Wire Wrap

Wire wrap technology represents a significant stage in the evolution of electronic assembly methods, originating in the telecommunications sector and later becoming a mainstream technique for constructing reliable and easily modifiable electronic circuits. Its development, characteristics and applications have had substantial influence on prototyping practices, high-reliability system design and early computer engineering.
Wire wrapping is based on the creation of mechanical and electrical connections without solder, using tightly wound bare wire around a square metal post. The resulting gas-tight, cold-welded joints offered a compelling alternative to solder-based connections and found wide use in digital systems, computer backplanes and microwave-frequency prototypes throughout the mid-twentieth century.

Origins and Historical Development

The origins of wire wrap lie in early twentieth-century point-to-point wiring, when components were frequently connected by manually wrapping wires around binding posts or spade lugs before soldering. However, the modern technique took shape after the Second World War, when Bell Laboratories sought a robust and practical method for interconnecting contacts in a new relay intended for the Bell Telephone system. A design team led by Arthur C. Keller created the Keller Wrap Gun, a hand tool enabling consistent, high-pressure wraps. The technology was later transferred to Western Electric for industrial deployment.
A procurement decision at Western Electric led to the manufacture of the tool by Keller Tool of Grand Haven, Michigan. Beginning in 1953, the company introduced refinements that improved manufacturability and user operation, and subsequently gained a licence to supply the tools commercially. The method spread rapidly through industry. IBM’s first transistorised computer systems in the late 1950s, including those using the Standard Modular System, relied on wire-wrapped backplanes. The approach was also employed in early prototyping at Apple Computer for the original Macintosh, demonstrating its value for small runs and design iteration.
Wire wrap reached peak popularity during the 1960s and early 1970s, becoming a standard for high-density digital assemblies. Its use declined sharply in subsequent decades with the emergence of surface-mount technology, affordable printed circuit board (PCB) fabrication and solderless breadboards, although it remains relevant for specific applications requiring mechanical reliability or rapid modification.

Principles of Construction and Electrical Features

Wire wrap works by forming a tight mechanical bond and low-resistance electrical connection around a square, typically gold-plated, post. For 28 or 30 AWG wire, a standard wrap involves approximately seven turns of bare wire above half to one and a half turns of insulated wire that act as strain relief. The sharp edges of the post bite into the silver-plated copper conductor, displacing air and oxides to create up to twenty-eight discrete contact points. This structure produces cold-welded joints capable of resisting vibration, corrosion and mechanical stress more effectively than many soldered connections.
Most wire wrap systems rely on Kynar-insulated, silver-plated copper wire. The insulation tolerates heat and does not emit hazardous gases when warmed. Standard posts are made from bronze or beryllium-copper alloys, with premium versions plated with gold to ensure long-term corrosion resistance. The sockets into which components are placed are generally bonded to glass-fibre-reinforced epoxy plates using silicone or cyanoacrylate adhesives.
Mechanical stability is reinforced through systematic layering: long wires are typically placed first, and shorter wires subsequently applied to secure them. Repairability is enhanced by ensuring that all wire ends terminate at the same height on each post, enabling straightforward replacement. In aerospace-rated assemblies, additional measures such as wax conformal coatings are used to damp vibration, with epoxy avoided because it prevents later rework.

Manual Tools and Wrapping Procedures

Hand wrapping tools contain two holes: one for feeding the wire and insulated segment, and a central hole that fits over the post. Rapid twisting produces a controlled sequence of insulated and bare wire wraps. Typically a post accommodates up to three separate connections, although one or two are more common. The mechanical stress exerted by a correctly designed tool can reach pressures of around twenty tons per square inch at the wire-to-post interface, which is key to maintaining the gas-tight cold-weld.
Manual wire wrapping remains useful for repairs, modifications and low-volume prototyping. Its flexibility allows engineers to reroute or replace connections without redesigning a PCB layout, a feature that distinguished it from other automation-compatible processes of its time.

Automated Wire Wrap Machinery

Automation significantly expanded the possible scale and complexity of wire-wrapped assemblies. Beginning in the 1960s, the Gardner Denver Company produced computer-controlled wire wrap machines capable of routing, measuring, stripping and wrapping wires with precision.
Early horizontal-format machines such as the 14FB and 14FG employed hydraulically driven servos, ball-screw carriages, extensive relay logic and card-reader instruction systems. Boards were mounted pins-up, and the machinery allowed four rotational and eleven longitudinal positions for tool access. These units were physically large and required specialist maintenance.
Later vertical-format machines, notably the 14FV, replaced hydraulic units with direct-drive motors and rotary encoders. Improved visibility and reduced mechanical complexity enabled wrap rates of up to 1,200 wires per hour, compared with 500–600 wires per hour for earlier systems. Automated systems allowed exact wire lengths, paired routing and the construction of intricate wiring topologies suitable for supercomputers, digital backplanes and microwave circuits.

Applications, Suitability and Technical Considerations

Wire wrap has historically been most suitable for digital circuits with relatively few discrete components. Integrated circuits mounted in wire wrap sockets can be easily reconfigured, and the technique proved valuable for experimental logic systems, minicomputers and telecommunications switching. Its strengths include:

• high reliability under vibration and mechanical stress
• rapid, design-free prototyping without the need for PCB fabrication
• low electrical resistance due to cold-welded joints
• controlled wire lengths and the ability to route twisted pairs or quads