On crimping

Ah, crimping wires! Such an exciting and useful skill to have.  But less obvious than you might imagine.

I feel compelled to write about crimping because of a recent interaction I had with a professional contract engineering team. First, they wanted to bill me over $300 for a special crimping tool. After I brought it up, they decide to waive the charge because the tool will go in their toolbox, not mine. Fine by me.

After testing it, they send me the device I had them working on. The device works fine for about two minutes before the battery dies. Due to time restrictions, they were unable to provide a way to charge the device without opening its shell, so I opened it up, connected the battery to its charger, and waited.

I had an important meeting the next day with an investor, and I was hoping to have the device ready for demo. The battery was charged – I verified voltage with a multimeter – so I figured it would be fine.

It wasn’t. I still cringe thinking about the demo I attempted. The indicator LEDs, which are supposed to flash when the device is turned on, remained silent and unblinking.

It’s pretty hard to perform electrical troubleshooting while pitching someone important, so when I got home I was able to take an analytical look at the device.  Almost by chance, I noticed the lights would activate when I gently moved one of the connecting wires.  Clearly, this was the result of a poor contact, and I just found which contact was bad: one of the leads from the power switch!

I hardly had to pull on the wire at all, and it popped out of the connector – not just the plastic housing, but the actual tin crimp connector! That’s what we call a bad crimp.

Note the loose, dangling green wire (out of focus). It’s missing a terminal!

For clarity, wires for micro-electronics and prototyping are often crimped into tin crimp connectors, which are then mated into plastic housings that lock into matching plastic housings. The molex connector is a relatively large version of this general interconnect system. 2.54 mm headers are common with microcontrollers, and it’s extremely cheap to buy crimp connectors and crimp connector housings instead of paying too much for pre-fab ‘jumper cables.’

Detail view of a decent crimp using a female crimp terminal for 2.54 mm header. Image credit: pololu.com

There are two key parts to this interconnect system: first (and most importantly), the crimp connector has to be secure; second, the crimp connector must exactly match the plastic housing.

Let’s take a quick pictorial look at the crimping process.

It doesn’t take any special tools, just some $2 Harbor Freight needle-nose pliers, generic $10 Radioshack crimpers, and a pair of needle-point forceps borrowed from the biology lab.

Above: crimping tools, cut-tape reel of female crimp connectors, 2.54 mm header pins with female-female jumper cable

Use the pliers to bend the outer jacket clips (far right end of pictured connector) into a tighter, rounder shape.

If you tighten them too much, the wire jacket won’t fit. Just slide the forceps inside and loosen the circle until the wire can slide in.

Strip the wire, making sure there is only enough exposed wire to make contact with the wire clips. If you strip too much, then the excess length of stripped wire will extend past the wire clips. Slide the wire into the connector to test fit, and trim any extra bare wire if necessary.

Above: too much stripped wire. Also, note the proper position of the wire jacket – it should extend slightly past the jacket clips. When the jacket clips are crimped, the connector will be secured to the wire jacket and cannot be pulled off, even if you use significant force.

Crimp ONLY the jacket clips first. I crimp once, then turn the connector 180 degrees in my crimpers and press down hard, using both hands.

Half of a secure crimp. See how the jacket clips come together?

At this point, check the crimp – are the jacket clips secure? Can you pull the connector off at this point? If so, you have the foundation for a bad crimp.

Use the forceps and push the stripped wire back against the back of wire clips. Make a second crimp at the wire clips – again, rotating the connector 180 degrees in order to create an even crimp.

The result? A connection that won’t fail.  I can pull on it with pliers, using extreme force, and it won’t come apart. There’s good reason the wiring connections in the International Space Station are all crimp connections.

Slide the tin connector into the appropriate plastic connector housing, ensuring proper orientation.  It should make a satisfying ‘click’ when mated properly, otherwise, it may be easily pulled out of the connector housing.

Simple, right?