INFLUENCE OF VARYING H2S CONCENTRATIONS AND HUMIDITY LEVELS ON ImAg AND OSP SURFACE FINISHES

Corrosion impacts electronic systems by attacking boards or individual components. Of particular concern is corrosion of the metallization on printed wiring board assemblies due to attack from sulfur-containing species, most notably sulfurous gases. Sulfurous gases are emitted by a diverse range of processes, ranging from paper and pulp bleaching to the warming of clay used in industrial modeling facilities. However, the impact of varying sulfur concentrations and humidity levels on corrosion needs further examination. In this study, corrosion induced by exposure to H2S gas is examined for copper printed wiring board metallizations coated with Immersion Silver (ImAg) and Organic Solderability Preservative (OSP) surface finishes at different levels of humidity and H2S concentration. Optical images of the boards revealed that boards with the OSP surface finish had more signs of copper corrosion than boards with the ImAg surface finish. These images also revealed that corrosion on the boards did not stop after 3 days of testing since boards exposed for 10 days had more signs of corrosion than boards exposed for only 3 days. Optical images indicate that ImAg is more sensitive to sulfur concentration than to relative humidity, while OSP is more sensitive to humidity. Uniform corrosion of the ImAg surface was observed with no sign of creep corrosion or dendrite formation. Pure copper coupons were also subjected to the corrosive tests; the weight gain of the copper coupons indicated a constant rate of corrosion over the test duration.