"gold/nickel galvanic couple"

Question:

"I am looking for a compelling reason not to utilize a nickel plated aluminum shield in direct contact with an immersion gold plated area on a printed circuit board. The estimated thickness of the gold plating is 3-5 microns.The underplate is nickel.Environment is outdoor use. The unit is moisture resistant, not hermetic. I would appreciate any input. We have cast aluminum shields for RF that are nickel plated. We had been receiving PC boards that were immersion gold over nickel except in the shield contact area. These areas were stripped of the gold to expose the nickel, creating a nickel/nickel interface. Our vendor will no longer strip the gold from these areas. The immersion gold is required elsewhere as the board must be solderable. Our board cleaning process is very good, (<2 ug NaCl/sq in), regarding salt residue. The environment is outdoor and worldwide. The nickel plating is considered passive moving it closer to gold on the galvanic scale. Our concern is regarding the potential for galvanic activity between the nickel plated shield and the gold contact area on the PC board. There is no significant difference in area between the shield and the PC board. We don't anticipate any severe degradation regarding the fact that some of the nickel plating may corrode. Just a few more questions if you don't mind... Can the immersion gold layer be virtually ignored due to it's thickness, (2-7 u inches)? Is there any known level of salt concentration that would initiate agressive galvanic action?"

Answer:

Aluminum is at opposite poles with respect to Gold on the galvanic scale. Depending on the severity of the environment, and other paramenters, such as the surface area ratio between the two components, the corrosion rate of aluminum, with respect to aluminum not in electrical contact with Gold, could be substantial. As you suggest, the nickel/gold couple reduces the potential driving force, compared with an aluminum/gold couple. Electroplated nickel is also reasonably corrosion resistant to atmospheric environments except possibly those high in sulfur compounds and should provide satisfactorily service, if the plating is sound and nonporous. If I understand you correctly, even if the nickel is breached in places, some finite corrosion of the RF shield should not compromise its effectiveness? As long as there is an electrical couple that promotes a driving force in the direction of the nickel, I don't believe that you can ignore the effect of the gold plating. Remember, though, an electrolyte has to be present also. I don't know of a "minimum" level of (aqueous) salt. The aggressiveness of the galvanic couple will be a function of the conductivity of the solution and the area ratios of the coupled components. If you want quantitative information, consider salt fog testing.

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