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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.
Device and Materials Reliability, IEEE Transactions on (Volume:10 , Issue: 1 )
Date of Publication: March 2010