Assuming you are talking about solderable surfaces:
People specify various materials to protect the solderability of the pads on the board. Gold, actually, Electroless Nickel - Immersion Gold [ENIG], is popular. * Board fabricators like it because they get rich. * Assemblers like it because it has a �flat surface�. Flat surfaces are important when assembling complicated boards. [And they like complaining about poor solderability. And they�re too lazy to evaluate imm tin and imm silver.]
The Merix site [ http://merix.com ] has a pretty good comparison of various materials used in protecting the solderability of board surfaces.
Assuming you are talking about wear surfaces [yano, edge connectors, button contacts, etc.]: Search for discussion on �fretting corrosion� in a recent �OnBoard Forum� hosted by Jim Zanolli from Teka.
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