Crimp Pull Test & Crimp Height for Buyers

What makes a good crimp

A crimp joins a wire to a terminal or contact by deforming the terminal barrel around the conductor strands, compressing them enough to form a gas-tight, low-resistance, mechanically strong joint. Too little compression leaves gaps and a weak joint. Too much cuts strands and weakens the wire. The window between them is set by the terminal maker for each terminal, wire size and tool.

A good crimp needs the right combination of four things: the terminal, the wire (size and strand construction), the tool or applicator, and the set-up. Change one and the result changes.

Terms you will meet

Conductor crimp: the part of the terminal that grips the wire strands.
Insulation crimp or support: the part that grips the insulation for strain relief.
Bellmouth: the flared edge of the conductor crimp that avoids cutting strands.
Brush: the strand ends protruding past the conductor crimp.
Applicator: the tooling in a crimp press, matched to a terminal family.

The checks, and what each proves

Check	How it is done	What it proves	Limits
Visual inspection	Magnified view against workmanship criteria such as IPC/WHMA-A-620	Bellmouth, strand visibility, insulation position, no cut strands visible	Cannot see compression inside the barrel
Crimp height	Crimp micrometer across the crimped barrel, compared with the terminal maker's specification	The applicator is set up correctly for that terminal and wire	A correct height does not detect every fault, such as missing strands
Pull test	Pulling wire from terminal on a calibrated tester until it breaks or pulls out, compared with a minimum force	Mechanical strength of the joint	Destructive; done on samples
Cross-section	Cutting, polishing and photographing the crimp under magnification	Compression, strand distribution, voids, symmetry	Destructive, slower, needs equipment or a lab
Crimp force monitoring	Sensors on the press record the force curve of every crimp and flag outliers	Every crimp in a run, not a sample	Needs set-up per terminal; mainly on automatic and bench presses

Where pull force values come from

Minimum pull force values depend on wire size and are published in several places. IPC/WHMA-A-620 includes a table of minimum pull forces for crimped terminations. UL 486A-486B covers wire connectors for safety certification. In automotive, SAE/USCAR-21 is a widely used crimp performance specification. Many terminal makers publish their own values, which can be higher. Aerospace crimp contacts are usually governed by their own military or industry specifications.

Because these sources differ, the drawing or purchase order should name which one applies. A pull test "passed" against one table may not pass against another. This page deliberately does not reproduce force values; take them from the current edition of the specification you call out.

What a capable shop does

Set-up verification. At the start of each run and after any change of terminal, reel, wire lot or applicator, crimp samples are checked for height and pulled, and the results recorded.
In-process checks. Further samples at set intervals and at the end of the run.
Tool control. Every applicator and hand tool has an identity, a calibration or verification record, and a record of which terminals it is approved for.
Terminal maker data on file. The crimp height and pull values used on the floor come from the terminal maker's application specification, at its current revision.
Hand tools matched to contacts. For military and aerospace contacts, the tool, positioner or turret and selector setting are those specified for the contact.
Records kept. Results are traceable to the job, so a later problem can be traced to a set-up.

What to ask a supplier

How is a crimp set-up approved before production starts, and who approves it?
How often are samples taken during a run, and what is done with parts made since the last good sample if a sample fails?
Which pull force table do you work to by default, and can you work to the one on my drawing?
Do you use crimp force monitoring, and on which machines?
Can you provide cross-sections for the first article, in-house or through a lab?
How are hand tools verified, and how often?

What to put in your requirements

The workmanship standard and class, for example IPC/WHMA-A-620 Class 3. See choosing a class.
The pull force specification, if it differs from the workmanship standard's default.
Whether pull test and crimp height records must be supplied, or only retained.
Whether cross-sections are required for the first article, and how many.
For automotive, the crimp performance specification required by your customer.

Common questions

Is a pull test enough on its own?

No. A crimp can meet a pull force minimum while having poor electrical properties, or a height outside the terminal maker's window. Pull tests, height checks and visual inspection work together.

Why is crimp height measured with a special micrometer?

The crimped barrel has a curved profile. A crimp micrometer has a blade on one side and a point on the other, so it measures the height at the right place without being thrown off by the crimp's shape.

Can I ask for crimp records with every shipment?

Yes, if you specify it in the purchase order. Many buyers ask for records to be retained and available on request, which is less paperwork for both sides.

Does a hand-crimped contact need the same checks?

Hand tools also need verification, and hand-crimped samples are pull tested too. For military contacts, tool gauging with a go/no-go gauge is a common check on the tool itself.