No-Clean Solder Flux Chemistry and Temperature Effects on Humidity-Related Reliability of Electronics

The use of no-clean flux technology for the wave soldering process of a printed circuit board assembly (PCBA) influences the humidity-related robustness of the electronic devices due to the ionic residues remaining on the PCBA surface after soldering. This paper investigates the effect of various no-clean solder flux chemistries on the formation of a water layer on the PCBA surface studied as a function of climatic conditions relevant for the electronics industry. The activating part of the commercial flux formulations used for the investigations was based on single weak organic acid (WOA) and WOA mixtures. The hygroscopic nature of flux residues, reflecting their ability to uptake moisture and lead to water film build-up, was investigated using the gravimetric moisture sorption test and AC electrochemical impedance. The effect of flux composition and ambient climatic conditions on the corrosion reliability was studied using DC leakage current measurements upon varying potential bias, humidity, and temperature levels. The study shows that solder flux residues containing a mixture of WOA activators in their formulation facilitate the formation of water layer on the PCBA surface at lower humidity levels, compared to the residues composed of a single activator, and absorb a higher amount of moisture. Corrosion occurrence is accelerated by the presence of highly hygroscopic residues of WOA mixtures, compared to significantly less corrosive contaminants comprised of single activator. Increasing ambient temperature enhances moisture interaction with flux residues and changes its deliquescent behaviour, leading to the formation of a thicker water layer and severe ion-induced corrosion at lower humidity levels.