Engineering solder paste performance through controlled stress rheology analysis

The rheology of solder paste significantly affects the qualities of stencil printing, tack and slump performance. This paper describes a series of tests performed on solder paste to investigate and determine the rheological properties of a group of solder pastes and fluxes, and the correlation of those properties with paste performance prior to reflow. Data indicate that: the incidence of print defects are proportional to the material’s compliant qualities (J1 and J2) and are inversely proportional to the elastic properties (G¢/G¢¢ and recovery) and meta‐rigidity (yield stress); slump resistance is proportional to elastic properties (recovery), solid characteristics (stress [G¢ = G¢¢]), and rigidity (|G*|); and that high elastic properties (recovery), low compliance (J1 and J2), and low solid characteristics (stress [G¢ = G¢¢]) are required in order to achieve high tack value. Good correlation between fluxes and solder pastes were observed for yield stress and recovery only, suggesting that those two properties are primarily dictated by fluxes.