Terminal Crimp Quality: Crimp Height, Pull Force and Defects
Crimp quality is decided by three measurable controls: crimp height within about ±0.05 mm of the terminal maker’s window, pull force at or above the minimum for the wire gauge (for example roughly 50 N for 20 AWG strands), and a compression ratio near 80–90%. Specify these three values in writing and most field failures disappear.
A crimp is also the one joint in a wire harness that can no longer be inspected once assembled, so buyers have to judge it up front — on paper, then on samples. Yet crimping is the step most often left vague in RFQs. Drawings arrive with a terminal part number and a wire part number but no crimp height window, no pull force requirement and no acceptance criteria. That gap is where intermittent opens, high contact resistance and vibration failures are born.
Suzhou Fuxi Electromechanical Equipment Co., Ltd. (Xinya) has manufactured custom wire harnesses since 2008 and, as an authorised distributor of Xinya Electronic Co., Ltd., supplies more than 100 electronic wire styles with UL, VDE, PSE, CE, TUV, RoHS and REACH options. This article sets out the crimp numbers we put on our own drawings for products such as terminal crimped wire leads, so you can hold any supplier — including us — to the same standard. The full harness product line shows where these crimp standards are applied.
Why Does Crimp Height Matter So Much?
Crimp height is the single best predictor of a reliable crimp because it controls how tightly the barrel compresses the strand bundle. A correct crimp deforms the strands into a gas-tight mass at roughly 80–90% of their original cross-section: tight enough to expel air and block oxidation, loose enough that no strand is sheared.
Terminal manufacturers publish a crimp height window — typically a few hundredths of a millimetre wide, with working tolerances around ±0.02 to ±0.05 mm depending on gauge and barrel type. Below the window the barrel cuts strands; above it the bundle stays loose and pull force collapses. Because crimp height can be measured in seconds with a micrometer on a sectioned sample, it is the control checked most often: at set-up, after every reel change and at intervals during the run, with destructive pull testing reserved for sampled parts.
How Much Pull Force Does a Crimp Need?
Pull force is the pass/fail test: the crimped joint is pulled at a controlled speed until it fails, and the breaking value must exceed the minimum for the wire size. The table below lists values widely used across UL 486A-style acceptance criteria and major terminal maker application notes. Treat them as typical industry minima — the terminal manufacturer’s datasheet always takes precedence for a specific part.
| Wire size | Typical minimum pull force | Approximate imperial |
|---|---|---|
| 28 AWG | 10 N | 2.2 lbf |
| 26 AWG | 15 N | 3.4 lbf |
| 24 AWG | 22 N | 5 lbf |
| 22 AWG | 30 N | 6.7 lbf |
| 20 AWG | 50 N | 11 lbf |
| 18 AWG | 70 N | 15.7 lbf |
| 16 AWG | 100 N | 22.5 lbf |
| 14 AWG | 120 N | 27 lbf |
| 12 AWG | 180 N | 40 lbf |
| 10 AWG | 220 N | 49 lbf |
Two practical notes. First, the failure mode matters as much as the number: a correct crimp pulls the strands apart inside the barrel — the wire should never slide out of it. Second, test speed influences results; pull testers commonly run at 25–100 mm/min, and numbers from different suppliers are only comparable at similar speeds.
What Should a Crimp Specification Contain?
A complete crimp specification is short — about half a page — but it removes nearly all ambiguity. For each wire and terminal combination, ask your supplier to state:
- Terminal and wire part numbers, including plating and strand class
- Conductor crimp height window, plus the insulation crimp window where the terminal has one
- Minimum pull force for the gauge
- Strip length tolerance and the maximum allowed number of nicked or cut strands
- Bellmouth requirement at the barrel entrance
- Sample size and frequency for crimp height checks and pull tests
Our online harness builder captures exactly these fields — terminal, wire style, gauge and protection — so what you configure there converts directly into a specification our engineers can quote. If you already have a drawing, the same fields are listed step by step in our custom wire harness specification checklist, and the wire selector helps you lock down the wire side first.
Which Crimp Defects Cause Field Failures?
Six defects account for most crimp-related returns we see on crimped lead assemblies and panel harnesses, and all of them are visible on a sectioned or pulled sample:
| Defect | Visual sign | Main risk |
|---|---|---|
| Under-crimp (height too high) | Loose strands, wire pulls out of the barrel | Intermittent opens, high resistance |
| Over-crimp (height too low) | Strands sheared or notched at the barrel bottom | Broken strands, vibration failure |
| Stray strand outside the barrel | A single strand escapes the conductor crimp | Short circuit to adjacent contacts |
| Insulation crimp piercing strands | Insulation bellmouth closed onto the conductor | Wicking, fatigue at the crimp edge |
| Nicked strands from stripping | Visible cut marks before the barrel | Reduced flex life, premature break |
| Missing bellmouth | Sharp barrel edge against bare strand | Work hardening and fracture at the exit |
Most of these trace back to process setup rather than operator skill: a worn applicator anvil, the wrong strip length, or a terminal reel with different plating thickness. That is why a supplier’s crimp process — setup sheets, applicator maintenance, first-article discipline — tells you more than its marketing page.
How Is Crimp Quality Verified in Production?
Verification runs on three layers. Set-up: the applicator is set against the terminal maker’s window, and a first article is sectioned, measured and pull-tested to destruction before the batch is released. In-process: crimp height is checked with a micrometer on sample parts at fixed intervals and recorded; crimp force monitoring on the press flags any drift in the force curve even while height still reads in window. Batch: pull tests run at a defined sample rate — commonly three samples per reel or per shift for critical harnesses such as automotive and transport programs.
You can see this equipment and the operators who use it in our factory videos. The same verification flow applies whether the harness is destined for industrial automation panels, energy storage racks or appliance volume production.
What Should Buyers Check Before Approving a Supplier?
Before placing a first order, ask for three documents. First, a crimp cross-section or crimp height record from a sectioned first article — the fastest way to see whether the process is actually controlled. Second, pull test data with the failure mode noted, not just a pass stamp. Third, the setup sheet showing the crimp height window the operator works to. Questions about any of these are answered on our FAQ page.
Also ask how wire supply is handled, because a harness is only as consistent as its wire. Common styles such as UL1015 hook-up wire are stocked in volume, while specialty insulations typically need about 7 days of procurement. Drawing on the PVC and SR-PVC hook-up family and other UL, VDE, PSE and TUV-listed styles as an authorised distributor keeps both sampling and repeat orders short. How these choices move the number on your quotation is covered in wire harness cost factors, and the approval workflow itself in build to print vs build to sample.
Frequently Asked Questions About Crimp Quality
How often should pull force testing be performed?
A pull test belongs in every first article, after any reel, terminal or applicator change, plus a defined in-process sample rate — commonly three samples per reel or per shift. Recording every result, including failures, is what turns the test into a trend you can audit.
Can buyers specify their own crimp height windows?
Yes. If your terminal maker’s datasheet defines a window, we build to it and copy it onto the setup sheet. If the data is not available, we work from the terminal manufacturer’s published specification and state the applied window on the first-article report so it can be checked on receipt.
What is the difference between open-barrel and closed-barrel crimps?
Open-barrel terminals — the stamping-style contacts common in JST, Molex and automotive connectors — are closed in a press applicator, which allows crimp height measurement and high-volume production. Closed-barrel terminals are closed with a hand or pneumatic tool and suit low and medium volumes. The pull force requirement is the same for both; only the measurement method differs.
Do you accept customer-specified terminal brands?
Yes — JST, Molex, TE and customer-furnished terminals are routine. Genuine parts and the correct applicator keep crimp height inside the published window; no-name substitutes or wrong tools are the usual root cause behind “identical” terminals that fail pull testing.
What comes with a crimp qualification report?
Crimp height measurements against the stated window, pull force values with the observed failure mode, wire and terminal lot references, and photographs of the sectioned sample on request. Keep this report with the harness drawing so repeat orders can be compared against the approved baseline.
How fast can crimped samples be supplied?
Harnesses built on stocked wire styles — UL1007, UL1015, UL1061, UL1569 and similar — move into sampling quickly; wires outside stock are typically sourced in about 7 days. Production scheduling then follows the drawing and quantity, which is why complete drawings shorten the whole cycle.
Get Your Crimp Specification Reviewed
Send us your drawing or terminal list and we will return a quotation with crimp heights, pull force values and the test plan written out. Request a quote or configure your requirements in the harness builder, and our engineers will confirm the numbers before any sample is made.