Study of Interaction Between Reflow Solder Flux and Humidity in Relation to Failures in Electronics

This article shows the humidity interaction between different reflow solder flux chemistry and the effect on corrosion-driven failures using a custom-designed test printed circuit board assembly (PCBA), which simulates a dummy "component" mounted at a known standoff distance from the base PCB. Five reflow solder flux systems were exposed to a standardized harsh climate profile after reflow soldering on the PCBA, and changes in surface insulation resistance (SIR) were measured during exposure. Trends in SIR measurements were correlated with other properties of the flux residue such as moisture absorption, released ionic contamination, and activator chemistry. Results showed that the higher the flux activity (driven by both the overall quantity of weak organic acids used and the proportion of more active glutaric acid used as an activator), in combination with higher moisture-absorbent polymer tackifier compounds (like poly(isobutene:isoprene) and synthetic polyterpenes that spread and "opened up" more to release contained activator compounds) resulted in lowered SIR values. This study provides an insight into the climatic reliability-based performance of different flux systems based on the properties of various chemical constituents by developing a methodology of solder paste flux analysis.