Revealing the nucleation and growth mechanism of a novel solder developed from Sn-3.5Ag-0.5Cu nanoparticles by a chemical reduction method

Sn-3.5Ag-0.5Cu nanoparticles were synthesized by chemical precipitation with NaBH4. By using x-ray diffraction and transmission and scanning electron microscopy, the microstructural characteristics of particle growth were evaluated. The results indicated that the primary particles after precipitation were (Ag,Cu)4Sn, with a size of 4.9 nm. (Ag,Cu)4Sn was transformed into (Ag,Cu)3Sn, 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 final particle size of polycrystalline particles was 42.1 nm owing to the depletion of Sn atoms in the solution. Nucleation and growth mechanisms of Sn-3.5Ag-0.5Cu nanoparticles are also discussed and proposed.