High reliability high melting mixed lead-free BiAgX solder paste system

In the current work, a mixed powder BiAgX solder paste system with the melting temperature above 260°C and comparable, or better, reliability to the high lead-containing solders has been studied. The mixed powder solder paste system is composed of a high-melting first alloy solder powder as a majority and the additive solder powder as a minority. The additive solder is designed to react preferentially with various surface finish materials before, or together with, the melting of the majority solder to form a controllable IMC layer. The IMC layer of the mixed powder system is controllable by the species and quantity of the additive solder, and it is observed to be insensitive to thermal aging and thermal cycling in current tests, while the high lead-containing solders show a considerable increase in IMC layer thickness. Both micron-sized Ag-rich particles and AgSn phases along the Bi colony boundaries in the joints have been observed. The exposed Ag-rich particles and the surrounding stepwise pattern in the Bi matrix on the fracture surface indicate that these Ag-rich particles constrain the dislocation movement in Bi matrix, and thus enhance the strength and the ductility of the joint.