An experimental study of the variation of wettability of SMDs using the micro-globule wetting method

Surface mount devices (SMDs) from 10 different sources have been tested using the globule/balance or micro-wetting method; they included both chip capacitors and resistors. Wettability indices measured include incubation time, eventual force normalized by theoretical maximum force, the wetting rate and Schouten´s figure of merit. These indices enable a comparison of both the wettability and the variation in the wettability between suppliers. The variability is an important criterion not previously investigated. Variability in wetting is directly responsible for significant classes of solder defects because it cannot be factored out by tuning the process parameters during reflow. A more soundly based measure of wettability, the wet rate, shows discrimination between different batches similar to that of other indices. It is related to, but distinct from, the speed at which solder moves up the component metallization at quasi-steady state after wetting has begun but before end-effects associated with the geometrical extent of the metallization are reached. Schouten´s index is shown to be misleading. The effects of metallization geometry on the maximum equilibrium force are discussed in the light of experimental results