Wire wrap
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- This article deals with electronics manufacturing and prototyping techniques, see Wire wrap jewellery for the jewellery related topic
Wire wrap is a technique for constructing small numbers of complex electronics assemblies. 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, radar, sonar, pipe organs, and other complex pieces of equipment that are needed in small volumes; the Apollo Guidance Computer, among many other historically relevant computers, was constructed using wire wrap technology.
Wire wrap construction can produce assemblies which are more reliable than printed circuits — connections are less prone to fail due to vibration or physical stresses on the base board, and the lack of solder precludes corrosion, dry joints, etc. The connections themselves are firmer and possibly have lower electrical resistance due to cold welding of the wire to the terminal post at the corners.
Wire wrap construction became popular around 1960, and use has now sharply declined. Surface-mount technology and the increase in electronic switching speed have made the technique much less useful than in previous decades. Solderless breadboards and the decreasing cost of professionally made PCBs have nearly eliminated this technology.
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[edit] Overview
The electronic parts sometimes plug into sockets. The sockets are attached with cyanoacrylate (or silicone adhesive) to thin plates of glass-fiber-reinforced epoxy.
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.
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, 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".
There are three ways of placing wires on a board.
[edit] Manual Wire Wrap
A manual wire wrap tool resembles a small pen. It is convenient for minor repairs. Wire wrap is one of the most repairable systems for assembling electronics. Posts can be rewrapped up to ten times without appreciable wear, provided that new wire is used each time. Slightly larger jobs are done with a manual "wire wrap gun" having a geared and spring loaded squeeze grip to spin the bit rapidly. Such tools were used in large numbers in American telephone exchanges in the last third of the 20th century, usually with a bigger bit to handle 22 or 24 AWG wire rather than the smaller 28 or 30 AWG used in circuit boards and backplanes. The larger posts can be rewrapped hundreds of times. They persisted into the 21st century in distribution frames where insulation-displacement connectors had not taken over entirely. Larger, hand held, high speed electric guns were used for permanent wiring, when installing exchange equipment between the late 1960s when they replaced soldering, and the middle 1980s when they were gradually replaced by connectorized cables.
[edit] Semiautomated Wire Wrap
Semiautomated powered 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 allows the operator to place wires without concern about whether they are on the correct pin, since the computer places the gun correctly.
Semi-automated wire wrapping is unique among prototyping systems because it can place twisted pairs, permitting complex high frequency computer and radar systems.
[edit] Automated Wire Wrapping
Automated wire-wrap machines, as manufactured by the Gardner Denver Company in the 1960s and 1970s, were capable of automatically routing, cutting, stripping and wrapping wires onto an electronic "backplane" or "circuit board". The machines were driven by wiring instructions encoded onto punch cards, Mylar punched hole tape, and early micro computers.
The earliest machines (14FB and 14FG models, for example) were initially configured as "horizontal", which meant that the wire wrap board was placed upside down (pins up) onto a horizontal tooling plate, which was then rolled into the machine and locked onto a rotating (TRP table rotational position of four positions) and shifting (PLP = pallet longitudinal position of 11 positions) pallet assembly. These machines included very large hydraulic units for powering the servos that drove the ball screw mounted "A" and "B" drive carriages, a 6' tall electronics cabinet loaded with hundreds of IBM control relays, many dozens of solenoids for controlling the various pneumatic mechanical subsystems, and an IBM 029 card reader for positioning instructions. The automatic wire wrap machines themselves were quite large, 6 ft (2 m) tall and 8 ft (3 m) square. Servicing the machines was extremely complex, and often meant climbing inside them just to work on them. This could be quite dangerous if safety interlocks were not maintained properly; there were rumors throughout the industry that some fatalities/serious injuries had actually occurred.
Later, somewhat smaller machines were "vertical" (14FV) which meant the boards were placed onto a tooling plate with pins facing the machine operator. Gone were the hydraulic units, in favor of direct drive motors to rotate the ball screws, with rotary encoders to provide positioning feedback. This generally provided better visibility of the product for the operator, although maximum wrap area was significantly less than the Horizontal machines. Top speeds on horizontal machines were generally around 500-600 wires per hour, while the vertical machines could reach rates as high as 1200 per hour, depending on board quality and wiring configurations.
Wires would be routed over the board, using "dressing fingers", and carriages would lower the A and B wrapping bits onto the board. The process for wrapping a wire was as follows (Note: the "A" carriage was on the right, while the "B" carriage was on the left). Machine carriages would meet at the next "A" carriage X/Y wire routing position, and the wire feed and stripper assembly located just under the "A" carriage would clamp the supply wire and feed it (push) to the "B" carriage. The "B Gripper" on the B carriage would accept the wire by clamping it, once limit switches in the strip and feed assembly indicated they had completed the feed cycle. Next, the "B" carriage would move "X" (to the left) to the first wire routing position, pulling the supply wire as it moved through the feed assembly from the supply reel, and the "B" dressing finger would pivot down over the wire. Once the limit switch for the dressing finger indicated it was down, the "B" carriage would move "Y" to the target pin. The "A" carriage dressing finger would then pivot down, and the "A" carriage would move "Y" to its target pin, still pulling supply wire as it moved. Once all wrapping bits and dressing fingers were in position, the cut and strip assembly would retract, stripping the trailing edge of the wire on the "A" side (and simultaneously stripping the leading edge of the next wire). The "A" gripper would clamp the wire against the wrapping bit, and the wrapping tools would close the bits, which meant the outer bit sleeves would retract, pulling both wire ends up into the bits. Once the wire was safely loaded into the wrapping bits, the "A" and "B" grippers would open, and the A and B tools, along with the dressing fingers, would lower "Z" onto the pins. Once the designated "Z" level had been reached (again, sensed by more limit switches) the pneumatic tools would spin, and back pressure would allow the tools to rise up slightly as the wire wrapped around the pins. Waste insulation (transferred from the "A" carriage cut and strip assembly to the "B" carriage during wire feed) is ejected into the waste container at the far left side of the "B" carriage while the wires are wrapped. Finally, the "A" and "B" tools are raised "Z", dressing fingers are retracted, and the carriages regroup for the next cycle.
[edit] Use of Electronic Design Automation
In wire-wrapping, electronic design automation can design the board, and optimize the order in which wires are placed.
The first stage was that a schematic was encoded into a netlist. This step is now done automatically by EDA programs that perform "schematic capture". A netlist is conceptually a list of pins, with each pin having an associated signal name.
The next step was to encode the pin positions of each device. The easy way to do this is to encode lettered rows and numbered columns where the devices should go. The computer then assigns pin 1 of each device in the bill of materials to an intersection, and renames the devices in the bill of materials by their row and column.
The computer would then "explode" the device list into a complete pin list for the board by using templates for each type of device. A template is map of a device's pins. It can be encoded once, and then shared by all devices of that type.
Some systems optimized the design by experimentally swapping the positions of parts and logic gates to reduce the wire length. After each movement, the associated pins in the netlist would be renamed. Some systems could also automatically discover power pins in the devices, and generate wires to the nearest power pins.
The computer program then merges the netlist (sorted by pin name) with the pin list (sorted by pin name), transferring the physical coordinates of the pin list to the netlist. The netlist is then resorted, by net name.
The programs then try to reorder each net in the signal-pin list to "route" each signal in the shortest way. The routing problem is equivalent to the travelling salesman problem, is therefore NP complete, and therefore not amenable to a perfect solution. One practical routing algorithm is to pick the pin farthest from the center of the board, then use a greedy algorithm to select the next-nearest pin with the same signal name.
Once routed, each pair of nodes in a net becomes a wire, in a "wire list". The computer then reads incidental information (wire color, order in the net, length of the wire, etc) in the netlist and interprets it to renumber the wire list to optimize the ordering and direction of wires during production. The wire list is then resorted by the wire numbers.
For example, wires are always "top and 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.
Long wires are usually 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.
Wires of different colors can also be placed together. Most wires are blue. Power and ground wires are often made with red and black. Clock wires (or other wires needing special routing) are often made yellow or white. Twisted pairs are usually black and white.
Another optimization is that within each size and color of wire, the computer selects the next wire so that the wrap head moves to the nearest pin. This can save up to 40% of the wrap time, almost getting two wire-wrap machines for the price of one. It also reduces wear on the wire-wrap machines.
Finally, the direction of placing a wire can be optimized 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.
Lastly, the sorted, optimized wire list is then printed out for use by machine operators, and turned into a tape or card deck for the machine. Machine-readable copies of this valuable production data are often archived at the same time.
