Introduction to wire wrap

20 06 2012

So, I bet a lot of you who dabble in breadboard prototype electronics projects use things like these:

Jumper wires are popular and gets the job done alright for making quick and dirty connections, but in my experience they’re not practical for high-complexity projects or anything that is supposed to last more than a few days:

  • Big mess – tend to form big loops over your breadboard
  • Easy to yank out by accident
  • Generally too expensive to be left inside permanent projects

Here’s how I usually prototype – a typical breadboard project:

Random project I can’t even remember the purpose of.

Let’s have a closer look at these connections:

Wirewrapped pin headers

The wires are standard AWG 30 Kynar-jacketed single-core, silver-coated wires wrapped tightly around gold-coated 0.025″ square pins spaced 0.1″ (2.54mm). This method of making connections is called wire wrap.

Wirewrapping is a still-useful but very, very old technology. It was pioneered by American engineers sometime back in the early 1960s and was used for decades to form wiring backplanes and bus connections for supercomputers and telephony equipment.

It is cheap in use, quickly applied, extremely rugged – the wire connections can withstand vibrations more reliably than even soldered equipment. For example, the Apollo Guidance Computer used in the lunar landings consisted of modules connected by epoxy-encased wirewrap. It can be used in highly complex projects also: The custom video ICs in the Commodore Amiga were prototyped using this technology..

For more specifications and application examples, read the Wikipedia article.

Tools and equipment needed:

Pin headers, wire (Kynar jacketed spool of AWG 30 ), wire wrap pen, cutter

The wire comes in different colors – just google for wire wrap AWG30 and you’ll find many good deals. Typical is about $3-$5 for 100 feet. Get different colors so you can identify power buses and distinguish signal classes. The cutter should be a comfortable and sharp type with a small beak.

VECTECH CB-30 M

The wrapping tool itself comes in several varieties, but this type is my favorite: The VECTECH CB-30M is common in Asia, and you can walk into any Sim Lim Tower or Akihabara electronics shop and find these off the shelf. There are also wire wrap tools in Radio Shack and Fry’s, but the kinds I’ve seen there have round barrels and lousy grip.

For this demonstration I’ll be putting wires on the pins of one of these:

Dark and mean – the original Wiring board

How many people still remember the original Wiring board that begat the whole Arduino craze? That’s right, this brilliant thing is the spiritual and technological granddaddy of all the countless Arduinos in use today in their hundreds of variations. It shipped with those 0.025″ square posts pre-soldered and was clearly designed for exactly such wirewrap connections. This one looks a little grubby since it has been used and reused for dozens of projects over the years. Anyway, I’ll use this to demonstrate the application of wirewrap. Here we go:

First, insert one inch of wire through the stripper blade that’s embedded in the tool. Gently “hook” the wire on the blade and pull back to make a clean cut.

You should end up with something like this. Then, insert the wire through the opening on the long end of the tool so that it goes into the slot along the top:

This is the tricky part. The stripped wire goes into the slot along the top.

The wire should be pushed all the way in so that the jacket is visible in the hole:

Once you get the hang of it, the above steps should have taken you only a few seconds to do. It’s really very easy!
Next, you place the tool tip over the target pin that you want the wire attached to:

Then, while holding the wire down with one finger, gently twirl the tool around about 7 complete turns in order to apply the wire to the post. You can tell that you’re done when the wire has completely disappeared from the slot, and you can remove the tool:

That’s it! That was easy! Look at that beautiful connection! A correctly wrapped pin has about 30 points of contact each held firmly in place with a remarkable twenty tons of force per square inch. There is a turn and a half (or so) of wire jacket on the bottom to provide strain relief. The silver-plated wire cold welds to the gold coated pin and so the connection has virtually zero electrical resistance. And there’s enough work space to wire wrap another pin next to, or in between other pins that have already been wrapped:

You can also apply several wires to one pin – there is usually room for 3 wires on a common-size pin, although the ones on the Wiring board shown here are a bit shorter and so can fit only 2 each. Multiple connections on one pin lets you daisy-chain signals from one pin to the next. Some strategy is required for most efficient use of this approach, however – make sure you apply the wire in a logical order if you need to undo the connections later.

It is possible to use wire wrap on the pins of other components even if they don’t have the standard square shape. The hold will be slightly less strong, but usually works perfectly fine still. Here wire wrap has been applied to LEDs – note that the cathode pins (black wires) have two wires attached to each.

LED pins solderlessly connected with wire wrap

Undo

The connections are firmly made and don’t come off by accident. However, intentionally removing a connection from a pin is even easier than putting it on, and leaves the pin fully reusable for another connection:

To remove the wirewrap connection from a pin, place the short “undo” end of the tool over the pin and hold it gently against the top of the wire. Then, twirl the tool around in the opposite direction of the application direction to loosen the wrap a bit. After just a few turns you can usually pull the wire right off the pin leaving no marks on it at all.

The ghosts of connections past

I usually prefer clockwise application and so counterclockwise twirl to remove, but that’s completely arbitrary. Wires removed can sometimes be reused – cut off the spiral knots at the ends.

Miscellaneous

Different size breakaway wirewrap pin headers

You can easily obtain the standard-size pins as breakaway strips easily cut or snapped to size and mashed into the breadboard sockets. In places like Sim Lim Tower you can find many different types for cheap.

IC sockets with long pins for wire wrapping

These things are stupidly useful too and were used by the hundreds in the Commodore Amiga custom graphics co-processor mockups: You can use them with wirewrapped prototype PCBs to fit ICs in a circuit and using only the wire wrap pins on the underside for making connections.


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