Viscosity of a no-flow underfill during reflow and its relationship to solder wetting

There is much interest in understanding viscosity changes of a no-flow underfill during reflow. Viscosity changes that result from the onset of cure provide resistance to the formation of solder interconnections, and can result in a decrease in yield. The difficulty has been tracking viscosity changes during temperature profiles that vary as much as 4 degrees/second. We used microdielectrometry to make real-time ion viscosity measurements of various no-flow underfill formulations during a standard reflow temperature profile. The results clearly show the initial viscosity drop as temperature ramps up, and the point at which the effect of cure dominates and causes viscosity to rise. The data are related to DSC analysis and to solder joint yield. Overall, the results show how viscosity changes during the reflow process can potentially have a dramatic effect on assembly yield rates. The information gained is useful for developing a reflow process to form reliable solder joints