Crimp Quality and Pull Testing
Why Crimps Matter
A crimp is a cold weld formed between the wire strands and the terminal when the tool compresses them. Done correctly, it is more reliable than a soldered joint, because it is ductile, it resists vibration and it does not embrittle the strands. Done incorrectly, it is one of the least visible and most damaging defects in an assembly: a crimp with the wrong tool creates a joint that passes a quick look and fails after a few thermal cycles, and the failure appears in the field as an intermittent fault that is very hard to trace. Crimp quality is therefore a process control issue rather than an operator skill issue.
Tooling Is the Process
The tooling decides the result. A crimp tool is designed for a specific terminal, a specific wire gauge and a specific insulation diameter, and it produces a controlled compression rather than a crush. A generic tool, a tool set to the wrong position or a tool that has drifted will produce a joint that looks acceptable and performs badly. The most reliable arrangement is a tool with a ratchet that will not release until the crimp is complete, used with the correct die set, and with the wire gauge and insulation diameter within the terminal’s specified range. Where a terminal requires a different die for the insulation crimp, the two must be used together.
What a Good Crimp Looks Like
The visual criteria are established and should be written down. The wire should be visible in the bell mouth of the terminal, the strands should not be cut or nicked, and the insulation should be captured by the insulation crimp rather than the conductor crimp if the terminal has one. The terminal’s wings should be rolled into the wire rather than lying on top of it, and the crimp should be centred. There should be no loose strands outside the terminal, no flash, and no crack in the plating. The insulation should not be pushed so far forward that the strands are compressed against the terminal’s stop. Each of these is easy to check with a small magnification and a good light.
Cross Section and Measurement
The definitive check on a crimp is a cross section. A sample is mounted, ground and polished, and the image shows how the strands have deformed and whether the compression is correct. The measurement of the crimp height, taken with a crimp micrometer, is the production control that correlates with the cross section, and it is specified by the terminal manufacturer. The crimp height is a better control than the crimp width because it directly reflects the compression, and it should be measured on a sample from the beginning, the middle and the end of each batch. A height that drifts indicates that the tooling is wearing or the wire has changed.
Pull Testing
The pull test measures the force required to separate the wire from the terminal, and it is the acceptance test that confirms the process. The test is destructive, so it is performed on samples, and the standard gives minimum forces by wire gauge. Where a crimp is made correctly, the failure should occur in the wire rather than at the crimp, or the wire should break before the termination releases. A crimp that pulls out cleanly has not formed a weld, and a crimp whose strands break at the entrance indicates a damaged conductor. The test should be recorded with the batch, and the results should be monitored as a process control rather than treated as a one-off qualification.
Common Defects and Their Causes
The typical defects are recognisable. A crimp that is too low, caused by an over compressed tool or a wire that is undersized, cuts the strands and reduces the strength. A crimp that is too high, from a tool that is worn or from a wire that is oversized, leaves the strands loose. A crimp on the insulation rather than the conductor leaves the strands untouched, and it is often the result of a strip length that is too short. A strand folded back over the insulation escapes the crimp and can bridge to a neighbouring terminal. Contamination from handling or from a lubricant reduces the weld. Each has a process cause and a process cure.
Records and Traceability
A wire harness is difficult to inspect after it is assembled, so the record is the evidence. The terminal part number, the wire gauge, the tool and the die set, the crimp height, the pull test result and the operator should all be recorded for the batch. Where the harness is part of a product that will be traced, the record should be linked to the product’s identity so that a field failure can be investigated against the process data. Where a harness is supplied by a subcontractor, the same records should be required, and the requirements should be written into the specification rather than assumed.
FAQ
Why is a good crimp more reliable than a soldered joint? It is ductile and resists vibration, and it does not embrittle the strands.
What is the production control? The crimp height, measured with a micrometer on samples and correlated with a cross section.
What does a pull test prove? That the crimp formed a weld: the wire should break before the termination releases.
What causes loose strands? A crimp height that is too high, from a worn tool or an oversized wire, or a strip length that is too long.
Why keep records for a harness? Because the assembled harness cannot be inspected, so the process data is the evidence of quality.
Conclusion
A crimp is a controlled cold weld, so use the correct die, control the crimp height and verify with pull tests and cross sections. Record the batch. Wire termination practice belongs to PCB assembly and cable integration, the assembly it connects to sits in SMT PCB assembly, and the acceptance criteria are part of quality management. Termination requirements for a new product are specified during prototype PCB assembly in 2026.
