Wobble and Minimum Test Pad Size

Due to the necessary play between the Plunger and the Barrel of a Test Probe, the tip can be deflected out of the ideal (i.e. vertical) position. This deflection, the so-called "wobble", was measured on INGUN Test Probes using a state-of-the-art optical measuring machine. This machine can, on request, also be used to verify customized Test Fixtures. To define the minimum test point size, the various tolerances of the Test Fixture were also taken into consideration.

Basically, the statement can be made that no direct dependency between the wobble and the contacting accuracy of a Test Probe exists. Important is especially the position of the probe tip at the time of actual contacting. For the subsequent stroke it is even often an advantage when a greater wobble exists, because this then helps to reduce the wear when the plunger is pressed down into the barrel. Furthermore, the tendency to bend the Test Probe is reduced and thus the life-expectancy prolonged. The minimum test point size is stated for all common Test Probes and is related to the usage in INGUN Test Fixtures.

The following diagram shows the deflection of the Plunger from the ideal position (i.e. the centre of the assembly hole) of various Test Probes. The experimental set-up was made with a standard INGUN probe plate, and the Test Probes were inserted in Receptacles. The row of experimental tests was carried out a number of times. Between each measurement the Test Probes were activated a few times.

The result of the investigation offers no information about other factors, which must also be taken into consideration, i.e. the tolerances of the PC-Board and of the Test Fixture assembly as well as the uncertainty and faults, which can be incurred during insertion and removal of Receptacles and Test Probes. Therefore, INGUN recommends the usage of a guide plate (especially in the case of critical applications, i.e. Test Point Size < 0,8 mm) with which the plunger tips are guided. Then, the majority of the tolerances can be ignored.