Topic

Wire wrapping

Wire wrapping uses the fact that, when a wire is wound tightly around a square post with sharp corners, the corners cut through both insulation or any surface layer, at the same time breaking down the oxide on the post, to create a metal-to-metal contact. By controlling the wire tension, and the way the wire is wrapped around the post, it is possible to make a joint that is reliable for long periods. Applications range from quite simple prototype boards to complex military assemblies.

The posts around which the wires are wrapped are either component sockets or connectors into which assemblies are plugged. The technique is therefore more widely used for backplanes than for board assembly. Posts are square and typically 0.017" square 1" high and spaced at 0.1" intervals. The better-quality posts are hard-drawn beryllium-copper with a flash plating of gold, but bronze posts with tin plating are also used where cost is an issue.

Typically the wire is silver-plated soft copper, and insulated with Kynar, a fluorocarbon resin. The normal wrap has 1½–2 turns of insulated wire wrapped around the post, in order to help keep the wire from fatiguing where it meets the post, followed by 5–10 turns of bare wire, the number depending on the gauge of wire.

The process can be carried out manually, which makes wire wrapping very suitable for repair and modification in the field; the wires need to be replaced, but posts can be re-wrapped a number of times.

Totally manual operations are prone to variation, so semi-automated wire-wrap systems, as illustrated here, are popular. As with any assembly process, the level of automation will depend on the application and funds available ranging from semi-automated systems that identify the post to be wrapped but use the operator to make the final approach, to totally-automated systems that will both strip and wrap the wire.

A number of practicalities associated with wire wrapping are quite closely associated with a harness practice. One of the few books with any content on this is Gerry Herrick’s Electronic assembly: soft soldering and wire wrapping published by Prentice-Hall in 1992 as ISBN0-13-248766-7, though this is unfortunately out of print. However, an indication of the requirements can be gauged from this NASA workmanship standards pictorial reference

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