FLUXES WITH DECREASED VISCOSITY AFTER REFLOW FOR FLIP-CHIP AND SIP ASSEMBLY

A series of flux systems have been developed which would result in a reduced viscosity after reflow. This enables a high viscosity, high tack flux to be used to secure components at the component placement and reflow stage, but ends up with a low viscosity flux residue after reflow, thus facilitating the flux residue to be cleaned. A technique for forming such special fluxes is to establish a temporary association force within the materials themselves, such as an acid-base association. This kind of association force can increase the apparent molecular weight and cause material viscosity to increase. After a heating process, one of the critical ingredients was evaporated, thus eliminating the association force, causing a decrease in the apparent molecular weight, and consequently a decrease in viscosity or an increase in mobility. The evaporation of one ingredient can be the result of one ingredient having a lower boiling point, or the decomposition of one ingredient during heating. A strong association force is desired to allow this acid-base combination approach to work. In this work, the volatile ingredient approach was less effective than a decomposable ingredient approach, presumably due to the formation of a bigger association cluster from the decomposable ingredient. Accordingly, the decomposable ingredient was the best approach to lower flux viscosity after reflow.