Crimping Fundamentals

Posted by SZFRS Engineering Team

Crimping is the dominant termination method in cable manufacturing globally. Fast, repeatable, and capable of producing gas-tight electrical connections, crimping has displaced soldering across most volume cable applications for good reasons — speed (sub-second cycle times), consistency (statistical process control on key dimensions), and reliability (gas-tight crimps maintain low contact resistance across decades and harsh environments). The science behind a good crimp is more interesting than it first appears. Compression of stranded conductor inside a copper barrel, when done correctly, produces a metal-to-metal cold weld at the strand surfaces — atoms from adjacent strands actually bond, creating a solid metallic connection without solder. This guide walks through crimp formation, measurement methods, tooling, standards, and the workmanship details that drive crimp quality.

TL;DR — What Makes a Good Crimp

A good crimp produces a gas-tight connection — the conductor strands are compressed enough that no air can reach the metal-to-metal contact zones. Gas-tight crimps maintain low contact resistance for decades because there’s no oxidation occurring at the buried interface. Three measurements verify crimp quality: crimp height (the dimension across the crimp barrel after crimping), pull force (the tensile force the crimp can sustain before failure), and micrograph cross-section (visual inspection of internal crimp structure). Crimp height comes from the crimp tool specification; pull force from the contact specification (typically 80-120% of conductor tensile strength); micrograph from periodic destructive inspection. Standards (UL 486A, IPC/WHMA-A-620, MIL-T-22520, SAE-AS22520) define acceptance criteria. Tooling families (Schleuniger, Komax, Mecal, TE, Molex) drive the manufacturing process. Below covers each in detail.

The Science of Crimp Formation

When a metal contact is crimped onto a conductor, several mechanical processes happen simultaneously:

• Compression of conductor strands. The crimp barrel deforms inward, pressing strands tightly together. Strand-to-strand voids are eliminated as strands flatten against each other.
• Plastic deformation of conductor metal. Individual strand surfaces deform under compression. Tin oxide layers (on tinned copper) crack and break; pure copper surfaces are exposed to adjacent strand surfaces.
• Strand-to-strand cold welding. Where freshly-exposed copper surfaces contact each other under sufficient pressure, atoms migrate across the boundary creating metallic bonds. The bond is real — strand-to-strand metallurgical fusion at low temperature (“cold welding”). The completed crimp is no longer just a mechanical compression; it’s metallurgically bonded.
• Compression of contact barrel. The barrel itself deforms, with the crimp tool’s anvil and die determining the final cross-section shape (typically square, hex, or B-shape).
• Stress-induced contact pressure. The compressed state retains residual stress that maintains contact pressure for decades. The crimp doesn’t loosen over time under normal conditions.

The result, when done correctly: a gas-tight metallic connection with electrical resistance often below 1 milliohm and mechanical strength approaching the conductor’s tensile strength. Field-failure rates for properly-made crimps are extremely low — well-made crimps in protected environments routinely run 30+ years without failure.

The tooling and process details that separate “properly made” from “problematic” are subtle. Compression that’s too light leaves voids and air pockets — gas-tight is lost, oxidation begins, contact resistance climbs over time. Compression that’s too aggressive can crush strands, breaking conductor cross-section and weakening the connection. The crimp tool specification defines the right compression range for each contact-conductor combination.

Crimp Height — The Primary Measurement

Crimp height is the dimension of the crimped barrel measured perpendicular to the crimp axis. It’s the most-monitored single dimension in cable crimping because it correlates strongly with crimp quality:

• Too high. Insufficient compression. Voids remain inside the crimp, gas-tight is lost, pull force is below specification.
• Too low. Over-compression. Conductor strands may be crushed; barrel may be damaged; pull force can paradoxically decrease as conductor cross-section is reduced.
• Just right. The crimp tool specification provides a target value with tolerance. Crimp height in the target range produces gas-tight connections with full pull force.

Typical crimp height tolerance is ±0.05 mm (±0.002 inch) around the target. For example, a 0.50 mm² wire with a particular contact might specify 1.55 mm crimp height with tolerance 1.50-1.60 mm. Crimp tools are calibrated to produce this dimension consistently.

Measurement methods:

• Optical measurement. Vision systems on automatic crimp machines measure crimp height in-line during production. Sub-second measurement, statistical tracking, and automatic rejection for out-of-tolerance crimps.
• Manual gauge. Hand-held crimp height gauges (typically Mitutoyo or similar precision micrometers) for sample inspection. Used for first-article verification and periodic checks.
• Automated micrometer. Stand-mounted micrometer with quick-change anvils. Faster than hand-held; standard for production sample inspection.

For Class 3 work (per IPC/WHMA-A-620), crimp height monitoring frequency is high — typically every 5-10 crimps for production runs. For Class 2, sampling frequency is lower (every 50-100 crimps). For Class 1, sampling is even less frequent. Statistical process control (SPC) charts track crimp height over time; sustained drift triggers tool inspection or recalibration.

Pull Force Testing — The Verification

Pull force is the tensile force the completed crimp can sustain before failure. It’s the most direct measure of crimp mechanical strength. Testing involves clamping the cable in a test fixture, gripping the crimped contact, and pulling until something fails:

• Failure modes. Wire pulls out of crimp (insufficient compression); conductor breaks at strand level (over-compressed); contact deforms or breaks (test fixture issue or extreme tool problem).
• Acceptance criteria. Per UL 486A or contact-specific datasheet. Typical specifications target 80-120% of the conductor’s specified tensile strength. For example, 18 AWG wire (specified tensile around 50-60 lbf) crimped on a typical contact should pull above 40 lbf.
• Sample frequency. Per Class — Class 3 typically every 100-1,000 crimps; Class 2 every 1,000-5,000; Class 1 less frequent. Pull force tests are destructive — test samples don’t go into shipped product.

Pull force testers come in two main categories:

• Mechanical pull force testers. Cantilever or pneumatic actuator pulls cable through a load cell. Real-time force display; force-vs-displacement graph for analysis. Typical equipment: Mark-10, ChatilLion, Mecmesin.
• Inline pull force testers on automatic crimp machines. Modern automatic crimp machines integrate pull force testing into the production cycle for select samples. Faster than offline testing.