The process of applying a solder thermal interface material typically includes applying flux to a solder preform and reflowing it in a sandwich between two substrates. When a preform comes flux coated, its a no-brainer how to apply the flux to the preform. ItҒs already done for you.
There are so many great types of flux available for specific applications, that you might want to use a specialty flux formulation matched to the substrates you are soldering to and the reflow temperatures you will be running. This flux formulation is commonly a Tacflux® and the key to good flux application is not to overdo it. A little bit goes a long way.
This past week I ran a soldering experiment in which I was soldering a 1 preform of 121 alloy (96.5Sn3.5Ag) to copper and wanted to use TacFlux® 023 because it is specifically formulated for soldering lead free solders to standard soldering metallizations such as copper. I knew that in the soldering process, the flux needed to make contact with the entire substrate I was soldering to, but I didn't know how far out the flux would wet or how much flux I should apply. I ran a few trials to test out various dispensing methods and amounts. What I found in the end was that one dot of flux dispensed onto the center of the substrate was all that was needed to remove the surface oxides and wet the solder over the entire surface