Film determines probe accuracy
You can help make sure your in-circuit test fixture provides adequate contact to the board under test.
Sol Black, Celestica -- Test & Measurement World, 3/1/2002
When embarking on in-circuit testing, you must perform a "pins" test to ensure that your test-fixture probes accurately contact with the test points of your board under test. If probe-to-board contact is marginal, than some good boards can fail your test. Unfortunately, circuit boards are opaque, so it's difficult to see whether pins are hitting the centers of their targets with adequate force. Even if boards were transparent, modern fixture designs, such as clamshell fixtures, would preclude you from observing probe-to-board contact.
Some tried-and-not-so-true techniques can help you determine probe accuracy. For example, you can place transparent tape on the side of the PCB to be probed, and then place the board in the fixture. Upon removing the board, you can inspect the tape, but any impressions that the probes have left on the tape are difficult to see.
An alternative approach is to coat probe tips with a substance that will rub off on the board at points of contact; you can then examine board test points for the presence of the substance. Such an approach offers no guarantees: The substance can be so wet that it is very messy, or conversely, the substance you choose can dry before probes make contact with the board, making it unlikely that visible quantities of the substance would adhere to board test points. (In a related application, Ref. 1 recounts the use of typewriter correction fluid to measure probe travel during fixture operation.)
Pressure-sensitive film (Ref. 2) offers a better alternative. The film, from Sensor Products (Ref. 3), changes color when subjected to pressure. Its color changes indicate different levels of applied force, although for this application, the location of the force rather than the amount of force is what's critical.
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| Figure 1. Pressure-sensitive film applied to a PCB can help you determine the accuracy of your fixture probes for in-circuit-test applications. The dark-red spots indicate probe contact points. The circled spot indicates a missed target. |
The film is semitransparent, and when you place it on a PCB, you can clearly see the test-point targets. Figure 1 shows a board with the film applied after the board has been placed in a fixture, the fixture actuated, and the board removed. The dark-red spots show probe contact locations. The circled spot represents a probe that missed its target contact point.
The film comes in several pressure ranges. I've tried many of them, and each worked equally well in this application, so I recommend the low-range film, which costs the least.
| Author Information |
| Sol Black is the Lead Engineer of the Structural Test Support and Development Group at Celestica's Columbus, OH, location. |
| References |
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