The twice-monthly Lost Knowledge column explores the possible technology of the future in the forgotten ideas of the past (and those just slightly off to the side). Every other Wednesday, we look at retro-tech, “lost” technology, and the make-do, improvised “street tech” of village artisans and tradespeople from around the globe. “Lost Knowledge” was also the theme of MAKE, Volume 17
This week, we take a peek at the not-lost but fast-fading art of wire-wrapping. Wire-wrapping used to be a circuit prototyping and assembly technique found in the repertoire of every electronics geek, but increasingly, generations of wireheads are now coming of age knowing little-to-nothing about it. I, for one, have never operated a wrap tool or populated a wrap card in my life. I got a wire wrap tool in an electronics toolkit at the beginning of my interest in electronics and hadn’t the vaguest idea what it was until, years later, I saw it in a how-to book and thought: “Aha, so THAT’S what that funny-looking tool is for!”
Wikipedia has a good overview of wire-wrapping:
The sockets have square posts. The usual posts are 0.025 inches (635 micrometres) square, 1 inch (25.4 mm) high, and spaced at 0.1 inch (2.54 mm) intervals. Premium posts are hard-drawn beryllium–copper alloy plated with a 0.000025 inches (25 microinches) (635 nanometres) of gold to prevent corrosion. Less-expensive posts are bronze with tin plating.
The 30 AWG Kynar is cut into standard lengths, then one inch of insulation is removed on each end.
A “wire wrap tool” has two holes. The wire and one quarter inch (6.35 mm) of insulated wire are placed in a hole near the edge of the tool. The hole in the center of the tool is placed over the post.
The tool is rapidly twisted. The result is that 1.5 to 2 turns of insulated wire are wrapped around the post, and atop that, 7 to 9 turns of bare wire are wrapped around the post. The post has room for three such connections, although usually only one or two are needed. This permits manual wire-wrapping to be used for repairs.
The turn and a half of insulated wire helps prevent wire fatigue where it meets the post.
Above the turn of insulated wire, the bare wire wraps around the post. The corners of the post bite in with pressures of tons per square inch (MPa). This forces all the gases out of the area between the wire’s silver plate and the post’s gold or tin corners. Further, with 28 such connections (seven turns on a four-cornered post), a very reliable connection exists between the wire and the post. Furthermore, the corners of the posts are quite “sharp”.
The backplane of a Zilog Z80 computer from 1977.
There are three main methods of wire wrapping:
Manual Wrapping: Here, a small pen-like tool is used to hold and twist the end of the wire onto a square terminal post that’s been inserted into a wire wrap board (or card). This manual method is especially common for small prototyping projects or when doing repairs on large wire-wrapped boards.
Semi-automatic Wire-Wrapping: For larger wrapping jobs, a manually-powered “squeeze tool” is used. A trigger grip is pumped, which twists the wire onto the terminal post. Used in medium-sized wire-wrapping applications and in field repair.
Automatic Wire-Wrapping: For really extensive wire-wrapping of large electronics arrays, such as those used on airplanes, telephone switching systems, etc, electronic wrapping guns are used, which greatly speed up the wrapping process.
One of the better intro articles available online is on The Citizen Scientist’s site. In it, author Allan Rydberg runs through the steps of using a manual wrapping tool: 1) stripping the wire, 2) inserting it into the tool, 3) wrapping it onto the terminal post, and 4) inspecting the finished wrap (all seen below).
One might think there’s little-to-no reason to use this technology anymore, that it’s been completely superseded by solderless breadboarding in prototyping and soldered electronics in more permanent applications. But there are still situations where wire-wrapping makes sense. It is a purely mechanical connection that’s extremely reliable (especially in situations where a solder joint might be prone to failure over time). Also, in field applications, it can be easier for a tech to use a wrapping tool instead of a soldering iron to do repair work. With the onward march of miniaturization, and surface-mounted components, wire-wrapping has greatly diminished, but it’s still an option in some situations, especially for high-reliability systems, such as in aviation, military tech, and telcos (tho I’m not sure how many new systems are being built these days with wire-wrap technology). Maybe some wire-wrap aficionados will chime in in comments and tell us where this tech is still in active use.
There’s amazingly little information online about wire-wrapping. Here are a few resources:
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