Investigation on the effect of different components of no-clean flux chemistry on coating performance for printed circuit boards

In the present work, the effect of different components of no-clean flux systems on the performance of a two-component Polyurethane (PU) coating was investigated. A Surface insulation resistance (SIR) pattern with interdigitated electrodes was used a test substrate. In order to investigate flux chemistry effect, the SIR pattern was contaminated prior to coating with different no-clean solder fluxes with and without resin component, high and low acid value, and different solvent (alcohol and water) in order to evaluate the effect of different parts of flux chemistry on coating performance. Additionally, the influence of resin was systematically investigated together with a Weak organic acid (WOA) as activator component. For testing, specimens were exposed to constant humidity (95%) and temperature cycling (40 ° C-65 °C), while Electrochemical impedance spectroscopy (EIS) was used for monitoring moisture transport, and DC potentiostatic measurement as accelerated testing for triggering failures. Moisture absorption properties of coating were evaluated using gravimetric method. Hygroscopic nature of solder fluxes was carried out by EIS with changing humidity conditions. Scanning electron microscopy (SEM) and Fourier-transform infrared spectroscopy (FT-IR) were implemented for inspection of the coating/substrate interface and any potential chemical degradations. The study revealed flux residues on the PCB surface as the major factor determining the reliability of the test PCBs under humidity exposure. No failures and good electrochemical performance was found when alcohol-based fluxes were utilized prior coating application, especially with the addition of resin, while water-based fluxes resulted in failures. The same outcomes were observed after application with Poly(ether-ester) coatings. • Components in No-clean flux chemistry determine coating performance. • Fluxes with higher acid content found to be worse for conformal coating performance regardless type of solvent base. • Presence of high-acid fluxes caused greater delamination for coating/substrate. • For Alcohol-based fluxes, addition of resin enhanced performance to a level similar with a clean board with or without applied coating. • Coating on PCBs with flux residues containing resin showed better performance and adhesion. • Tests using resin and WOAs activator showed that resin could mask effect of activator by encapsulation avoiding direct contact between humidity and activator component. Effect was found to be a function of relative amounts of activator part and resin.