Electrochemical Migration Issues Related to Improper Solder Mask Application

Abstract

This work aimed to analyze a reliability issue that occurred on printed circuit boards (PCB) primarily designed to study electrochemical migration (ECM) phenomena. The test boards were supplied directly from the PCB producer, and the copper traces were covered with a hot air solder leveling (HASL) surface finish. However, the solder layer was clearly inconsistent and poor, caused by contamination from improper solder mask application, as was confirmed by analysis using the scanning electron microscope. On these boards, a water drop test with distilled water (bias voltage of 10 V) and thermal humidity bias test (85 °C/90% R.H./25 V/168 h) was conducted to evaluate predisposition for electrochemical migration of boards with a such poorly fabricated solder mask. PCB without solder mask and with correctly applied solder mask was also included in this study for comparison. The results clearly showed that the test boards with the poorly fabricated solder mask were significantly more inclinable to electrochemical migration – in the case of the water drop test, the forming dendritic structures shorted the electrodes up to 6 times faster than on PCB without a solder mask, while the samples with correct solder mask exhibited the best resistance against ECM. During the thermal humidity bias test, the electrical short appeared after only 2 hours on PCB with the bad solder mask compared to PCB without the mask, where the dendrites grew after more than 27 hours. Energy dispersive spectroscopy confirmed that the migrating element was tin from the HASL cover layer.