The particle size of solder pastes can significantly affect the rheological properties of these products. If the rheological properties of solder paste are not in a good state, the reflow soldering process can degrade to a disastrous one. Therefore, it is important for electronics manufacturers to understand the properties of these materials. This will help avoid costly rework on assemblies.
The most common characteristics of solder pastes are the viscosity and the thixotropic behaviour. In this study, a Taguchi method was used to find an optimal combination of these parameters.
To determine the rheological behavior of different pastes, the viscosity of the samples was measured after 8 h of warm-up. Viscosity was found to be influenced by the flux and the vehicle system. However, time-dependent viscosity changes were not considered during the printing cycles.
Another important property of solder pastes is the distribution of particle sizes. The best pastes have small particles, which minimizes voids during reflow. Larger particles can prevent reflow defects.
Aside from the rheological properties of solder powder, the amount of metal particles in the paste also has a significant influence on the formation of a solder joint. High solid fluxes show high viscosity at room temperature, while low solid content can result in thermal collapse.
Several research works have been done on the rheological properties of solder. However, the results of these investigations had major shortcomings. Goodwin and Hughes (2008) concluded that the rheological parameters did not depend on the size of the metal particles, but on the volume fraction of the particles.