Finding Intermittent Errors in Fixturing

intermittentIntermittent errors can be difficult to find, not only because they are not consistent, but also because they could be in your fixturing or your cable. The first step to finding intermittent errors is to make sure your tester is capable of testing for them. Next, you should make sure to maintain and test your fixturing regularly to avoid intermittent problems. If your equipment is in good shape, you will be able to find intermittent errors in your cables faster and more accurately.

Don’t overlook some of the best techniques for finding intermittent errors.

  1. Use testing for feedback and take appropriate action to remove causes rather than just attempting to test the quality of your cables.
  2. Make sure that you maintain reliable fixturing. Worn contacts or wiring make it harder to detect intermittent problems within the cable.

 

intermittent test switchTest your Tester

To solve intermittent problems in your cables and harnesses, you have to be aware if you have them. Even inexpensive testers can detect intermittent errors, but it is wise to “test” your tester. (Note: All Cirris testers can test for intermittent errors as long as the test program is set up correctly.)

Use the following test to see if your tester can detect intermittent problems:

  1. Take a “known good” cable, break one wire and insert a switch that when open is labeled “OPEN” and when closed is labeled “GOOD.”
  2. Put the switch in the GOOD position and test your cable to make sure it passes. Next, put the switch in the OPEN position and test your cable to make sure it fails. Now you are ready to test your tester.
  3. Test the cable again with the switch in the Good position. This time, before the test finishes, move the switch from the GOOD position to the OPEN position. The tester should indicate an error. Move the switch back to the GOOD position. Your tester should report an intermittent error, indicating that your tester can show intermittent errors in real time.

This same test of your tester could be made more challenging by using a 100-ohm resistor in parallel with the switch and again checking the results in these steps (to catch intermittent high resistance failures). You can also do these same tests with a switch that introduces shorts instead of opens. Finally, you could experiment with how briefly you can activate and deactivate the switch to create the defect before the error is detected, thus determining how brief of an intermittent error you can detect.

If your tester did not behave in the way described in the steps above, you might have to adjust your test program, equipment, or process. Here are two common tester shortfalls that may inhibit your ability to find intermittents:

Tester Shortfall #1 “Single Test”: If your tester indicated an open and the status did not change after removing the open, your tester is operating in a “single test” mode. If you use a tester in “single test” mode, you have a slim chance of recognizing an intermittent error during the one brief moment the error occurs. Since the operator has no feedback that they are experiencing an intermittent error, they may inspect and test the cable without finding the problem. To solve this, change the test mode to “continuous test.” The tester will run a series of tests, giving you more time to find an intermittent error.

Tester Shortfall #2 “Wiggle Until Good”: If the tester indicates a good cable instead of an intermittent error, it could be operating in a test-until-good mode, sometimes called “wiggle until good” or “wiggle and jiggle.” If your process allows the operator to “wiggle until good,” you will not find intermittent problems, even if your cable tester can find them.

This shortfall also allows production testing using test fixtures with intermittent errors. If the operator knows an intermittent problem is present in the fixturing, they may “wiggle” the cable until the error turns to a good. Testing with a known intermittent problem in your fixturing can be dangerous if an intermittent error is also present in the cable. The unknown intermittent error often gets missed because of the “wiggle until good” method.

To solve this, make sure your tester can recognize an intermittent error. If you move the switch to GOOD after an error has occurred, your tester should read INTERMITTENT to indicate that although now good, an intermittent error has occurred. The best performing testers would also capture what the type of intermittent error and which connections were involved, even if you can’t re-create the intermittent error. You should also regularly check your test fixturing for issues and replace any parts that can cause problems.

The “Test your Tester” process is also shown in our video, "Testing for Intermittents 2."

Test your Fixturing

Using a tester that effectively finds intermittent errors often exposes intermittent problems in test fixturing. Solving this problem may be more work than simply getting the right tester or using your tester in the right operating mode.

The key question here is, “How can you GUARANTEE that the intermittent problem is in your test fixturing and NOT in the cable assembly under test?” Test fixtures such as interface cards, adapter cables, test blocks, etc. all have the same characteristics as the actual harness being tested: connectors, wires, and terminations. The problem is that the test connectors see many more mating cycles than the assembly being tested. Ignoring intermittent errors (wiggle until good) saves on fixture costs and maintenance, but at the expense of being blind to real intermittent problems in your assemblies.

Upgrading Test Fixture Quality

Before you can eliminate intermittent errors in the device-under-test, you must eliminate intermittent errors in your test fixturing. Typically, there are two types of test fixtures employed in testing cable/harness assemblies: actual mating connectors and test blocks using spring-loaded contacts that imitate the actual mating connector.

Actual Mating Connectors

In lower-volume, long-life connectors (typically computer, medical, military, communications) it is popular to use the mating connector in the test fixture. In this case, make sure to use gold plated versions of the mating connector; in high volume testing “gold flash” plating will not hold up. Refer to manufacturer’s data sheets to ensure adequate gold plating and for information on expected mating cycles. In the event of very high volumes, even gold-plated connectors will wear out. For these applications, you may want to choose a “replaceable” style adapter, where you can easily replace the connector on the same test fixture. You can see how to do this on the Replaceable Adapters page.

ECC block with spring loaded pins
ECC block with spring loaded pins.

Test Blocks Using Spring-Loaded Contacts

High-volume, low mating-cycle connectors (typically automotive and appliance) generally use tin plating, and the actual mating connectors are unsatisfactory for high volume test requirements. In this case, operators use milled or molded test blocks with spring-loaded contacts. These types of fixtures can be problematic where intermittent errors are concerned due to the pogo pin connection instead of an actual mating connector. In these instances, you can use spring-loaded pins with higher forces and designs that minimize the resistance in the spring-loaded contact. Barrel to shaft style pogo pins often results in higher resistance. Pogo pins with sharp points help deal with tin oxides on the surface of pins in the harness being tested. The drawback: possible damage to connector contacts and tests that pass despite contacts that are not “locked in,” damaged, or that have severe contamination on the surface (pushback test blocks can be used to test for pins not locked in). More information about connector selection is on our Choosing Connectors for Fixturing Page.

Maintaining Test Fixture Quality

To effectively catch intermittent errors in the product-under-test, the test fixturing must be free from intermittent errors. To assure ongoing intermittent-free test fixturing requires a consistent program of test fixture verification and maintenance. You can create a “shorting block” adapter for each different test connector. This shorting block should be plugged in at regular intervals to “test the test connector” for intermittent problems. When discovered, you should repair/replace the test fixture immediately. You can get instructions on how to do this on our Testing Adapters Page.

Recommended Action Plan

Assess if intermittent problems can escape your current production process.

If Yes:

  1. Change your test process to call intermittent problems BAD, not wiggle until they are GOOD.
  2. Upgrade fixturing, eliminating the possibility of intermittent errors on your test fixtures.
  3. Maintain your test fixturing, repairing and replacing as indicated by using fixture test blocks.
  4. Remove the causes of intermittent errors. Testing for intermittent errors is far from 100% effective. After making your intermittent errors visible, attack the causes of these intermittent errors at the source.