The Development Of Tin Based Composite Lead-free Solders

Lead-free solder plays crucial roles in the reliability of interconnects in electronic packaging. Two main trends have aroused researchers'interests. One is the multicomponent solder alloys. The multicomponent elements are added into the binary alloys, such as Sn-Ag, Sn-Cu, Sn-Zn or Sn-Bi, to improve their properties. The second one is composite lead-free solder. The second-phase particles are doped into the Sn-Ag, Sn-Cu or Sn-Bi lead-free solders by in-situ or mixture method in order to get the composites. The objective of composite method is to improve the properties of the solder joints by the stable microstructure during service.This work focuses on composite lead-free solders by particle strengthening. The particles include Fe which combined well with the solder matrix, Cu6Sn5 which combined worse with the solder matrix and oxide which combined the worst with the solder matrix. The solder powder, strengthening particles and flux are mixed well to get the solder paste. The effects of strengthening particles on melting temperature, wettability, microstructure, mechanical properties of solder joints are investigated.The main conclusions are summarized as follows:1. For the Sn-3Ag-0.5Cu-Fe composite lead-free solders, the melting temperature did not change so much, but the wettability on Cu substrate was degraded slightly when Fe was added into Sn-3Ag-0.5Cu solder. The Fe particles were surrounded by FeSn2 phase which was formed owing to the reaction between Fe and Sn during soldering. Moreover, they accumulated near the interface between solder and Cu substrate and inhibited the growth of Cu6Sn5 intermetallic compounds, so the size of Cu6Sn5 grains got smaller than that of Sn-3Ag-0.5Cu/Cu. The microstructures of composite solder matrix were refined due to the promotion of nucleation rate ofβ-Sn. Compared with Sn-3Ag-0.5Cu/Cu solder joint, the shear strength of Sn-3Ag-0.5Cu-Fe/Cu increased significantly. Ductile fracture was observed in all samples.2. For the Sn-58Bi-Cu6Sn5 composite lead-free solders, the Cu6Sn5 particles which were produced by mechanical alloying had little effect on the melting temperature of Sn-58Bi based composite solder. The wettability of composite solders was accepted although they were worse than that of Sn-58Bi. The Cu6Sn5 particles were well dispersed in the solder matrix and refined the grain size ofβ-Sn and Bi-rich phase. The thickness of IMC slightly increased. The shear strength of composite solder joints was higher than that of Sn-58Bi/Cu. The fracture modes of composite solder were mainly ductile while the fracture mode of Sn-58Bi/Cu was brittle.3. For the Sn-58Bi and Sn-3Ag-0.5Cu composite lead-free solders with Y2O3 and Al2O3 additions,(1) minor addition of oxide had little effect on the melting temperature of Sn-3Ag-0.5Cu and Sn-58Bi. Due to the oxide particles, the oxidation of the surface on solder joint was suppressed to improve the wettability of composite solders on Cu substrate. The oxide particles were well dispersed in the solder matrix. The growth of IMC on the interface was suppressed.(2) the shear strength of Sn-3Ag-0.5Cu-Y2O3/Cu was higher than that of Sn-3 Ag-0.5Cu/Cu and the fracture paths of solder joints were along the solder matrix. Only some broken IMC grains were detected.(3) during aging at 60℃,90℃and 120℃, the IMC growth of Sn-58Bi-xY2O3/Cu (x=0,0.5,1,3wt.%) were controlled by diffusion; the activation energies calculated for the growth of the total IMC were 72±5 kJ/mol of Sn-58Bi,74±4 kJ/mol of Sn-58Bi-0.5wt.%Y2O3,81±5 kJ/mol of Sn-58Bi-lwt.%Y2O3 and 81±7 kJ/mol of Sn-58Bi-3wt.%Y2O3, respectively.To sum up, the addition of second-phase particles on the Sn-based lead-free solders (in spite of combining well, worse or worst with solder matrix) can refine the microstructures, strength the solder alloy and improve the shear strength of composite solder joints; the growth of IMC was suppressed during soldering and aging. Although the composite lead-free solders still remain some disadvantages to resolve, the research on it still attracts interest.