Electrochemical characterization of plasma coatings on printed circuit boards

Printed Circuit Boards (PCB) are the backbone of electronic devices and communication technology. The corrosion of copper tracks in PCBs is the main reliability issue in case of exposure to humidity. To overcome this, several surface finishing methods have been introduced in the industry such as plasma coatings. In this study, several electrochemical characterisation methods are explored on blank and plasma-coated PCBs to identify reliable measurement techniques and to get insight in the protective properties of the coatings on an overall macroscopic visual scale, a dedicated surface-average scale, as well as on a localised microscopic scale. The macroscopic visual corrosion behaviour was evaluated by standard salt-spray exposure tests. Odd Random Phase multisine Electrochemical Impedance Spectroscopy (ORP-EIS) provided insight into the average barrier properties of the coatings. Scanning Vibrating Electrode Technique (SVET) and Scanning Kelvin Probe Force Microscopy (SKPFM) were used to evaluate the samples’ reactivity on a local scale. Scanning Electron Microscopy-Energy Dispersive X-ray (SEM-EDX) analysis was used as a complementary surface analysis method to characterise the coatings. Blank PCBs were studied as reference substrates, and their behaviour was compared to three plasma-coated PCB variants, based on fluorinated organic precursor chemistry. The salt-spray tests illustrated the protective behaviour of the coatings but did not allow to differentiate between the different coatings. With ORP-EIS, it was observed that the coatings act as a barrier against the electrolyte. This is confirmed on the local scale, where no activities were seen on the surface of the coated samples using SVET, except after 10 days of immersion when single layer plasma coatings showed higher activities. SKPFM revealed much higher surface potentials for the coated PCBs compared to the blank PCB allowing to differentiate between the various coated samples.