A Study on Process, Strength and Microstructure Analysis of Low Temperature SnBi-Containing Solder Pastes Mixed with Lead-free Solder Balls

As the traditional eutectic SnPb solder alloy has been outlawed, the electronic industry has almost completely transitioned to the lead-free solder alloys. The conventional SAC305 solder alloy used in lead-free electronic assembly has a high melting and processing temperature with a typical peak reflow temperature of 245ᵒC which is almost 30ᵒC higher than traditional eutectic SnPb reflow profile. Some of the drawbacks of this high melting and processing temperatures are yield loss due to component warpage which has an impact on solder joint formation like bridging, open defects, head on pillow, and other drawbacks which include circuit board degradation, economic and environmental factors, and brittle failure defects in the circuit board like pad cratering. To overcome this, a detailed study has been carried out on low temperature lead-free solder paste that utilizes Bi bearing alloys. Three low temperature lead-free solder pastes, Sn-58Bi, Sn-57Bi-1Ag and Sn-40Bi-Cu-Ni with the melting temperatures of 138ᵒC (which is 45ᵒC below eutectic SnPb and 79ᵒC below SAC) were printed on Cu-OSP finish test boards. These pastes were then assembled with SAC305, Sn99CN and Sn100C solder spheres. The range of Bi concentrations for various mixtures used in this study was calculated to be in the range of 2 to 4 wt%. The mixtures were reflowed under two different low temperatures reflow profiles; (a) a traditional SnPb profile with a peak temperature 217ᵒC and (b) a low temperature SnBi profile with a peak temperature 177ᵒC (recommended by the paste manufacturer). After the assembly process, the mixed solder joints were shear tested to study the failure modes and shear strength at rate of 27.50mils/sec. Cross sectioning was performed to evaluate the possible microstructural changes at room temperature and after aging conditions that may have led to the changes in failure mode observed in shear testing. The isothermal aging condition used in the study is 125ᵒC for 200 hours, which mimics 21 years of field storage at 25ᵒC degrees using Arrhenius extrapolation for Cu6Sn5 intermetallic formation. Our study suggests that high temperature reflow profile (217ᵒC peak profile) had better mechanical strength than the low temperature reflow profile (177ᵒC peak profile). A metallurgical explanation for the improvement is presented in this paper. Thus, this paper describes that by generating a robust reflow assembly process for SnBi solder paste, the shear strength can be increased, cost of manufacturing can be reduced and high temperature assembly process (SAC) issues can be minimized which may improve product yield in production