Spec helps ensure connection reliability

A Cadillac or Nexus can have more than 400 connectors with 3000 or more individual terminals. These connectors must mate cleanly, quickly, and reliably.

The force required to mate these connectors is of particular concern to the National Institute for Occupational Safety and Health (NIOSH; www.cdc.gov/niosh), which warns that employees performing repetitive wiring tasks are at risk of developing carpal tunnel syndrome and other musculoskeletal disorders. Autoworkers are especially at risk, because they often have to mate connections in hard-to-reach positions.

With this in mind, USCAR (www.uscar.org), a consortium formed by DaimlerChrysler, Ford, and General Motors, released a specification in April 2001 that cut the maximum allowable mating/unmating force to 75 N (16 lbs). This specification—SAE/USCAR-2, Standard for Automotive Electrical Connection Systems—not only specifies mating/unmating force requirements, but also sets standards for contact resistance and describes test procedures for measuring these parameters (Ref. 1).

The standard's connector-connector mating/unmating force test requires you to prepare a minimum of 10 fully populated connector assemblies. "Fully populated" means that each terminal position must have a terminal with a properly crimped wire. If the connector is to be used with any accessories, such as wedges, locks, or seals, you must include them in the assemblies under test.

Once assembled, you can begin to test the connectors. To perform the test, you will need a test fixture that aligns the connectors so they engage cleanly. If they are out of alignment, there may be side loads or the connectors may bind. Both conditions will affect the force measurement.

To run the test, you apply a force to the connector and ramp it up until mating occurs. The maximum insertion rate must not exceed 50 mm/min.

To measure the insertion force, you need a peak-reading force meter. While a peak-reading meter is all that is required, the specification recommends that you use a tester that can continually measure the mating force applied as well as the insertion distance. With this data, you can graph the insertion distance versus the mating force.

The graph will tell you whether you have a properly designed system (one that will show a smooth rise to a single peak force, then a decline until the connector is fully mated). If the chart shows two peaks, then the potential for a false lock exists, and this could lead to reliability problems once the connector is installed in a vehicle.

The unmating force test is a little more involved. The specification says to test five of the connectors with the locking mechanism engaged. You are to apply the unmating force and ramp it up until the lock breaks and the connectors disengage. The force required to unmate the connectors with the lock engaged is to be greater than 110 N.

For the remaining five connector assemblies, you have to measure both the force required to disengage the locking mechanism and to unmate the connectors. The force required to disengage the locking mechanism must be between 20 N and 70 N, and the force needed to unmate the connectors must be less than 75 N.

To help meet SAE/USCAR-2 specifications, automakers spray a lubricant on a connector's terminals. These lubricants enable workers to use less force in mating and unmating connectors, and they also reduce wear and retard oxidation, which can increase contact resistance and shorten the connector's life.

For example, lubricants reduce fretting corrosion, a type of mechanical wear caused by low-amplitude vibration that is frequently found in automobiles. Lubricants also protect against substrate corrosion in gold-plated connectors. Thin gold plating can be microscopically porous, and a film of lubricant seals the pores to prevent substrate oxidation; if substrate oxidation were permitted to build up, it could eventually exude through the pores, build up on the noble metal surface, and lead to high contact resistance.

Nye Lubricants (Fairhaven, MA) recently ran a series of tests to demonstrate how lubricants help keep mating forces down and improve connection reliability. The company measured the insertion force required to mate a 6.35-mm male terminal with a female terminal on an Instron 5566 tester. A test machine inserted the terminals into one another to a depth of 0.375 in. at a rate of 0.0595 in./s for 10 insertions. They tested unlubricated terminals and terminals coated with three of the company's lubricants—NyoGel 760G, UniFlor 8511, and UniFlor 8917 (Ref. 2). The figure shows the measurements.

The unlubricated terminal had an average insertion force of 4.4 lbs, and this force changed very little throughout the test. The terminal coated with NyoGel 760G had an insertion force of 3.8 lbs on the first mating and 2.8 lbs on the tenth mating. The terminal coated with UniFlor 8511 had an insertion force of 1.3 lbs on the first mating and 0.5 lbs on the tenth mating, while the one with UniFlor 8917 had an insertion force of 0.8 lbs on the first mating and 0.3 lbs on the tenth mating. Clearly, using a lubricant allows you to use connectors with more terminals than if you did not use a lubricant.

To verify that terminals coated with a lubricant would also meet the SAE/USCAR-2 contact-resistance specification, Nye submitted the terminals to an independent test lab. The lab ran the 1008-hr current-cycling test, which is run at an ambient temperature of 150°C. Technicians measured the contact resistance recorded before and after the test and found that the average resistance across the terminals lubricated with UniFlor 8917 was 0.489 mΩ. This easily meets the SAE/USCAR-2 specification of 10 mΩ.