Effect of PCB Manufacturing Process Step-Related Cleanliness on Performance of Conformal Coating

In the presented work, the accumulative effect of different manufacturing steps of printed circuit board (PCB) on conformal coating performance is evaluated. An interdigitated comb pattern on an FR-4 board was used as a test PCB. Manufacturing processes included base test PCB produced by three different manufacturers (Man) incorporating copper-clad lamination (CCL) and hot air solder leveling (HASL) steps. In addition, test boards underwent typical wave solder and selective mini-wave steps. Commercial co-polymer polyurethane/polyacrylate and urethane acrylate conformal coatings were applied on test boards before as well as after soldering process. The study aims to evaluate induced contamination on the PCB surface after these manufacturing steps and the effect of PCB surface cleanliness on the protection performance of conformal coating under humidity. The results revealed chloride residues prior to soldering on the PCB surface with variations across Man-1 (0.2μg/cm²), Man-2 (0.4μg/cm²), and Man-3 ($0.8μg/cm²). In surface insulation resistance (SIR) measurements under humidity exposure, Man-3 exhibited 100% failure caused by dendrite formation, with resistance levels consistently over a decade lower than Man-1, highlighting the quality of the base PCB materials as a major factor for humidity-related issues. Equal importance was found regarding different wave soldering methods and coatings. However, the initial contamination present dominated over subsequent manufacturing steps with the highest chloride contamination resulting in up to one decade difference depending on the flux and coating chemistries.