Page 18 - 2019 How-to-Specify E-Book
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in the spring material at temperature, and stress relaxation of the spring material. The second failure mode is insulating film formation on the contact material surface, which typically results in a complete barrier to conduction across the separable interface. Films may form due to the outgassing of plastic housings or other adjacent volatile materials, or via the diffusion and subsequent oxidation of non-noble metals on the contact surface. The common effect is an unacceptable rise in contact resistance. Reductions in contact force lead to reductions in the real area of contact and, as such, a rise in contact resistance. Mechanical stability is also affected by such reductions and may lead to intermittent interruptions.
Connector Spring Materials
The majority of connector spring materials are copper based due to the inherent conductivity of copper. Strength levels are achieved by work hardening: a combination of solid solution and work hardening or precipitation hardening. The strengthening mechanism and the degree of alloying determine the upper temperature application limit. High copper alloys with less than 5% alloying content tend to rapidly lose strength at elevated temperatures. For example, pure copper (Cu) will fully anneal in as little as 15 seconds at 315°C. During short-term exposures, C51000 phosphor bronze starts to demonstrate reduced normal force at 100°C, while more highly alloyed materials, such as C72500 (Cu-Ni-Sn), demonstrate only a moderate reduction at 200°C. Both materials are sensitive to stress relaxation during long-term exposure.
Precipitation-hardened copper alloys such as C17200 (beryllium copper or Alloy 25), C17410 (mill-hardened beryllium copper or Alloy 174), C7025 (Cu-Ni-Si-Mg), C7026 (Cu-Ni-Si), and C72900 (Cu-Zn-Sn-Fe-Pb-Mg-Mn- Ni-Nb) retain normal force at elevated temperatures and also demonstrate resistance to stress relaxation during long-term exposure to 200°C. However, both oxidation and strength reductions are concerns for temperatures above 200°C. Non-copper materials, such as stainless steel, are noted for their high strength and oxidation resistance and should be considered for spring material designs operating above 200°C.
Connector Contact Materials
Commonly used connector contact materials are applied to the spring material by electroplating or inlay cladding processes. In general, electroplated materials are limited to lower operating temperatures than clad metal inlays due to the inherent microstructure of the two materials. Electroplated materials tend to have extremely fine grains when compared to clad contact materials, and fine-grained materials have more grain boundary area and are therefore prone to grain boundary diffusion at elevated temperatures. The structure of cobalt- or nickel-hardened gold (Co-Au or Ni-Au) is a complex of gold grains surrounded by an organometallic material within the grain boundaries. These grain boundaries are much wider when compared to those in unhardened deposits. In fact, diffusion rates along grain boundaries in hard gold (i.e., gold with a Knoop hardness of 120–300 and a purity of 99.7–99.9%)2 approach those measured on free surfaces limiting the use of hard gold to temperatures below 125°C.
 Connector contacts are electroplated or inlay clad with various materials to achieve application-specific performance characteristics, including robust resistance to high-temperature operation over long periods of time.

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