Resistance Calculation of a Probe/Receptacle
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. However, 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 for all paths.
|I BRL x R BRL = I SPG x R SPG
= 2125.09 x I SPG
|= 255.42 I SPG
The current through the barrel is 255 times as great as the current through the spring or 99.6% of the current goes through the barrel and 0.4% goes through the spring.
For this example, we have ignored the contact resistance between the plunger and barrel, just as we have ignored the constriction resistance between the plunger and spring. The net effect of this simplification will not alter the fact that by far, the vast majority of the current will go through the barrel.
To simplify the calculation of the resistance of a probe, assume the current has traveled through the total length of the plunger and then directly to the barrel. Therefore, the plunger and barrel are in series. The current now must travel from the barrel to the receptacle. The detents in the receptacle supply a solid connection between the barrel and receptacle. The current will tend to 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 determined is an approximation of a Size 25 DuraGold® Series probe. The charts on the next page show the actual data recorded at IDI, on 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