The influence of solder mask and hygroscopic flux residues on water layer formation on PCBA surface and corrosion reliability of electronics

The presence of solder flux residue on the Printed Circuit Board Assembly (PCBA) surface compromises the corrosion reliability of electronics under humid conditions and can lead to degradation of the device’s lifetime. In this work, the effect of solder mask morphology and hygroscopic residues were studied towards assessment of their influence on the water film formation on the PCBA surface.

The in-situ observations of water layer build-up was studied on the solder mask substrates as a function of surface finish and residue type (adipic and glutaric acids). The effect of solder flux residues was described in terms of their varying hygroscopicity defined by chemical structure and test temperature.

The climatic testing of two acids was performed under relative humidity (RH) conditions varying from 30% to ~99% at 25˚C and 40˚C using gravimetric water vapour sorption/desorption and electrochemical impedance methods. The corrosivity of WOAs was evaluated via leakage current measurements using surface insulation resistance (SIR) comb patterns.

The corrosion studies were correlated with the hygroscopicity studies. The results show that the water layer formation depends on the PCBA surface topography and the type of post-production residue present on the surface. Moreover, the hygroscopic nature of typical flux residue depends on its chemical structure and temperature.

An increase of temperature shifts the critical RH level for water vapour absorption towards lower RH range accelerating the formation of well-conductive electrolyte and the occurrence of ionic-induced electrochemical migration (ECM).