Nickel Underplates: Then and Now

By Dr. Bob Mroczkowski | August 01, 2012

By Max Peel, Contech Research

August. 7, 2012

Over the last few years, I have talked to a number of engineers with less than five years’ experience in the connector field. These engineers were replacing the “gray beards” who were retiring to the local golf course or other such activities. What surprised me was the lack of knowledge these newcomers had on basic connector technology. I want to revisit the pertinent, basic issues that have been in place for the last 40 years, and why they are important in establishing acceptable connector performance and reliability. The first of these is the use of a minimum of 50 microinches of nickel underplate.

During the late ’50s and early ’60s, silver was the common underplate used for contacts. However, it was quickly found that silver had migration and tarnishing issues. By the mid ’60s silver was eliminated in favor of copper as the underplate.

Copper was also found to have a dark side in the form of major diffusion issues. Copper diffusion through gold was rapid, allowing copper corrosion products to form on the contact surface.

In the ’70s, extensive research, notably by Dr. Mort Antler of AT&T Laboratories, established that nickel would be an acceptable underplate. Nickel became the underplate of choice and had wide acceptance throughout the industry.

The benefits of nickel underplates are numerous:

  • It is a leveling medium, resulting in increased wear resistance and improved coverage of the prime plating, along with reduced porosity.

  • It acts as a diffusion barrier, preventing diffusion of copper and zinc through gold.

  • Nickel exposure to ambient condition will produce a surface nickel oxide. This oxide is passive and self-limiting in thickness (of the order of 100 angstroms).  

  • Nickel oxide will not creep, and it acts as a barrier to prevent ambient pollutants from interacting with copper alloy substrates.

  • Nickel underplates are particularly important when used in conjunction with thin gold or gold flash applications.

  • But as a word of caution, there are a few negative aspects of nickel underplates:

  • Nickel has a high modulus of elasticity. When contact beam thicknesses approach a few millimeters and the nickel thickness approaches 150 microinches, the mechanical and normal force characteristics of the contact beam may be affected.

  • Nickel plating bath and plating practices can affect the ductility of the underplate. Nickel ductility may affect wear and mechanical performance, making a ductile bath mandatory.

  • Nickel thicknesses less than 50 microinches will lead to increased plating porosity, having a negative impact in thin or gold-flash plating systems.

Nickel has been the underplate material of choice since the late ’70s. Nickel underplate thicknesses of 50 to 150 microinches are recommended, and deviations from this range should be avoided unless appropriate justification is in place, and that means testing.

Several papers have detailed studies validating the nickel performance benefits described here. These works were published in the ’70s and have disappeared from the visibility of engineering libraries (except from those files still kept by the few graybeards still active).

If you have questions or wish to debate any of the above, please feel free to contact me directly or through Connector Supplier.

One final note: Two years ago (on July 3, 2010), I went into a mourning period that lasted for a month or so. That was the day I smoked my last cigar, which now resides in a coffin of appropriate decor. I now have to find different ways to annoy my associates. (However, I refuse to eliminate good cognac or brandy from my menu at appropriate locations.)

Dr. Bob Mroczkowski
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