Crimp (electrical)

The technique of soldering wires has remained common for at least a century, however crimp terminals came into use in the middle of the 20th century. In 1953, AMP Incorporated (now TE Connectivity) introduced crimp barrel terminals, and in 1957 Cannon Brothers experimented with machined contacts integrating crimp barrels.[6] During the 1960s, several standards for crimp connectors were published, including MS3191-1, MS3191-4 and MIL-T-22520. In 2010, the predominant standard for crimp connectors changed to MIL-DTL-22520.[7]

Crimp-on connectors are attached by inserting the stripped end of a stranded wire into a portion of the connector, which is then mechanically deformed by compressing (crimping) it tightly around the wire.[8] The crimping is usually accomplished with special crimping tool such as crimping pliers. A key idea behind crimped connectors is that the finished connection should be gas-tight.

Effective crimp connections deform the metal of the connector past its yield point so that the compressed wire causes tension in the surrounding connector, and these forces counter each other to create a high degree of static friction which holds the cable in place. Due to the elastic nature of the metal in crimped connections, they are highly resistant to vibration and thermal shock.[9]

Two main classes of wire crimps exist:[10]

• Closed barrel crimps have a cylindrical opening for a wire, and the crimping tool deforms the originally circular cross section of the terminal into some other shape (see § Crimp shapes). This method of crimping is less resilient to vibration.
• Open barrel crimps have "ears" of metal that are shaped like a V or U, and the crimp terminal bends and folds them over the wire prior to swaging the wire to the terminal. Open-barrel terminals are claimed to be easier to automate because of avoiding the need to funnel stranded wire into the narrow opening of a barrel terminal.

In addition to their shape, crimped connectors can also be characterized by their insulation (insulated or non-insulated), and whether they crimp onto the conductor(s) of a wire (wire crimp) or its insulation (insulation crimp).[11]

Crimped connections are common alternatives to soldered connections. There are complex considerations for determining which method is appropriate – crimp connections are sometimes preferred for these reasons:

• Easier, cheaper, or faster to reproduce reliably in large-scale production
• Fewer dangerous or harmful processes involved in termination (soldered connections require aggressive cleaning, high heat, and possibly toxic solders)
• Potentially superior mechanical characteristics due to strain relief and lack of solder wicking

Crimped connectors fulfill numerous uses, including termination of wires to screw terminals, blade terminals, ring/spade terminals, wire splices, or various combinations of these. A tube-shaped connector with two crimps for splicing wires in-line is called a butt splice connector.

Single-wire crimp terminals

Blade connectors (bottom), ring and spade connectors (top), and bullet connectors (right).

• Blade or quick disconnect (e.g. Faston or Lucar)
• Bullet (e.g. Shur-Plug)
• Butt splice
• Flag tongue
• Rectangular tongue
• Hook tongue
• Spade tongue (flanged, short spring, long spring)
• Ring tongue (slotted, offset)
• Multiple stud
• Packard 56
• Pin (SAE/J928)[17]
• Wire pin

A crimped connection will only be reliable if a number of criteria are met:

• All strands have been deformed enough, into roughly a trapezoidal shape, to cold-flow into the terminal body[22]
• The compression force is not too light, nor too strong[23]
• The connector body is not overly deformed
• Wires must be in solid working condition, cannot have scrapes, nicks, severing or other damages
• Insulation should not show any signs of pinching, pulling, fraying, discoloration, or charring [24]
• Large voids are not left inside the crimp (caused by not enough wire inside the connector)
• The wire should have as many strands as possible, so that a few damaged or uninserted wires will not adversely affect the crimp density, and thus degrade the electrical and mechanical properties of the connection.[2]

Micrographs of the crimped connections can be prepared to illustrate good and bad crimps for training and quality assurance purposes. The assembled connection is cut in cross-section, polished and washed in nitric acid to dissolve any copper dust that may be filling voids leading to a false indication of a good crimp.

A wide variety of crimping tools exist, and they are generally designed for a specific type and size of terminal. Handheld tools (sometimes called crimping pliers) are most common, which may be ratcheting. For mass production operations, automated devices are available.[3]