You run the pull test. The crimp fails. You adjust the tool, run it again-it passes. Then next week, it fails again.
This cycle is frustrating, expensive, and dangerous. A crimp that measures 38 N pull force instead of the 55 N IPC/WHMA-A-620 minimum for 20 AWG passes visual inspection every time-and fails in the field within three to five years. In high-current applications, an under-crimped connection can drop a 12V solenoid below its activation threshold after just six months of thermal cycling.
The problem is rarely random. Crimp pull-test failures follow predictable patterns with identifiable root causes. Here are the most common ones-and how to fix them.
1. Crimp Height Is Out of Specification
Crimp height is the single most important controllable variable in the crimping process. If it is too high, voids remain between strands and the terminal barrel, reducing both pull strength and electrical conductivity. If it is too low, strands are nicked or severed-the crimp may pass a pull test initially but fail under vibration.
Solution: Measure crimp height with a micrometer using a point and blade-not calipers. Flash on the crimp surface can throw off the reading. Compare against the terminal manufacturer's specification. An occasional conductor crimp height measurement should be performed during every production run.
Using a crimping tool that doesn't match the terminal series and wire gauge is one of the most common causes of pull-test failure. Worn or misaligned dies create uneven compression-one side over-crimped, the other loose.
Solution: Verify that your die set is designed for the specific terminal and wire size you're running. Inspect dies for wear daily. According to IPC/WHMA-A-620B-S §19.6.1, crimp tools should not be used for longer than 30 days without verification. For high-frequency operations exceeding 1,000 crimps per day, accuracy checks every six months are the minimum-but monthly is safer.
3. Missing or Damaged Strands
Every strand must be captured inside the crimp barrel. Missing strands-whether from nicking during stripping, birdcaging during wire handling, or breakage during crimping-directly reduce pull force. Birdcaging occurs when wire strands are pushed back or splayed outward during stripping or terminal feeding, preventing all strands from entering the crimp barrel.
Solution: Use a precision stripper and replace blades regularly. After stripping, re-twist strands into a tight bundle before insertion. Inspect for birdcaging before crimping. If strands are floating outside the barrel, the crimp should be rejected-even if it passes a pull test. A single stray strand in a high-density connector can pierce adjacent insulation and cause a dead short.
4. Wire Not Fully Inserted
If the stripped conductor does not reach the barrel stop, the effective crimp length is reduced. Fewer strands make contact with the terminal metal, and pull force drops proportionally.
Solution: Push the wire in until the insulation meets the barrel stop. Verify that the conductor end is flush with or extends slightly beyond the barrel edge. Confirm that all strands are inside the barrel-none outside except the bellmouth flare.
5. Insulation Trapped in the Conductor Crimp
When the strip length is too short, or the wire is inserted at an angle, insulation can end up inside the conductor crimp zone. The insulation prevents metal-to-metal contact, creating high resistance and weak retention.
Solution: Set stripping length to the exact dimension specified by the terminal drawing. Discard wires with damaged or elongated insulation. Inspect the crimp transition zone-equal amounts of conductor and insulation should be visible at the boundary.
6. Wrong Wire Gauge for the Terminal
Using a wire that is too small for the terminal barrel means the crimp cannot exert enough compressive force to lock strands in place. Using a wire that is too large means strands cannot be fully captured.
Solution: Verify wire gauge against the terminal manufacturer's specification before production. If you must change wire size, use a splice-do not force a mismatched combination.
How to Verify You've Fixed It
Three checks confirm a proper crimp:
Crimp height measurement - the most accurate non-destructive test. Compare against the manufacturer's spec every shift.
Pull force testing - destructive. Measure with no influence from the insulation crimp. Pull at 50 mm/min. Minimum values per IPC/WHMA-A-620 range from 10 N for 30 AWG to 265 N for 8 AWG. Run at least 5 measurements per setup.
Micro-section analysis - for critical applications. A proper crimp shows conductor strands compressed to 75–85% of original cross-section, with a honeycomb structure and zero voids. Pull testing alone cannot detect internal voids caused by under-crimping.
Prevention: The Real Solution
Pull-test failures are symptoms, not causes. The real fix is process control:
Crimp Force Monitoring (CFM) measures force on every cycle and compares it to a golden waveform. If it detects a missing strand or high crimp, it locks the press automatically.
Pull test at every shift change, applicator change, and new wire reel-not once a week.
Daily die inspection and documented tool verification prevent drift before it produces scrap.
A crimp that passes continuity testing but fails pull force can still light a tester with only a few strands in contact. The green light proves a circuit exists-not that the connection is mechanically sound.
Crimp pull-test failures don't happen randomly. They come from out-of-spec crimp height, worn tooling, missing strands, improper insertion, or mismatched materials. Each has a specific fix. Measure crimp height. Verify tooling. Inspect every crimp for strand capture. And above all-test on samples, not just on gut feeling.
Because the most expensive crimp failure is the one you never caught.
