TE - White Papers

soft, malleable, easily displaced, and conductive. There are various anti-oxide coatings that can be applied to minimize the formation of these oxides, but they cannot be eliminated altogether, so mitigating the effects of these oxides falls to the experience of the connector designer. So what happens when oxides form on non-noble platings? The biggest degradation mechanism affecting tin and, to a much lesser extent, silver is fretting corrosion. Fretting corrosion occurs through micro-motions on the contact interface and can be thermally or mechanically induced. Where these micro-motions cause problems is in oxide formation. To use a medical analogy that all of us are familiar with, when a micro-motion induced movement occurs, it opens a fresh "wound" in the plating surface. As expected, the wound eventually "scabs" over with a non-conductive oxide layer. As shown in Figure 5, the problem occurs when a micro-motion induced movement moves the contact interface back over the oxide "scab." The contact normal force partially re-opens the scab so the interface becomes a higher resistance mix of oxide and metal contact areas. In the meantime, the area that the interface just moved from also forms an oxide "scab." Micro-motion again moves the contact on top of this oxide layer. With each successive micro-motion, the oxide "scab" layer builds while the conductive metal contact areas diminish resulting in increasing interface resistance. In a signal application, the resistance eventually builds to the point where the connector no longer conducts the signal. In a power/current-carrying application, the results are much more dramatic since increased interface resistance results in increased joule heating of the interface and contact. The increased heating of the contact eventually builds to the point where it anneals the contact material and gradually reduces the normal force exerted at the interface. With decreasing normal force comes increased resistance that further increases the heating at the interface. Eventually, this vicious heating cycle will cause the interface to catastrophically fail. Mature connector suppliers are well aware of this phenomenon and take steps in their manufacturing and plating processes to minimize the fretting corrosion that leads to this failure mechanism.

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