Letter 079

Alternatives to gold for contact surfaces

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I would like to replace the gold plating of an electrical spring contact with another material to save money. Currently we use two gold plated surfaces that are electrically connected by a gold wire spring which makes surface contact with the two gold pads. The spring is in place to compensate for expansion and contraction driven by temperature changes. I really do not know of any need to replace the spring but would like to replace the plating on the two contact surfaces. The processing problems associated with plating the gold on the contact surfaces is the main driver for change. Currently they are brush plated and this will not facilitate the production volumes anticipated. The intent of the package is to function for a minimum of twenty years and will be hermetic. The current subcontractor for the two contact surfaces has suggested replacement by simple solder plate (Pb/Sn). I am concerned that this would be unreliable due to migration or dendrites or something. I have never seen Pb/Sn used as a contact surface. Could carbon be used when applied by a thick film process? Would Palladium work? Would it be any cheaper? Any other ideas?

Gordy S
Honeywell Inc.

IC's with tin or solder plated leads (I don't know how much lead they use) are commonly used both in zero-insertion-force sockets and conventional spring sockets, even for low-voltage DC circuits. On the other hand I've seen circuit boards with tin or solder plated fingers used in control systems for plating machines, and the experience was miserable. In a plating shop environment, it simply doesn't hold up; I can't count how many times I pulled boards out and reinserted them so that the scraping action would present a clean contact surface so they would work again.

I suspect that if the contacts are sealed, and the application is not very low voltage, that it probably would work for 20 years. But this is based on the anecdotal evidence of my very limited observations, and I hope that somebody who really knows what they're talking about pipes in.

As for paladium, that should be a satisfactory substitute for gold. AT&T Nassau Metals division used to market it, but with the constant changes in corporate structures these days, I wouldn't know offhand how to locate the supplier.

Ted Mooney, P.E.
finishing.com
Brick, New Jersey