Humidity-related failures in electronics: effect of binary mixtures of weak organic acid activators

Abstract

The ionic residues of activator compounds used in the no-clean solder flux systems often remain on a Printed Circuit Board Assembly surface after the soldering process and may compromise the corrosion reliability of electronic device upon exposure to humid environment. The solder flux formulations contain weak organic acids (WOAs) activators of different types, sometimes present in the form of mixtures of varying compositions. This paper presents the results of parametric hygroscopicity studies of binary mixtures of weak organic acids that simulate the activating part of the typical no-clean solder flux formulation. Three types of binary blends of varying ratios between the acids were tested: adipic:succinic, adipic:glutaric, and succinic:glutaric. The hygroscopic and corrosive behaviour of the mixtures were investigated under relative humidity (RH) varying from 30 to ~ 99% and at test temperatures of 25 °C, 40 °C, and 60 °C. The moisture sorption and desorption isotherms were determined using a gravimetric method, while AC electrochemical impedance was employed for monitoring the solid–liquid transition of residues. The influence of residues on corrosion reliability was investigated using DC leakage current measurements on a contaminated surface insulation resistance comb patterns with tin solder alloy. The results of corrosion testing were correlated with hygroscopicity studies and visualized by ex-situ colorimetric analysis using a tin ion indicator in a gel. The results show that the critical humidity level for moisture absorption shifts to lower RH values for mixtures, compared to pure WOAs. Water absorption behaviour varied based on the relative content of WOAs in the mixtures and ambient temperature, which also influenced the leak currents and electrochemical migration.