This paper studies the creep corrosion of immersion silver (ImAg) finished printed circuit board (PCB) under high sulfur environments. Clays containing sulfur were heated with water to simulate high sulfur environment and to drive creep corrosion on ImAg finished pads. Except the silver sulfide film formed on ImAg finish, creep corrosion began by growth of dendrites on pad edges, which was caused by galvanic corrosion. Copper sulfide and silver sulfide were the main components of creep corrosion products. The different coverage extent of ImAg finish on pad edges caused different reaction speed, the morphology and the creep distance of corrosion products, and different content of copper sulfide and silver sulfide in corrosion products. The impact of creep corrosion products on surface insulation resistance (SIR) was measured. Weibull distribution with two parameters was used to analyze the length of dendrites and to quantitatively evaluate the corrosion resistance of ImAg finish. The methods to improve the reliability of ImAg finished PCB was also discussed in the paper.

Keywords: immersion silver, creep corrosion, sulfur, surface insulation resistance, Weibull distribution

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