The effect of solder paste viscosity on porosity and mechanical properties of surface mount solder joints

This paper reports, for the first time, the results of a systematic experimental investigation of solder paste viscosity on the porosity and mechanical properties of surface mount solder joints. By means of X-ray radiography, it is generally observed that solder joints have greater porosity area fraction for higher solder paste viscosity, with increasing viscosity from 35.3-213 Pa.s, the porosity area fraction varies from 3.0-9.9%, respectively. Thermogravitational analyses on these solder pastes were performed to explain the observed phenomena. It is found that the solder paste with lower viscosity range has a larger evaporation rate of organics than those with higher viscosity range before metal alloys melting, but a smaller rate after metal alloys melting. Shear test results demonstrate that solder joints fabricated with lower viscosity solder paste have higher shear strength by up to 30% (increased from 35.3-46.3 MPa for viscosity change from 213-35.2 Pa.s, respectively).