these types of environments. A connector’s plating is a crucial element in long life as well as its ability to handle shock and vibration and resist corrosion. CORROSION Copper will oxidize and create contact resistance, so it needs to be finished with another layer, such as tin, gold, palladium, or silver (for very high temperatures). Tin is inexpensive and highly conductive, but it can be subject to fretting corrosion [1] and tin whiskers which can lead to dangerous conductivity [2]. Fretting corrosion occurs with vibrations over time. These micro movements can lead to oxidation where the tin is making a connection, and ultimately lead to a failed connection (specifically, an oxide forms, breaks, and then eventually corrodes the connector away). This can be avoided with lubrication, but tin is typically not used on contact points for high-performance connectors.
The next best plating choice is gold, which is one of the best conductors of electricity. It is a soft metal, so it is often hardened into an alloy with an element such as cobalt. Unfortunately, copper can migrate through gold within a year or even months. The industry standard is to use a layer of nickel between the copper and the gold to prevent copper migration. Nickel is desirable because of its resistance to wear and conductivity [3]. To complicate things, harsh or high temperature conditions can speed up the oxidation of copper. This can be addressed with thicker nickel layers or thicker gold layers. Gold plating in the range of 10 micro inches is very porous, and, while it looks continuous, the pores can provide a way for the base metal to oxidize. Increasing the gold thickness to 30-50 micro inches can greatly improve corrosion resistance [4]. For instance, in its Extended Life Product (ELP) version of the Tiger Eye connector, Samtec uses up to 30 micro inches of cobalt gold over a minimum of 50 micro inches of sulfamate nickel over a copper alloy substrate (Figure 1). For any other application, in standard conditions, 10 micro inches of gold would be sufficient. Some new extended life applications are requesting 50 micro inches of gold plating.
DESIGNERS MUST CONSIDER THE
MATERIALS SELECTED IN THE CONNECTOR DESIGN PROCESS TO PROTECT AGAINST SHOCK, VIBRATION, AND CORROSION, AND WEIGH THE TRADEOFFS IN TERMS OF PRODUCT LIFECYCLE AND COST.
Figure 1: Cobalt gold contacts are plated over sulfamate nickel and BeCu in the Samtec Tiger Eye product. This product features three points of contact and is qualified to 2,500 mating cycles [5].
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