Wire wrap
Encyclopedia
Wire wrap is a technology used to assemble electronics. It is a method to construct circuit boards without having to make a printed circuit board
Printed circuit board
A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board or etched wiring...

. Wires can be wrapped by hand or by machine, and can be hand-modified afterwards. It was popular for large-scale manufacturing in the 60s and early 70s, and continues to be used for short runs and prototypes. It is unusual among prototyping technologies in that very complex assemblies can be produced by automated equipment, and then easily repaired or modified by hand.

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 soldering faults such as corrosion, cold joints and dry joints. The connections themselves are firmer and have lower electrical resistance due to cold welding
Cold welding
Cold or contact welding is a solid-state welding process in which joining takes place without fusion/heating at the interface of the two parts to be welded. Unlike in the fusion-welding processes, no liquid or molten phase is present in the joint....

 of the wire to the terminal post at the corners.

A correctly made wire-wrap connection is seven (7) turns of wire with 0.5-1.5 turns of insulated wire at the bottom for strain relief. The square hard-gold-plated post thus forms 28 redundant contacts. The silver-plated wire coating cold-welds to the gold. If corrosion occurs, it occurs on the outside of the wire, not on the gas-tight contact where oxygen cannot penetrate to form oxides. A correctly designed wire-wrap tool applies up to twenty tons of force per square inch on each joint.

Wire wrap was used for assembly of high frequency prototypes and small production runs, including gigahertz microwave circuits and super computers. It is unique among automated prototyping techniques in that wire lengths can be exactly controlled, and twisted pairs or magnetically-shielded twisted quads can be routed together.

Wire wrap construction became popular around 1960 in circuit board manufacturing, and use has now sharply declined. Surface-mount technology
Surface-mount technology
Surface mount technology is a method for constructing electronic circuits in which the components are mounted directly onto the surface of printed circuit boards . An electronic device so made is called a surface mount device...

 has made the technique much less useful than in previous decades. Solder-less breadboard
Breadboard
A breadboard is a construction base for prototyping of electronics. The term is commonly used to refer to solderless breadboard ....

s and the decreasing cost of professionally made PCBs have nearly eliminated this technology.

In telecommunications wire wrap is in common high volume use in modern communications networks for cross connects between copper wiring plant. For example, most phone lines from the outside plant
Outside plant
In telecommunication, the term outside plant has the following meanings:*In civilian telecommunications, outside plant refers to all of the physical cabling and supporting infrastructure , and any associated hardware located between a demarcation point in a switching facility and a demarcation...

 go to wire wrap panels in a central office, whether used for POTS
POTS
POTS may refer to:* Plain old telephone service, basic wireline telecommunication connection** POTS codec, a digital audio device** DSL filter, also known as a POTS filter* Postural orthostatic tachycardia syndrome, a medical condition...

 phone service, DSL or T1 lines. Typically at a main distribution frame
Main distribution frame
In telephony, a main distribution frame is a signal distribution frame for connecting equipment to cables and subscriber carrier equipment . The MDF is a termination point within the local telephone exchange where exchange equipment and terminations of local loops are connected by jumper wires...

 Internal Cross Facilities Assignments and External Cross Facilities Assignments, are connected together via jumpers that are wire wrapped. Wire wrap is popular in telecommunications since it is one of the most secure ways to attach wires, and provides excellent and consistent data layer contact. Wirewrap panels are rated for high quality data services, including Category 5
Category 5
Category 5 may refer to:*Category 5 , an album from rock band, FireHouse*Category 5 cable, used for carrying data*Category 5 computer virus, as classified by Symantec Corporation*Category 5 Records, a record label...

 grade wiring. The principal competitor in this application is punch block
Punch block
A punch down block is a type of electrical connection often used in telephony. It is named because the solid copper wires are "punched down" into short open-ended slots which are a type of insulation-displacement connectors...

s, which are quicker but less secure.

Overview

The electronic parts sometimes plug into socket
Jack (connector)
In electronics and electrical assemblies, the term jack commonly refers to a surface-mounted connector, often, but not always, with the female electrical contact or socket, and is the "more fixed" connector of a connector pair...

s. The sockets are attached with cyanoacrylate
Cyanoacrylate
Cyanoacrylate is the generic name for cyanoacrylate based fast-acting adhesives such as methyl 2-cyanoacrylate, ethyl-2-cyanoacrylate , and n-butyl cyanoacrylate...

 (or silicone
Silicone
Silicones are inert, synthetic compounds with a variety of forms and uses. Typically heat-resistant and rubber-like, they are used in sealants, adhesives, lubricants, medical applications , cookware, and insulation....

 adhesive) to thin plates of glass-fiber-reinforced epoxy
Epoxy
Epoxy, also known as polyepoxide, is a thermosetting polymer formed from reaction of an epoxide "resin" with polyamine "hardener". Epoxy has a wide range of applications, including fiber-reinforced plastic materials and general purpose adhesives....

 (fiberglass
Fiberglass
Glass fiber is a material consisting of numerous extremely fine fibers of glass.Glassmakers throughout history have experimented with glass fibers, but mass manufacture of glass fiber was only made possible with the invention of finer machine tooling...

).

The sockets have square posts. The usual posts are 0.025 in (0.635 mm) square, 1 in (25.4 mm) high, and spaced at 0.1 in (2.54 mm) intervals. Premium posts are hard-drawn beryllium copper
Beryllium copper
Beryllium copper , also known as copper beryllium, beryllium bronze and spring copper, is a copper alloy with 0.5—3% beryllium and sometimes with other alloying elements. Beryllium copper combines high strength with non-magnetic and non-sparking qualities. It has excellent metalworking, forming...

 alloy plated with a 2.5E-05 in (635 nm) of gold to prevent corrosion. Less-expensive posts are bronze
Bronze
Bronze is a metal alloy consisting primarily of copper, usually with tin as the main additive. It is hard and brittle, and it was particularly significant in antiquity, so much so that the Bronze Age was named after the metal...

 with tin
Tin
Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...

 plating.

30 gauge
American wire gauge
American wire gauge , also known as the Brown & Sharpe wire gauge, is a standardized wire gauge system used since 1857 predominantly in the United States and Canada for the diameters of round, solid, nonferrous, electrically conducting wire...

 silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...

-plated soft copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

 wire is insulated with a fluorocarbon
Fluorocarbon
Fluorocarbons, sometimes referred to as perfluorocarbons or PFCs, are organofluorine compounds that contain only carbon and fluorine bonded together in strong carbon–fluorine bonds. Fluoroalkanes that contain only single bonds are more chemically and thermally stable than alkanes...

 that does not emit dangerous gases when heated. The most common insulation is "Kynar".

The 30 AWG
American wire gauge
American wire gauge , also known as the Brown & Sharpe wire gauge, is a standardized wire gauge system used since 1857 predominantly in the United States and Canada for the diameters of round, solid, nonferrous, electrically conducting wire...

 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 1/4 in 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": they have a quite-small radius of curvature.

There are three ways of placing wires on a board.

In professionally-built wire-wrap boards, long wires are placed first so that shorter wires mechanically secure the long wires. Also, to make an assembly more repairable, wires are applied in layers. The ends of each wire are always at the same height on the post, so that at most three wires need to be replaced to replace a wire. Also, to make the layers easier to see, they are made with different colors of insulation. In space-rated or airworthy wire-wrap assemblies, the wires are boxed, and may be conformally-coated with wax to reduce vibration. Epoxy is never used for the coating because it makes an assembly unrepairable.

History

Wire wrapping technology was developed after WWII at Bell Laboratories as a means of making electrical connections in a new relay being designed for use in the Bell Telephone system. The design team at Bell was headed up by Arthur Charles Keller (18 Aug 1901 – 25 Aug 1983), a renowned inventor and audio engineer. A new hand tool to make the wrap, the “Keller Wrap Gun” was also designed at Bell Labs, and the entire wrap system was passed over to Western Electric
Western Electric
Western Electric Company was an American electrical engineering company, the manufacturing arm of AT&T from 1881 to 1995. It was the scene of a number of technological innovations and also some seminal developments in industrial management...

 for execution. After a “make or buy” committee at Western Electric decided to have the hand tool manufactured by an outside vendor, Western Electric sent the tool contract out for bids. Keller Tool of Grand Haven, Michigan, a supplier of rotary hand tools to Western Electric, won the contract and made several design changes to make the tool easier to manufacture and to use. Keller began manufacturing the tools in 1953, and subsequently obtained a license from Western Electric allowing sale of the technology on the open market. The tool was marketed under its original name – since the name of the manufacturer was coincidentally the same as the name of the inventor.

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 exchange
Telephone exchange
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...

s 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 frame
Distribution frame
In telecommunications, a distribution frame is a passive device which terminates cables, allowing arbitrary interconnections to be made.For example, the Main Distribution Frame located at a telephone central office terminates the cables leading to subscribers on the one hand, and cables leading to...

s where insulation-displacement connector
Insulation-displacement connector
An Insulation-displacement connector, insulation-displacement technology/termination or insulation-piercing connector is an electrical connector designed to be connected to the conductor of an insulated wire or cable by a connection process which forces a selectively sharpened blade or blades...

s had not taken over entirely. Larger, hand held, high speed electric wrap guns replaced soldering in the late 1960s for permanent wiring, when installing exchange equipment. In the middle 1980s they were gradually replaced by connectorized cables.

The Apollo Guidance Computer
Apollo Guidance Computer
The Apollo Guidance Computer provided onboard computation and control for guidance, navigation, and control of the Command Module and Lunar Module spacecraft of the Apollo program...

 with its short production run and stringent reliability requirement was one of the early applications of wire wrap to computer assembly.

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 pair
Twisted pair
Twisted pair cabling is a type of wiring in which two conductors are twisted together for the purposes of canceling out electromagnetic interference from external sources; for instance, electromagnetic radiation from unshielded twisted pair cables, and crosstalk between neighboring pairs...

s, and twisted magnetically-shielded quads, permitting the assembly of complex radar and high speed digital circuits.

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 punched card
Punched card
A punched card, punch card, IBM card, or Hollerith card is a piece of stiff paper that contains digital information represented by the presence or absence of holes in predefined positions...

s, 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 ft (1.8 m) 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 (1.8 m) tall and 8 ft (2.4 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 screw
Ball screw
A ball screw is a mechanical linear actuator that translates rotational motion to linear motion with little friction. A threaded shaft provides a helical raceway for ball bearings which act as a precision screw. As well as being able to apply or withstand high thrust loads, they can do so with...

s, with rotary encoder
Rotary encoder
A rotary encoder, also called a shaft encoder, is an electro-mechanical device that converts the angular position or motion of a shaft or axle to an analog or digital code. The output of incremental encoders provides information about the motion of the shaft which is typically further processed...

s 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.

Design automation

In wire-wrapping, electronic design automation
Electronic design automation
Electronic design automation is a category of software tools for designing electronic systems such as printed circuit boards and integrated circuits...

 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 gate
Logic gate
A logic gate is an idealized or physical device implementing a Boolean function, that is, it performs a logical operation on one or more logic inputs and produces a single logic output. Depending on the context, the term may refer to an ideal logic gate, one that has for instance zero rise time and...

s 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
Travelling salesman problem
The travelling salesman problem is an NP-hard problem in combinatorial optimization studied in operations research and theoretical computer science. Given a list of cities and their pairwise distances, the task is to find the shortest possible tour that visits each city exactly once...

, which is NP complete, and therefore not amenable to a perfect solution on a reasonable time scale. 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.

See also

  • Point-to-point construction
    Point-to-point construction
    Point-to-point construction refers to the method in which electronics circuits were constructed before the 1950s. Point-to-point construction is still used to construct prototype equipment with few or heavy electronic components....

  • Stripboard
    Stripboard
    Stripboard is a widely-used type of electronics prototyping board characterized by a 0.1 inch regular grid of holes, with wide parallel strips of copper cladding running in one direction all the way across one side of the board...

  • Wire sculpture
    Wire sculpture
    Wire sculpture refers to the creation of sculpture or jewelry out of wire. The medium was experimented with by Alexander Calder....

  • Wiring pencil
    Wiring pencil
    A wiring pencil is a tool for making electrical connections.A small reel of copper wire coated with a special insulating varnish is mounted on the end of the tool...

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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