Nanoscale monitoring of the initial stage of water condensation on a printed circuit board

Abstract

Electrochemical migration is a critical factor contributing to failures in electronics due to humidity. When moisture accumulates on conductor-dielectric-conductor systems under bias voltage, electrochemical processes can be triggered, leading to the growth of metallic dendrites that may ultimately result in system failure. Despite its significance, many aspects of electrochemical migration remain unresolved, particularly regarding the physical characteristics of liquid buildup that facilitate dendrite growth and short circuit currents. While there are a few techniques that can measure water adsorption on the nanoscale, most conventional methods focus on water droplets within the size range of visible light wavelengths. In this study, we implemented a combined electrical-optical-ellipsometric measurement on FR-4 printed circuit boards featuring Sn surface finishes. Our experimental setup allowed for the measurement of water condensation across a wide range of thicknesses, while simultaneously monitoring the solder mask and metal electrodes during cooling. The ambient temperature of 25∘C and a relative humidity of 60% were constant during the measurement. By employing this approach, we elucidated the mechanisms of dendrite formation and short circuit currents, demonstrating that the water film remains continuous between droplets on the solder mask surface. Compared to Sn the nucleation was delayed on the solder mask with a larger surface coverage at smaller thicknesses. This comprehensive methodology provides crucial insights into the electrochemical migration process, enhancing our understanding of the underlying phenomena that contribute to electronic failures due to humidity. Our work highlighted the complementary nature of ellipsometry and optical imaging.

Keywords

• Electrochemical migration
• Sn surface finish
• Condensation
• Ellipsometry
• Water adsorption