Electrical Contact Resistance: Causes and Alleviations

The Causes of Electrical Contact Resistance and How to Isolate Them

While methods of measuring resistance are second nature to most technicians in a laboratory setting, it’s worthwhile to note that a contact measurement (i.e., probing) differs from the inherent resistance of the object due to the imperfections between the contact surfaces and the added resistance of the leads and connections. The equipment manufacturer usually well-describes the latter and, while inescapable, is accountable during measurements with a simple offset value (this value may drift over the life of the equipment). For the contact action, there are two primary mechanisms to keep track of:

• The area of the surfaces in contact at the interface is a function of force.
• The measurement's contact resistance distributes over the surfaces in contact.

The simplicity of these statements belies the complexity of a thorough determination of the electrical contact resistance. What can appear as two smooth and feature-free surfaces to the naked eye become considerably rougher under magnification. At the microscopic level, weak contact between the asperities (rough microscopic surface features) results in a capacitive interaction from the troughs and valleys of the surface. Further complicating contact is the presence of oxides and adsorbed moisture at the surface of the metal contacts, which tend to increase the resistance unless sufficient force causes direct contact between the underlying metal.

The Beneficial Effects of Resistance Creep 

Over repeated contact measurements, the resistance begins to fall in a process known as resistance creep. The material experiences current-induced welding and dielectric breakdown at expanding, localized spots. Until this point, various techniques can more accurately measure the resistance of the measurement subject, most often by finding the difference between a two-terminal and four-terminal measurement: the latter method adds a second pair of probes in parallel with the first that measures the potential difference between the terminals without injecting current. Dividing the current injection and potential measurement between two pairs of leads ensures the potential drop across the terminals does not include that of the leads themselves.

It’s worth mentioning that a four-terminal resistance measurement results in a sheet resistance value, not just a resistance value. While the two have equivalent units, sheet resistance indicates ohms/square (Ω/⬜), functionally a resistance of uniform thickness. Technicians can multiply the sheet resistance value by the surface contact area to obtain a more precise electrical contact resistance.

A Deeper Look Into Surface Mechanics

Unfortunately, the simple model of asperities doesn’t hold under improved magnification; instead, a continuous fractal roughness appears until the atomic scale, where rough lengths become discrete atomic steps. Fractal models can better define surface interactions but have profoundly more rigorous mathematics involved. Broadly, a fractal model for electrical contact resistance uses asperities  to define the surface recursively (i.e., asperities of asperities). Characterizing the asperities of the surface would require advanced magnification capabilities, which is beyond the scope (and perhaps the means) of most motivations for electrical contact resistance. 

A Gaussian distribution of asperity height is acceptable for a couple of reasons. One, contact between the surfaces results in elastic and eventual plastic deformation of the asperities, reducing the maximum height of the surface defects under load bearing (think, “the nails that stick out get hammered down”). Additionally, it’s reasonable to expect that some deformations cause the material to efill the gaps in the plane of contact; the result of these two contributions is an overall normalization of the valleys and troughs of the surfaces.

The Change in Electrical Contact Resistance Over Time

With a careful setup, it’s possible to map the applied force to the voltage at a silicon transistor-structure level. Initially, new components will exhibit relatively high contact resistance, rapidly falling with repeat power/measurement cycles. Furthermore, the variability of the contact resistance shrinks, decreasing the maximum and average resistance measurement error simultaneously. 

An electrical contact resistance of zero is preferable but impractical in real-world conditions. When troubleshooting measurement errors, this resistance can be a benefit: technicians can use it to assess the mechanical and chemical quality of the connection. For example, an electrical contact resistance greater than a previous measurement could indicate surface corrosion, under-torqued fasteners, or a higher-than-expected operating temperature. High-current systems with excessive contact resistance would generate excess heat by dissipating more power across the resistive load in the process; regular upkeep and maintenance of electromechanical systems ensure maximum power efficiency, as designed.

Cadence Solutions Optimize System Performance

Electrical contact resistance is a complex material subject of significant importance to electromechanical systems. Serendipitously, the effect of contact resistance tends to decrease with repeated use, and its reappearance can act as a flag for some continuity malfunction. Systems with high current or power demands must ensure that the design intent meets the minimum requirements and ideally optimizes performance by curtailing loss. Cadence’s PCB Design and Analysis Software suite electronic development teams unparalleled simulation capabilities with constraint-driven design. Once modeling is complete, the board layout can easily incorporate data into OrCAD PCB Designer for a seamless ECAD experience.

Leading electronics providers rely on Cadence products to optimize power, space, and energy needs for a wide variety of market applications. To learn more about our innovative solutions, talk to our team of experts or subscribe to our YouTube channel.