Comparative study of the microstructure and mechanical strength of tin-copper (Sn0.7Cu) solder modified with silver (Ag) by both alloying and doping methods

In order to compare the effectiveness of alloying methods with doping methods, we have studied the microstructure and mechanical strength of Sn0.7Cu solder and two kinds of Ag modified Sn0.7Cu solders, i.e., Sn0.7Cu0.3Ag (prepared by alloying pure Sn, Cu, and Ag metals) and Sn0.7Cu + 0.3Ag (prepared by doping Ag nanoparticles into Sn0.7Cu). Solder joints were sent for mechanical tests and microstructure observation after aging at 90 °C for up to 500 h. Electromigration (EM) test models were built to evaluate the resistance of solders to EM degradation at an average current density of 0.79 × 10⁴ A/cm². Experimental results revealed that both Ag modified Sn0.7Cu solders had higher mechanical strength and stronger resistance to EM than Sn0.7Cu. This improvement was due to the introduction of dispersed Ag3Sn intermetallic compounds (IMCs), which acted as a second phase to refine the bulk solder, stop dislocations and hinder atomic migration. Sn0.7Cu0.3Ag was found to be more effective in reinforcing the mechanical strength than Sn0.7Cu + 0.3Ag, since the shear strength and microhardness of the former solder were respectively 12.79 % and 1.01 % higher than the latter one. The products of diffusivity and effective charge number DZ*, which represented the EM rate, for Cu atoms were measured to be 3.49 × 10⁻⁹, 1.91 × 10⁻⁹, 2.64 × 10⁻⁹ cm²/s for Sn0.7Cu, Sn0.7Cu0.3Ag, and Sn0.7Cu + 0.3Ag, respectively. The superior performance of Sn0.7Cu0.3Ag was ascribed to the smaller size and wider distribution of Ag₃Sn IMCs in the solder matrix. To conclude, the alloying method produced greater improvement than the doping method in the microstructure and mechanical strength of Sn0.7Cu solder.