When designing a bed-type testing fixture, the choice of probes is crucial, as they are commonly used. The probes for the testing fixture have been standardized, with parameters such as size, length, height, and stroke all having a series of numerical values. Therefore, our task is similar to selecting screws—only the model and specifications of the probes can be chosen, not arbitrarily deciding on quantities or types.

In general, the model of the probe and the importance of the test fixture lie in the diameter of the probe. The size of the probe is measured in mils, which is a British unit. The conversion is 100 mils = 2.54 mm = 0.1 in.

In PCB board design, the common pin pitch is typically 1 inch, or 2.54mm, thus the standard probe size is 100mil. However, 100mil does not mean the probe's diameter is 2.54mm; rather, it refers to the distance between two test points being 100mil. In reality, the diameter of the test fixture is approximately 1.7mm.