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Testing For Intermittents in Cable and Harness Assemblies

By Brent Stringham, Cirris Systems Corporation
For a video on intermittents
see:
"Testing for Intermittents."
To learn how to make sure your
tester and your test processes
will catch intermittents
see our video:
"Testing for Intermittents 2."

Most people today wouldn’t even think about using cable harness assemblies in their products that weren’t 100% electrically tested. But just because a cable has been “tested” does it mean that it is definitely a good cable? This article will discuss one of the most troubling aspects of electrical cable assemblies and wire harnesses, intermittent problems.

Like most electronic components, cables and wire harnesses are subject to manufacturing defects, and like most components, it is standard procedure to electrically test them. Hard (repetitive) failures are easy to detect, and testing is an effective means to eliminate them. Soft (intermittent) failures however can be much more difficult to find. It is possible to improve your products through testing by understanding the source of intermittents and attacking the root cause to eliminate them.

Since intermittent problems are just that, intermittent, they are much more difficult to find than solid defects and the best techniques for finding them are often overlooked. 1) Make sure that your harness tester, and your interface fixturing, support the detection of intermittent problems. 2) Use testing for feedback, and take appropriate action to remove root causes, rather than just attempting to test quality into your cables.

What is an intermittent?

There are three basic problems that occur in wire harnesses. Opens, shorts and mis-wires. Since wires generally don’t change location by themselves, intermittent shorts and opens are the problem.
Damaged wires from connector housing Pin not properly seated into connector which could cause an intermittent open on the cable assembley
Wires shown with red arrows are damaged from connector housing causing an intermittent short. Pin not properly seated into connector which could cause an intermittent open.

Intermittents are usually caused by some mechanical change; temperature, mechanical vibration, or physical flexing, which changes (temporarily) the electrical characteristics of the device under test.

Intermittent Shorts: Failures in insulation between conductors that should be isolated.

In most cases intermittent shorts are caused by some compromise of the insulating material intended to keep wires isolated from each other. When insulation is breached, by whatever means, there is now only an “air-gap” isolating wires from each other for some portion of the cable. When some force (bending, flexing, vibration, aging, etc.) causes the conductors isolated only by an air-gap to come into physical contact with each other, a short-circuit occurs. This short may become a hard failure, or remain an intermittent failure, depending on the nature of the force, however it can become the worst possible scenario, a field failure. Many intermittent shorts may be found by continual-scan flex-testing in production, however a more thorough test is using non-destructive (low current) high voltage tests (Dielectric Withstand Voltage - DWV). To get an idea of the voltage required to detect missing insulation, see our “High Voltage Arc Gap Calculator”.

Intermittent Opens/High Resistance Connections: Failures in the continuity of a conductor.

An intermittent open, or high-resistance connection, occurs when there is temporarily no electrical connection, or weak electrical connection (high resistance) from one end of a wire to the other. This can occur in the contact-to-contact mating at either end, the contact-to-wire bond at either end, or the wire itself.

Intermittent Contact-to-Contact Problems:

  1. Micro-fretting. Some metals, when they come into contact with each other, form surface oxides over time. This is common for tin/lead contacts and is often recognized as micro fretting. Tin contacts are especially susceptible to this problem unless they have high mating forces (good wiping action) or the application uses more than 10 volts. Micro-fretting is not a problem for gold plated contacts as long as the plating is intact. Often this problem does not show up in test. As soon as connectors are remated it goes away. A common field solution of remating connectors can cause this problem to disappear only to later reappear. See Tyco's technical report "The Tin Commandments: Guidelines for the Use of Tin on Connector Contacts" for more information on problems with micro-fretting.

  2. Improper mating forces. Intermittent opens can occur when contacts do not mate with the proper force. This can be caused by:

    1. Wear that weakens the spring forces in female contacts or reduces the size of male contacts.

    2. Contacts that are not fully mated. (crimped pins aren’t fully locked into the connector housing and push back when mated)

    3. Connector shells/mating hardware that interfere with full engagement of the mating connectors.

    4. Mechanical damage/ deformation of contacts.

    5. Foreign contamination on contacts. This can be caused by flux left from soldering, cleaning residues, dirt, etc.
      Connector pins contaminated with flux  
      Magnification of D-Sub contacts contaminated with flux residue.    
           

       

Intermittent Contact-to-Wire Problems - Bond is not Gas Tight

  1. In crimped connections intermittent opens (or high resistance connections) are often caused by poor crimps due to improper mechanical adjustment or misuse of crimping tools. Consider crimp pull testing or crimp force monitoring.

  2. Soldered connections. Cold solder joints and poor hand soldering can cause intermittent problems.

  3. IDC connectors can have intermittent problems due to bent tines, cut strands, misalignment of planar cable, including variation in spacing of wires in ribbon cable, and improper use of application tools.

Intermittent Problems in the Wire Itself.

Apart from problems caused at the point of wire-to-contact termination as discussed above, intermittent problems in the wire itself usually occur after the assembly has been put into use. This can be caused by flexing, stretching, or bending a cable more than expected. These types of problems are usually not solved by improving testing, rather by design changes such as using more strands of wire in the conductors, limiting the flexing and stress experienced by the assembly with improved strain reliefs, etc., and by protecting the assembly with better jacketing.

Expensive Solutions for Intermittent Opens/High Resistance

High-speed event detectors (discontinuity testers that monitor resistance simultaneously on each conductor) may be used as part of a rigorous set of tests under controlled conditions, including temperature cycling and vibration over a range of frequencies on a shake table. This solution, while effective for catching intermittent problems, lacks popularity as a production test solution because:

  1. Production processes must be streamlined for productivity. It takes time and expensive equipment (temperature chamber, shake table, event detector) to accurately perform these kinds of tests, hence they are usually reserved for connector testing qualification testing by connector manufacturers.

  2. Concern of degradation of the assembly because of the rigor of the test.

  3. Difficulty of replicating an event that causes real problems.

  4. Lack of “complete” test (discontinuity testers don’t test for shorts or insulation problems).

  5. False failures due to ESD (was it a real fault or just an ESD event in the vicinity of the test equipment?)

Inexpensive Solutions for Intermittent Opens/High Resistance

In order to solve intermittent problems in your cables and harnesses, you have to be aware of if you have them. Even inexpensive testers can detect intermittents but it is wise to “test” your tester:

Test your Tester for Intermittent Detection. Use the following test to see if your tester is capable of detecting intermittent problems. Take a “known good” cable, break one wire and insert a switch that when open is labeled “OPEN” and when closed is labeled “GOOD.”

Step 1. Put the switch in the GOOD position and test your cable to make sure it passes.

Step 2. Now put the switch in the OPEN position and test the cable again. After the tester displays an error move the switch to GOOD, eliminating the open.

Tester Shortfall #1 “Single Test”: If your tester indicated an open in step 2, and the status did not change after removing the open, your tester is operating in a “single test” mode.

Some cable testers only perform a “single test” each time you press a button. If you use a tester in “single test” mode you have a slim chance of recognizing an intermittent at the one brief moment the wire with the intermittent is truly open. Since the operator has no feedback that they are experiencing an intermittent, the cable may be inspected and hand tested with no problem found. The cable may be re-tested with the likelihood that this time the intermittent is missed.

Step 3. If your tester passed Shortfall #1, the switch is in the GOOD position and the tester should be indicating good cable. Now put the switch in the OPEN position again.

Tester Shortfall #2 “Wiggle Until Good”: If your tester still indicates a good cable, it is 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 are blind to intermittent problems, even if your cable tester is capable of finding them. An operator just wiggles the cable until it passes. This type of testing has been popular in automotive harness testing where the assemblies tested are “not expected” to have intermittents because manufacturing processes have eliminated any chance of them occurring. It also allows production testing using test fixtures with intermittents.

Step 4. If your tester passed Shortfalls #1 and #2, the switch is in the OPEN position and the tester should be indicating an open again, indicating that your tester is capable of showing intermittents in real time. If you move the switch to GOOD now, your tester should read INTERMITTENT, indicating that, although now good an intermittent has occurred. The best performing testers would also capture what the intermittent problem was, allowing you to know specifically which connections were involved, even if you can’t re-create the intermittent. 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 four 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 you can detect.

Cirris 1100R+ showing an intermittent cable error

Cirris 1100R+ showing an intermittent cable error during testing.

Using a tester that effectively finds intermittents 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.

Is the Intermittent in the Test Fixture, or in the Cable Itself?

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

Upgrading Test Fixture Quality

Before you can eliminate intermittents in the product under test, you must eliminate intermittents in your test fixturing. Typically there are two types of test fixtures employed in testing cable/harness assemblies; actual mating connectors, or 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 just 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 insure 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 the connector can be easily replaced on the same test fixture. You can see how this is done on the Replaceable Adapters Page.

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 milled, or molded test blocks using spring-loaded contacts are used. These types of fixtures can be problematic where intermittents are concerned due to the pogo pin connection in lieu 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, such as Everett Charles’ bias ball. Barrel to shaft style pogo pins often result 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 test 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. Ecc Test Block  
  ECC block with spring loaded pins  

Maintaining Test Fixture Quality

It has been established that, in order to effectively catch intermittents in the product under test, the test fixturing must be absolutely free from intermittents. To assure ongoing intermittent-free test fixturing requires a consistent program of test fixture verification and maintenance. A “shorting block” adapter can be created 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 the test fixture should be repaired/replaced immediately. You can get instructions on how this can be done on our Testing Adapters Page.

Recommended Action Plan

Assess if intermittent problems can escape your current production process.

If Yes:

  1. Update test equipment to that capable of catching very short-duration intermittent events (fast continuity scanning/ high-voltage testing)

  2. Upgrade fixturing, eliminating the possibility of intermittents on your test fixtures.

  3. Change your test process to call intermittent problems BAD, not wiggle until they go good.

  4. Maintain your test fixturing, repairing and replacing as indicated by using fixture test blocks.

  5. Remove the root causes of intermittents. Testing for intermittents is far from 100% effective. After making your intermittents visible, attack the causes of these intermittents at the source.