The current rating of the Spring Contact Probe is determined by the power (heat) generated by the current and resistance (I²R) and the ability of the probe and mounting plate to dissipate this heat. The base material, plating, and bulk size of the probe are critical in determining the current rating. Additional factors considered include the mounting centers, mounting material, ambient temperature, and duty cycle.
The resistance of a Spring Contact Probe depends on the base materials, platings, and physical design. The typical current path of a probe is from the plunger to the barrel, then through the receptacle and out to the wire. Approximately 99% of the current follows this path. The remaining 1% of the current flows through the spring.
Coaxial probes are available with operating frequencies up to 3 GHz for shielding plunger designs and 500 MHz for designs without shielding plungers. Spring probe connector designs can operate at frequencies over 20 GHz at -1dB. Signal path length and field array population are key considerations when determining the operating frequency of connectors.
The contact resistance of a spring contact probe/receptacle assembly is critical to successful testing. Listed below for reference are calculations of the approximate resistance:
.040″ diameter rod, .700″ long, Beryllium Copper base material, Gold over nickel plating: 1.24 mΩ
1.000″ long cylinder, 0.042″ inside diameter, .054″ outside diameter, DuraGold® material and plating: 8.32 mΩ
.006″ wire diameter, 7.500″ in length, Music Wire base material, Gold over nickel plating: 2125.09 mΩ
1.200″ long cylinder, .055″ inside diameter, .066″ outside diameter, Nickel/silver base material, Gold over nickel plating: 13.20 mΩ
The values listed above are approximations, but they are sufficient for the intended purpose. When determining the current path of the probe, it is important to note that current in parallel paths will divide itself between the paths such that the products of current and resistance in each path are equal.
The current through the barrel is 255 times greater than the current through the spring, meaning 99.6% of the current flows through the barrel, while 0.4% flows through the spring.
In this example, we have ignored the contact resistance between the plunger and barrel, as well as the constriction resistance between the plunger and spring. The net effect of these simplifications does not alter the fact that the vast majority of the current will flow through the barrel.
To simplify the calculation of the resistance of a probe, assume the current travels through the total length of the plunger and then directly to the barrel. Therefore, the plunger and barrel are in series. The current must then travel from the barrel to the receptacle, with the detents in the receptacle providing a solid connection between the barrel and receptacle. The current will transfer at this point. Assume all the current transfers from the barrel to the receptacle at the detents.
The plunger, barrel, and receptacle are in series with each other. Therefore, Ohm’s Law for resistors in series applies.
The total resistance calculated is an approximation of a Size 25 DuraGold® Series probe. The charts on the next page show the actual data recorded during the 4-wire Kelvin test. It should be noted that the recorded value includes the following additional resistances:
Constriction resistance between the probe tip and the sterling silver contact plate.
The solder joint on the sterling silver contact plate.
The solder joint on the receptacle.
The constriction resistance between the plunger and barrel.
The constriction resistance between the barrel and receptacle.
Oxide layers on material surfaces.
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