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 Cu
₆
Sn
₅
doped Sn-Ag-Cu solders. The lead-free solders with Sn-Ag-Cu nanopowders were synthesized by chemical precipitation with NaBH
₄
. 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)
₄
Sn with a size of 4.9 nm. It was also revealed that (Ag,Cu)
₄
Sn transformed into (Ag,Cu)
₃
Sn, when the total amount of Sn contributed from both (Ag,Cu)
₄
Sn and Sn covering the (Ag,Cu)
₄
Sn overtook that of (Ag,Cu)
₃
Sn. The nano-sized Cu
₆
Sn
₅
doped Sn-Ag-Cu solder paste was produced by mixing Cu
₆
Sn
₅
nanopowder into commercial SnAg solder paste. To realize the effect of Cu
₆
Sn
₅
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- Cu
₆
Sn
₅
doped composite solder was higher than that of commercial Sn-Ag-Cu solder, although the creep hardness of both solders was nearly identical