Test Probe Description
General Description of Test Probes
The purpose of this description is to give a basic introduction to test probes. For a detailed description please ref. the manufactures description.
The accuracy of the fixture is depending on the quality of the test probes and the fixture itself. In general the quality of the test probes is high. Despite of this the accuracy of the probes has, during the recent years been improved further due to improvement in the mechanical design. The accuracy of the fixture depends on the
- Positioning of the holes for the probe sockets
- Holes must be perpendicular to the surface of the probe plate
- Fixing accuracy of the PCB
- Accuracy of the movement of the probe plate
- Accuracy of the position of PCB fixing holes and test pads
- Precision of the test probe and the receptacle
Sockets. The test probe consists of two parts, a socket (receptacle) and a probe. The sockets are mounted into holes in the probe plate. The wire end of the socket has a solder cup or a wire wrap pin. When probes are mounted in sockets you get advantages as:
- Easy replacement of damaged test probes
- Test probes with other tips may be mounted
- Worn test probes are easy to replace
Probes. The test probe itself has a built in spring which allows the probe tip to move. A typical maximum travel length of the probe tip is 6mm and the recommended working travel is 4mm. The probe spring is preloaded i.e. when the probe tip is pressed at the very beginning of the contact the spring force is not zero but approx. 50% of the force of the full loaded spring (approx. 2N). The advantage is a high probe contact pressure even if the probe is only compressed a few millimeters.
Test probes come with a wide range of tips accommodating different needs. The main purpose is to penetrate contamination and layers of oxide. The probes should be inserted using a piece of plastic pressing on the probe tips. Use of metal tool to insert probes into sockets may damage tips. The tip should be self cleaning to avoid poor electric contact from contamination on the tip.
The typical life cycle of a test probe is some hundred thousands operations.
For the test probe center to center distance please ref. the page "Design for Testability".
Some common test probes. The one next to the match is a socket (receptacle).