sn grains surrounded by a eutectic mixture of Ag3Sn and a Sn rich matrix. An intermetallic compound of Ni3Sn4 particles was dispersed throughout the eutectic. For 5 wt-%Ni addition, the Ni3Sn4 phase and the remaining Ni particles were agglomerated. In the case of copper substrate dipped with a thick layer of composite solder, water quenched and then aged at 150°C, the induced (Ni, Cu)3Sn4 particles coarsened and agglomerated. Additionally, the intermetallic (Cu, Ni)6Sn5 compound layer formed at the solder/Cu interface thickened with increasing Ni content. [ABSTRACT FROM AUTHOR]