The Influence of $\hbox {H}_{2}\hbox {S}$ Exposure on Immersion-Silver-Finished PCBs Under Mixed-Flow Gas Testing

This study focuses on the corrosion of immersion silver (ImAg)-finished copper land patterns on printed circuit boards (PCBs) under H₂S exposure. Eight test conditions were examined with varying levels of H₂S, temperature, relative humidity, and exposure time. The results indicated both direct chemical-reaction corrosion and electrode-reaction corrosion, particularly galvanic corrosion. H₂S gas was a stronger driving force for corrosion on ImAg-finished PCBs than SO₂ gas. Temperature had a significant influence on ImAg-finished surface PCBs. Tests found extensive corrosion on ImAg-finished PCBs at 40??C, even in very low humidity. On ImAg-finished surfaces, the corrosion was nonuniform during the early period of exposure, with the corrosion modes being mainly pitting, open mouth, and particles. The corrosion products at this early stage mainly included Cu₂O and Ag₂S. As time progressed, the corrosion products mainly included Ag₂S, Cu₂S, CuS, and CuO. These formed a passive film on the surface. ImAg-finished PCBs are vulnerable to H₂S gas; nonuniform severe corrosion was found to occur at defects on the ImAg surfaces. The mixed-flow gas test produced creep corrosion on ImAg-finished PCBs using only H₂ S gas. Dendrite corrosion products growing from an edge with a solder mask were generally longer than those growing from an edge without a solder mask. The corrosion products of these dendrites mainly included Cu₂S or CuS.