Mechanical Characteristics of Sn-Ag-Based Lead Free Solders

Recently, preventing environmental pollution, lead-free (Pb-free) solders are about to replace tin-lead (Sn-Pb) eutectic solders. However, the mechanical properties of Pb-free solders have not been clarified. Hence, the following study was conducted; First, a rate-dependent plasticity was characterized to represent the inelastic deformation behavior for Sn-Ag-based lead free solders. The material parameters in a constitutive model was determined in a direct method combining both rate dependent and rate independent plastic strains. The constitutive model unifies both rate-dependent creep behavior and rate-independent plastic behavior occurring concurrently at the same time in the solders. Secondly, the strength of solders with a variety of plating materials was studied. Intermetallic compounds between solder and electrical pads are formed during reflow process and gradually grow in service. By using the Cu-plates on which Cu or Ni or Ni/Au plating was deposited, the specimens of solder joints were fabricated with Sn-Ag-based lead free solders. After aging the specimens in an isothermal chamber, tensile tests were performed. From SEM microscope observation and EDX microprobe analysis, the growth and components of the intermetallic compounds layer were also examined. Based on the experimental tests, the relations between solder joint strength and the aging period were discussed.