A proper crimp creates a gas-tight, mechanically strong connection that can outlast the equipment it serves. A bad crimp fails in the field—sometimes spectacularly, always expensively. The difference often comes down to following proven practices consistently.
Whether you're crimping a few wires on a prototype or thousands on a production line, these ten practices apply. They're drawn from IPC/WHMA-A-620 standards, terminal manufacturer specifications, and decades of production floor experience.
Match the Terminal to the Wire
Every terminal is designed for a specific wire gauge range—typically spanning 2-3 AWG sizes. Using the wrong terminal is the most common crimping mistake and the easiest to prevent.
| Terminal Color Code | Wire Range (AWG) | Wire Range (mm²) |
|---|---|---|
| Red | 22-18 AWG | 0.5-1.0 mm² |
| Blue | 16-14 AWG | 1.5-2.5 mm² |
| Yellow | 12-10 AWG | 4.0-6.0 mm² |
Common Mistake
Using the largest terminal that "fits" the wire. A 20 AWG wire in a blue (14-16) terminal will crimp, but without proper compression, it'll fail under vibration. Always use the smallest appropriate terminal.
Use the Correct Crimping Tool
Crimping tools aren't interchangeable. Each terminal manufacturer specifies approved tooling—using the wrong tool voids the terminal's performance ratings and creates unreliable connections.
Hand Tools
- Ratcheting for consistent force
- Cycle-controlled release
- Manufacturer-specific dies
- Best for: Prototypes, low volume
Bench Presses
- Pneumatic or manual
- Consistent force application
- Quick die changes
- Best for: Medium volume
Automatic Machines
- Strip, crimp, verify in one cycle
- Crimp force monitoring
- 100% process control
- Best for: High volume
"I've seen engineers grab whatever crimper is handy and wonder why their connections fail. That ratcheting tool you bought at the hardware store? It's designed for electrical contractors, not for crimping Molex terminals. Use the manufacturer's specified tool—no exceptions."
Hommer Zhao
Production Engineering Manager
Set Proper Crimp Height
Crimp height is the most critical measurable parameter of a finished crimp. Too high means inadequate compression; too low means over-compressed, weakened strands. Terminal manufacturers provide specific height ranges for each wire size combination.
Crimp Height Measurement
How to Measure:
- 1Use a crimp height micrometer (not standard calipers)
- 2Measure at the center of the conductor crimp barrel
- 3Ensure anvil is flat, not at an angle
- 4Compare to manufacturer specification
Typical Tolerance:
Most terminals specify ±0.05mm to ±0.10mm crimp height tolerance. For example:
Strip to the Right Length
Strip length determines where the conductor sits in the terminal barrel. Too short leaves conductor not fully engaged; too long extends past the barrel creating shorts or poor insertion.
| Terminal Type | Typical Strip Length | Critical Consideration |
|---|---|---|
| Open Barrel | Barrel length + 0.5-1.0mm | Conductor visible in inspection window |
| Closed Barrel | Per manufacturer spec (varies widely) | Conductor bottoms out in barrel |
| Insulation Displacement | No stripping required | Full insertion depth critical |
| Splice | Meets in center ± 1mm | Equal engagement both sides |
For precision stripping equipment, see our wire cutting and stripping capabilities.
Position Wire Correctly
Even with correct strip length, improper wire positioning creates defects. The conductor must be centered in the barrel, with no stray strands outside the crimp zone.
Correct Positioning
- • Conductor centered in barrel
- • All strands captured
- • Insulation meets barrel edge
- • No conductor visible between crimps
Incorrect Positioning
- • Wire off-center (uneven compression)
- • Stray strands outside barrel
- • Gap between insulation and barrel
- • Wire angled, not straight
Create Proper Bellmouth
Bellmouth is the slight flare at the front and back edges of the conductor crimp. It prevents sharp edges from cutting strands and provides a smooth transition into the crimp zone.
Bellmouth Requirements
Front Bellmouth (Wire Entry)
- Visible flare, not flat or sharp
- Protects strands during insertion
- Typically 0.25-0.5mm height
Rear Bellmouth (Wire Exit)
- Prevents flexing damage at crimp edge
- Critical for vibration resistance
- Required for Class 2/3 per IPC-620
Verify Insulation Crimp
The insulation crimp (also called strain relief crimp) grips the wire jacket, not the conductor. It prevents the wire from pulling out under stress and provides a secondary barrier against moisture.
Inspect Every Crimp
Visual inspection catches most crimp defects before they become field failures. Train your eyes—or your operators' eyes—to check these attributes on every termination.
Visual Inspection Checklist
For production environments, establish go/no-go visual standards with physical samples of acceptable and defective crimps. Our testing methods guide covers inspection procedures in detail.
Perform Pull Force Testing
Pull force testing is the ultimate validation of crimp quality. It verifies that the mechanical connection can withstand the stresses of installation and service life.
| Wire Size (AWG) | Minimum Pull Force (N) | Minimum Pull Force (lbf) |
|---|---|---|
| 24 | 22 | 5.0 |
| 22 | 31 | 7.0 |
| 20 | 44 | 10.0 |
| 18 | 67 | 15.0 |
| 16 | 89 | 20.0 |
| 14 | 133 | 30.0 |
"We test first-off samples from every production setup and random samples throughout the run. It adds time, but it's nothing compared to the cost of a recall. One automotive customer told us their field failure investigation costs $50,000 minimum—our testing adds maybe $50 to a harness."
Hommer Zhao
Quality Manager
Document and Control
Even perfect technique means nothing without process control. Document your crimp specifications, train operators, and maintain equipment to ensure consistency over time.
Crimp Specification Sheet
Document terminal part number, wire size, strip length, crimp height, and tooling for each application.
Operator Training & Certification
Train operators on technique, have them demonstrate proficiency, and certify per IPC standards.
Tool Calibration
Calibrate hand tools annually, automatic machines per manufacturer schedule. Log all calibrations.
First Article Inspection
Verify first piece of each production run meets all specifications before proceeding.
Statistical Process Control
Track crimp heights over time to detect tool wear before it causes defects.
Quick Reference: Crimp Quality Checklist
Frequently Asked Questions
Can I re-crimp a terminal if I made a mistake?
No. Once crimped, the terminal barrel is work-hardened and won't compress properly a second time. Cut off the damaged terminal and start fresh. Re-crimping creates weak connections.
How often should I calibrate hand crimpers?
Annual calibration is standard for most applications. High-volume or safety-critical work may require semi-annual or quarterly calibration. Always calibrate after dropping or obvious damage.
What causes 'bird caging' in crimped wires?
Bird caging (strands splaying out) typically results from pulling the wire through the terminal before crimping or from over-stripping. Position the wire gently and verify strip length.
Is soldering a crimp ever acceptable?
Generally no—solder wicks into the stranded wire, creating a stress point that fails under vibration. Some specifications explicitly prohibit it. If you need both mechanical and solder connections, use terminals designed for it.
Related Resources
About the Author
Hommer Zhao is a Production Engineering Manager specializing in high-reliability wire harness manufacturing. With IPC-A-620 CIT certification and over 15 years of hands-on crimping experience, he has trained hundreds of operators and troubleshot thousands of crimp-related quality issues.
Connect with Hommer