Process related contaminations causing climatic reliability issues

Some level of solder flux residue is inevitably found on electronics no matter whether the Printed Circuit Board Assembly (PCBA) manufacturing is carried out by hand, wave or reflow soldering process. The current use of no-clean flux systems should in principle only leave benign surface contaminants during the wave and re-flow soldering process; however variation in temperature on the PCBA surface during soldering can result in considerable amounts of active residues being left locally. Typical no-clean flux systems used today consist of weak organic acids (WOA) and active residues left on a PCBA are a potential climatic reliability risk for electronic devices. Reflow soldering is the process employed for PCBAs with surface mount components. No-clean flux is applied in a solder paste via a stencil followed by placing of components and PCBA passage through a soldering oven. Although a reflow soldering process leaves lower amounts of flux residue than a wave solder process, the morphology of the flux residue from re-flow is also different. Re-flow flux residue on the PCBAs is concealed initially due to the presence of a binder and resin part of the flux, therefore WOAs are not immediately released. Nevertheless, upon exposure to high humidity for prolonged intervals our results show that an opening of the flux residues will release WOAs and thus promote leakage current issues and corrosion processes on the PCBA surface. This paper summarizes our investigations of the release of WOAs from reflow solder paste (malic, adipic, succinic, and glutaric acid) and its effects on leakage current and corrosion of Sn and Cu. Leakage current due to flux residue was investigated using a localized cleanliness test system C3 (Foresite Inc., USA). The system extracts residue contaminants to a vial using steam purging to a localized area. Leakage current through the extracted solution is measured using a standard dual copper electrode pattern. Corrosion behavior of Sn and Cu are investigated using polarization experiments using a novel localized cell with solutions of malic, adipic, succinic or glutaric acid. Local release of WOAs on the PCBA from the flux residue was analyzed using a gel method with pH indicator. Extracted solution from re-flow solder flux residue can be chemically analyzed using ion chromatography (IC). Morphology of solder paste after exposure to humidity was analyzed using optical microscopy and scanning electron microscopy. Results revealed significant effect of flux residues and WOA on the corrosion at concentration levels usually found on PCBA surface.