3.2 Crimp quality assurance

The fundamental challenge with a finished crimp is that its true internal quality is concealed. To the unaided eye, a loose crimp and a perfect, gas-tight cold weld frequently appear identical from the outside. Therefore, quality assurance cannot rely on visual inspection alone. It demands a rigorous, tiered validation strategy: Crimp Height (for process control), Pull Testing (for mechanical strength), and Micro-Sectioning (for internal metallurgy). Validating the process quantitatively at the start of every shift and every new batch ensures the reliability of the connections produced during that run.

Crimp height measurement (CHM): primary control

Crimp Height is the primary non-destructive variable that correlates directly with the gas-tight nature of the connection. It is the measure of the vertical compression applied cleanly to the terminal.

The measurement guideline

The Specialized Tool: Measurements are taken with a specialized blade-micrometer (measuring from point-to-flat). Standard flat-jaw calipers are inadequate because they tend to bridge the curving bottom of the crimp, resulting in an artificially high reading.
The Manufacturer Target: The required height is defined by the terminal manufacturer’s engineering specification for the exact wire gauge being used (e.g. 1.15 mm ± 0.05 mm). This is a mandated operational target.
Active Process Control: Crimp height serves as a Critical-to-Quality (CTQ) characteristic. Modern automated presses utilize Crimp Force Monitors (CFM) that detect subtle height variations in real-time by precisely measuring the physical force required to fully close the die on every machine stroke.

Application: The setup operator must measure 5 consecutive pieces at the start of the production run. If the calculated mean height is not centered within the specification window, the applicator micro-dial is adjusted. Process stability requires operating well within the limits, rather than at the edge of the specification window.

Pull testing: destructive verification

The Pull Test verifies the mechanical tensile strength of the cold weld. It serves as evidence that the wire will not separate from the terminal under expected field tension or vibration.

The pull test protocols

The Frequency: Destructive pull testing is mandated at Setup, at any significant Material Change (such as loading a fresh wire spool), at any major Tool Change, and at clearly defined intervals (e.g. the start and end of every shift) to bracket the production run.
The Method: The wire is pulled axially straight from the terminal body at a constant, controlled speed (typically designated between 25 to 50 mm/minute). Jerking or snapping the wire yields invalid, inflated force results.
The Failure Modes:
- Pull Out (Review Needed): The wire slips entirely out of the crimp barrel. This indicates a severe under-compression condition (the crimp height is likely set too high).
- Wire Break (Success): The wire snaps outside the crimp area. This is the preferred failure mode, demonstrating the crimp joint is mechanically stronger than the raw copper wire.
- Terminal Tear: The terminal tears or breaks apart. This is considered acceptable only if the recorded force precisely at the moment of failure exceeds the mandated minimum limit.

Minimum force guidelines (reference: UL 486A / IPC/WHMA-A-620)

22 AWG: 36 N (8 lbs)
20 AWG: 58 N (13 lbs)
18 AWG: 89 N (20 lbs)
16 AWG: 133 N (30 lbs)

Pro-Tip: A delicate crimp can pass the mechanical pull test but still fail electrical resistance checks in the field (for instance, if the crimp is loose electrically but holding mechanically on the insulation segment). Therefore, Pull Testing must always be tightly paired with a precise Crimp Height Measurement.

Micro-section analysis: the ultimate validation

Micro-sectioning is a destructive laboratory process that involves sectioning the crimp perfectly in half, polishing the cut face, and inspecting the interior metallurgical structure under magnification. It is the only reliable method to visually verify a genuine cold weld.

The analysis criteria

The Void Percentage: Large, visible voids indicate insufficient physical compression. The correct target is a solid “honeycomb” structure boasting minimal to zero internal gaps.
Symmetry: The two curled crimp wings must roll down symmetrically and meet at the center of the barrel floor.
Safe Wing Closure: The metal wings must mechanically support each other tightly, but must definitely not pierce through the bottom floor of the terminal barrel.
The Strand Count: This verifies that no individual copper strands were missed or folded back outside the barrel during the high-speed insertion process.

Requirement: Formal micro-sections are mandated for all Class 3 setup validations and whenever a new wire-to-terminal combination is introduced to production.

Defect atlas: visual cues

A visual inspection by the trained operator serves as the final, critical quality gate. Inspectors must identify subtle machine setup errors that distort the terminal body slightly during the crimping strike.

Observation	Visual Appearance	Likely Root Cause	Potential Field Risk
Banana (Bending)	The terminal body is bent up or down relative to the straight wire axis.	Physical damage to the carrier strip; excessive Crimp Force applied; or an incorrect or loose anvil alignment block.	Mating alignment failure; potential connector housing damage during terminal insertion.
Flag / Twist	The terminal is twisted or bent sideways slightly off-axis.	Physical misalignment in the applicator’s feed track mechanism.	The terminal will not fit easily into the plastic connector cavity.
Cut Strands	Several individual wire strands are visibly severed at the bellmouth entrance.	Zero bellmouth present (creating a sharp shear edge); or Crimp Height is set too low (causing over-compression).	Reduced current capacity; high electrical resistance; mechanical failure under future vibration.
Insulation Entrapment	The wire insulation is visibly pinched inside the primary conductor crimp area.	The strip length was too short; the wire was inserted too far forward during the crimp.	High Electrical Resistance; intermittent connection (the soft plastic insulator blocks vital metal-to-metal contact).
Insulation Support Failure	The rear insulation crimp pierces through the jacket or fails to grip it.	Incorrect insulation diameter setting is dialed in on the tool; incorrect terminal size was selected.	Wire breakage due to fatigue (insufficient mechanical strain relief protecting the cold weld).

Final Checkout: Crimp quality assurance

Focus Area	Engineering Guideline	Verification Action
Height Verification	The final crimp height must measure within the manufacturer’s specified tolerance window.	Accurate measurement with a calibrated Blade Micrometer must be performed at setup and again at batch changeover.
Mechanical Strength	The Pull Test force must exceed the UL/IPC minimums for that specific wire gauge.	Destructive test performed at Setup and logged. The preferred failure mode is a “Wire Break.”
Internal Integrity	The micro-section must clearly show dense honeycomb compression and symmetric wing closure.	Performed during New Product Introduction (NPI) or standard Class 3 high-reliability validations.
Visual Geometry	The terminal body must remain free of Banana, Twist, or missing Bellmouth issues.	A visual check by the operator immediately after the crimping strike.
Zero Entrapment	Absolutely no plastic insulation is permitted anywhere inside the conductor crimp barrel.	Visual inspection of the open “window” located exactly between the two crimp zones.