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Wire wrap

Wire wrap is a technique for constructing small numbers of complex electronics. It is an alternative technique to the use of small runs of printed circuit boards, and has the advantage of being easily changed for prototyping work. It has been used to construct telephone exchanges, computers, control consoles, radios, radars, sonars, and other complex pieces of equipment that are needed in small volumes.

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Overview

The electronic parts plug into sockets. The sockets are glued with cyanoacrylate to thin plates of glass-fiber-reinforced epoxy.

The sockets have square posts. The usual posts are 0.017 inches square, 1 inch high, and spaced at 0.1 inch intervals. Premium posts are hard-drawn beryllium-copper alloy plated with a 0.0025" of gold to prevent corrosion. Less-expensive posts are bronze with tin plating.

30 gauge silver-plated soft copper wire is insulated with a fluorocarbon that does not emit dangerous gases when heated. The most common insulation is "kynar."

The 30 AWG Kynar is cut into standard lengths, and then stripped of insulation for one inch on each end.

A special tool called a "wire-wrap" tool has two holes. The wire and a quarter-inch of insulated wire are placed in a hole hear 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 two are needed. This permits manual wire-wrapping to be used for repairs.

The turn and a half of insulated wire help keep the wire from fatiguing 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 tons-per-square inch pressures. 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.

There are three ways of placing wires on a board.

Manual wire wrap

Manual wire wrap tool resemble small pens. They are convenient for repair. Wirewrap is the one of the most repairable systems for assembling complex elecronics. Posts can be rewrapped up to ten times without appreciable wear, as long as new wire is used each time.

Semiautomated wire wrap

Semiautomated wire-wrap systems place "wire-wrap guns" on arms moved in two dimensions by computer-controlled motors. The guns are manually pulled down, and the trigger pressed to make a wrap. The wires are inserted into the gun manually. This system lets the operator place wires without worrying about whether they are on the right pin. The computer puts the gun over the right pin.

Automated wire wrapping

Automated wire-wrap machines have two wrap-heads, and two claws to route the wire. On each wire, the two wrap heads come together above a place between the two pins of the next wire. One end is stripped and inserted into the passive head, which grips the wire. The wire is unreeled from the active head. The claws pull the wire to the correct routing. Then the other end of the wire is stripped and clamped in the active head. Both heads and both claws descend, routing the wire and wrapping both pins at once. Automated machines can place a wire every second, almost five times as fast as the most skilled semiautomated wire-wrap operators.

In commercial wire-wrapping, computer programs are used to optimize the order in which wires are placed. The programs generate a list of wires, and then put numbers in the list. The list is then sorted by the numbers.

For example, wires are "top & bottomed." That is, wires alternate between high and low as they connect a series of pins. This lets a repair or modification occur with the removal of at most three wires.

Another optimization is that long wires are placed first within a level, so that shorter wires will hold longer wires down. This reduces vibration of the longer wires, making the board more rugged in a vibrating environment such as a vehicle.

Placing all the wires of a certain size, makes it easier for a manual or semiautomated wire-wrapping machine to use precut wire. This especially speeds up manual wrapping.

Another optimization is that within each size of wire, the computer selects the next wire so that the wrap head moves to the nearest pin.

Finally, the direction of placing a wire can be optmized for right-handed wire-wrap people, so that wires are placed from right to left. In a semi-automated wire-wrap system, this means that the wrap head moves away from the user's hand when placing a wire. The user can then use their strong hand and eye to route the wire.

Semiautomated wirewrapping is unique among prototyping systems because it can place twisted pairs, permitting complex high frequency computer and radar systems.

See also printed circuit board and electronics.



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