9. Electrical Resistance of Spring Contact Probes

Depending on size, form, material, and surface finish, spring contact probes show highly differing resistance values. The typical value for each product is listed on its data sheet. This value shall not be regarded as a warranted property. It is only a value which is typically found for a particular series when we regularly perform quality tests.

It is highly dependent on the path(s) which the current flows along on its way through the spring contact. Normally there’s a parallel connection of two current paths: one with a lower, and one with a higher resistance. The majority of the current flows from the contacting plunger via the housing (barrel) to the receptacle and finally into the connected wire. A very small percentage of the current (less than 5 percent) is propagated through the spiral pressure spring.

 

The resistance values in the catalogue are based on Kelvin measurement. Theoretically the total resistance of a spring contact probe consists of the individual resistances of its parts, plus the transition resistances of the effective contact areas between the parts.

9.1 Components of a Probe

A typical probe plunger has an inherent resistance of approx. 1 to 2 milliohms. A typical barrel, e.g. from a 100 mil standard pin, takes approximately 6-8 milliohms on the dosplay of the meter. And the spring - for example made of music wire - outweighs the other parts with 1 to 2 Ω. Finally, the receptacle is of low resistance. Typical values range from 5 to 10 mΩ.

9.2 Constriction and contamination resistance

Between plunger and barrel and between pin housing (=barrel) and receptacle there is a certain guide clearance or side-play which electrically means extra transition resistance. This transition resistance is composed of contact resistances and the resistance of contamination substances. The contact resistance is the resistance of each individual contact surface. It is comprised of constriction resistance and contamination resistance. The constriction resistance arises from microscopic unevenness of a contact surface. The actual effective contact area decreases and the current flow is constricted. The constriction resistance depends on the resistivity of the used material, surface unevenness (caused by e.g. friction, wear etc.) and the number of effective contact surfaces. Through oxidation and corrosion the contact surfaces build up a contamination layer which increases the resistance. To prevent it the surfaces are normally coated with precious metal – in our case mostly gold.