Synthesis and Application of Novel Lead-Free Solders Derived from Sn-based Nanopowders

Two kinds of lead-free solders derived from tin-based nanopowders were fabricated. One was the lead-free solders with Sn-Ag-Cu nanopowders, and the other was nano-sized Cu₆Sn₅ doped Sn-Ag-Cu solders. The lead-free solders with Sn-Ag-Cu nanopowders were synthesized by chemical precipitation with NaBH₄. The isolated particle exhibited a near spherical shape with particle sizes around 5 nm. The primary particle size of Sn-Ag-Cu nanopowders was in the range of 40 nm. Microstructural characteristics of particle growth were evaluated by TEM and FE-SEM. The primary particles after precipitation were (Ag,Cu)₄Sn with a size of 4.9 nm. It was also revealed that (Ag,Cu)₄Sn transformed into (Ag,Cu)₃ Sn, when the total amount of Sn contributed from both (Ag,Cu)₄Sn and Sn covering the (Ag,Cu)₄Sn overtook that of (Ag,Cu)₃Sn. The nano-sized Cu₆Sn₅ doped Sn-Ag-Cu solder paste was produced by mixing Cu₆Sn₅ nanopowder into commercial SnAg solder paste. To realize the effect of Cu₆Sn₅ nanopowder doping, the ball shear strength of the joint was further investigated. The fracture behavior of Sn-Ag-Cu solder joints was probed with respect to the fracture surfaces, interfacial morphologies, and ball shear strength. It was demonstrated that the creep strain rate sensitivity of nano- Cu₆Sn₅ doped composite solder was higher than that of commercial Sn-Ag-Cu solder, although the creep hardness of both solders was nearly identical