Observations of solder paste reflow by means of electrical measurements

This paper presents a method for exploring the changes occurring at the surfaces of solder particles during the reflow soldering process. The method involves measuring changes in electrical resistance of a sample of paste as a function of test voltage. The results are used to estimate the size and nature of electrical contact spots between the particles, and how these depend on temperature and time. The activation energy of the process responsible for increasing the size of contact spots is deduced for RA and RMA type fluxes and it is shown that sintering is not the dominant mechanism for increasing contact size. These results, together with a programme of CFD studies are expected to help improve solder paste formulations.