The Biggest Fixturing Mistakes

Is fixturing the cause of your failures? Fixturing can cause perfectly good cables to fail when it is built using the wrong materials, constructed incorrectly, or not maintained.  One customer experiencing failures sent their fixturing to Cirris and we immediately identified common mistakes. The errors were corrected and the failures stopped. Mistakes like this customer experienced can cost thousands of dollars in time and materials. If you experience repeated failures but know your cables are good, the reason could be your fixturing.  

Design – Mistakes in the Materials

During the design stage, consider the materials selected, the type of environment you operate in, and the number of cycles the fixturing can endure. If there is any question of suitability in materials, try looking at the manufacturer’s specifications.

Recommended Materials

  • Acetal or Delrin
  • G10 (Glass apoxy)
  • Polycarbonate
  • Pins with gold plating (30 micro-inches or thicker)
  • Wood
  • Hot melt (acts as insulator and holds pins in place)

Materials to Avoid

  • Nylon (absorbs moisture which can lead to IR failures)
  • G10 contaminated with Carbon Black or a mold release (Good if “non conductive”)
  • No-Clean Flux (a better name would be “can’t-clean" flux)
  • Pins with tin plating (can only be used for a limited number of cycles before plating wears away)
  • Plywood as an electrical fixture (can act as a conductor and cause IR failures)
Fixture with unpopulated pins and plywood structure.

Some fixturing mistakes include building fixtures out of plywood, not registering the shell, and not populating all of your pins.

Recommended Materials for wire insulation

  • Teflon
  • Corona resistant wire
  • Silicone
  • Polyolefin
  • Kapton

Note: Those who design fixturing should research each recommended or avoided material to ensure its usability in their own fixtures.

Build – Common Construction Practices

Once you have selected the correct materials it is time to build the fixturing. Similar to selecting materials, there are common mistakes that result in bad fixturing. To avoid these errors:

  • Document all your work – retain a simple and accurate schematic of your fixturing.
  • Keep the test fixture clean – If dirt and flux contaminate your connectors, it can create a path for current to move between pins.
  • Minimize exposure to cutting fluid – be aware that machine fluids, coolants, and oils can contaminate insulators and degrade their insulating properties.
  • Never use no-clean flux – a better name might be “can’t clean” flux. Read our article on Frustrations, Failures, and Flux to learn more.
  • Provide strain relief for wires – pulling, flexing, or yanking on wires can ruin connections.
  • Avoid floating metal in your fixture – metal in contact with conductive parts of the device-under-test but not connected to ground or to a test point can cause intermittent high voltage failures and potentially cause good cables to fail.
  • Populate all pins in interfacing connectors to ensure all pins in the device under test are connected to test points and to catch unintended shorts (similar to floating metal).
  • Connect each connector shell to a separate test point.
  • Be aware of gaps between pins – make sure the connectors you buy or build can handle the voltage and have sufficient clearance between pins.
  • Be aware that pogo pins with pointed tips can scratch and damage connectors.

Test – Often and Thorough

Test your fixturing regularly to check for good connections. Cirris provides an in depth article on verifying fixturing using shorting blocks and test plugs.

Fixture with dirt and coffee stain.

Keep your fixturing clean. Dirt and damage can affect your fixturing and cause good cables to fail.

Maintain – Don’t Get Complacent

To ensure efficiency and longevity, perform regular maintenance on your fixturing.

  1. Clean your fixturing
    Keep your fixturing clean. Fixturing that is regularly cleaned of dust and other debris will last longer and work better.
  2. Replace worn out connectors and bent pins
    Dozens of cables inserted and removed from fixturing can take a toll on the connectors. When a connector wears out or has a bent pin, replace the connector with a healthy part for best results.
  3. Keep track of the cycle count
    The cycle count is the number of times you insert and remove a connector from your fixturing. This number is a good reminder to check for wear on your connectors.
  4. Check strain relief on wires
    Strain relief is most common at the ends of cables. The strain put on cables when they are twisted, bent, flexed, and yanked can wear cables out. Check that little strain is being placed on the cables.

For further information on fixturing maintenance, read Cirris' articles, “5 Tools to Help with Fixturing,” and “5 Questions You Should Ask Yourself About Fixturing.”

Training

 If you’d like to learn more about fixturing and other topics, or you’d like a more hands-on experience, check out Cirris training options.