The risk and solution for no-clean flux not fully dried under component terminations

The miniaturization trend is driving the industry to adopt low standoff components or components in cavity. The cost reduction pressure is pushing the telecommunications industry to combine component assembly and electromagnetic shields in one single reflow process. As a result, flux outgassing/drying is getting very difficult for devices due to poor venting channels. This results in insufficiently dried/burnt-off flux residue. For a properly formulated flux, the remaining flux activity poses no issue in a dried flux residue for a no-clean process. However, when venting channels are blocked, not only solvents remain, but also activators which could not burn off. The presence of solvents allows mobility of active ingredients and the associated corrosion, thus posing a major threat to the reliability. In this work, a new halogen-free no-clean SnAgCu solder paste, 33-76-1, has been developed. This solder paste exhibited a SIR value above the IPC spec 100 MΩ without any dendrite formation, even with a wet flux residue on the comb pattern. The wet flux residue was caused by covering the comb pattern with a 10 mm × 10 mm glass slide during reflow and SIR testing in order to mimic the poorly-vented low standoff components. The paste 33-76-1 also showed very good SMT assembly performance, including QFN voiding and HIP resistance. The wetting ability of paste 33-76-1 was very good under nitrogen. For air reflow, 33-76-1 still matched paste C, which is widely accepted by industry for the air reflow process. The good performance on both non-corrosivity with wet flux residue and a robust SMT process can only be accomplished through a breakthrough in flux technology.