Water film formation on the PCBA surface and failure occurrence in electronics

The widespread use of no-clean solder flux technology compromises the corrosion reliability of electronics exposed to humid conditions, and can lead to degradation of the
device’s lifetime due to the presence of solder flux residues on the Printed Circuit Board Assembly (PCBA) surface after the soldering process. In this work, the effect of hygroscopic flux
contamination was assessed in terms of facilitating the water film formation and corrosion on the PCBA surface under varying humidity and temperature conditions. The hygroscopicity of flux residues was evaluated and discussed as a function of flux chemistry and climate conditions for three residue types: adipic acid, glutaric acid, and a binary mixture of adipic:glutaric acids (1:1 ratio). The climatic testing was performed under the relative humidity (RH) conditions varying from 60 % to ~99 % at 25° C and 40° C using gravimetric water vapour sorption/desorption and AC electrochemical impedance methods. The corrosivity of flux residues was evaluated through the DC leakage current measurements performed across the interdigitated surface insulation resistance (SIR) comb patterns. The results show that the extent of water layer formation strongly depends on the hygroscopic nature of flux residue, which originates from its chemical structure and temperature. The corrosion occurrence is determined by the residue hygroscopicity and solubility in water. Temperature increase strengthens the residue-moisture interaction, accelerating the formation of water film and corrosion occurrence on the PCBA surface.