4.3 Wiring Harness and Cable Assembly Pack

Cable assemblies are the literal “nervous system” of your product, yet they are frequently the most under-documented, under-specified components in the entire Bill of Materials (BOM). A vague cable drawing leads directly to intermittent connections, EMI failures, and potential fire hazards. Cables must not be treated as simple, off-the-shelf commodities; they are custom electromechanical assemblies demanding a strict, unambiguous manufacturing specification.

The Cable Drawing Package

A CAD schematic showing a simple black line connecting two connectors is not a manufacturing drawing. The cable vendor requires a precise, physical build map to assemble the harness correctly.

1. Pin-to-Pin Wire List (The Netlist)

A dedicated, explicit table must be created directly on the 2D drawing that maps every single connection.

• Format: Position A (Pin 1) ⭤ Wire Color / AWG ⭤ Position B (Pin 1).
• Twisted Pairs: If signal integrity requires differential pairs, they must be explicitly grouped in the drawing (e.g., “Twist Red/Black, 2 twists/inch”).
• NC (No Connect): “No Connect” (NC) must be explicitly stated for any unused pins. Leaving table cells blank invites questions, assumption errors, or incorrect wire population by the factory.

2. Dimensional Tolerances

Cables are flexible, but their length tolerances are limited. Too long, and it pinches in the housing. Too short, and you cannot mate the connector.

• Total Length: The exact measurement method (e.g., connector face-to-face, or explicit wire cut length) must be defined. Realistic, manufacturable tolerances (e.g., 100 mm ± 5 mm) must be applied.
• Breakout Length: The distance from where the main cable jacket or shield exits, to the back of the individual connector must be clearly defined.

3. Labeling and Orientation

• Labels: The exact text, font, and distance from connector A (e.g., “Apply Flag Label ‘P1’ 50 mm from Connector A”) must be defined.
• Orientation: For right-angle connectors or keyed strain reliefs, the “clocking” orientation relative to the connector on the opposite end must be defined.

Critical Assembly Specifications

The Crimp Quality

The wire crimp is an engineered cold-weld process, not simply crushing metal with pliers. If the compression force is incorrect, the electrical resistance increases, leading to localized heating, voltage drops, and ultimately, thermal failure.

• Requirement: The exact Terminal MPN and the explicit matching Housing MPN must be specified.
• Validation: Pull Force testing data and Crimp Height analysis reports must be formally demanded from the vendor during the FAI (First Article Inspection) phase.

Common Pitfall

Mixing contact vendors with housing vendors (e.g., putting Molex terminals into TE housings) should be avoided, even if they “seem to fit.” The retention barbs often differ by fractions of a millimeter, leading to “push-back” failures where the pin slides out during final assembly mating.

Shielding and Strain Relief

• Shield Termination: How the EMI shield connects must be exactly defined. Is it a full 360˚ crimp to the metal connector shell, or a “pigtail” drain wire soldered to a ground pin? Pigtails severely degrade EMI performance; a 360˚ termination is highly preferred for high-speed signals.
• Strain Relief: The exact strain relief method (e.g., custom Overmold or applied Heat Shrink) must be defined.
  • When using Heat Shrink, “Adhesive Lined” (Dual Wall) shrink tubing must be explicitly specified for rigorous environments to permanently prevent the tube from sliding off the jacket.

Workmanship Standard: IPC/WHMA-A-620

Clear expectations must be established rather than relying on subjective interpretations. IPC/WHMA-A-620 (Requirements and Acceptance for Cable and Wire Harness Assemblies) must be adopted as the governing quality standard directly on the print.

• The IPC Class must be defined:
  • Class 1 (General Electronic): Consumer toys, cheap disposable devices.
  • Class 2 (Dedicated Service): Laptops, home appliances, general industrial equipment. (This is the most common default).
  • Class 3 (High Performance): Medical, aerospace, life-support electronics.
• Rationale: Without explicit classification, vendors may default to Class 1 or internal standards.

Electrical Test Requirements

Visually inspecting a micro-short circuit hidden inside a plastic connector housing is impossible. Automated electrical testing at the vendor is mandatory.

• 100% Continuity Test: Every single cable produced must undergo an automated continuity check for opens, shorts, and miswires before being shipped to your factory.
• Hi-Pot (Dielectric Withstand):
  • When the application is High Voltage (> 50V), explicitly require Hi-Pot testing to check the insulation integrity between wires.
  • When the application involves AC Mains Power, Hi-Pot testing is a mandatory requirement for safety compliance.

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Recap: Wiring Harness and Cable Assembly Pack

Parameter	Requirement	Value / Condition	Action / Document
Pin-to-Pin Wire List	Complete, explicit connection map.	Format: Position A (Pin #) ↔ Wire Color/AWG ↔ Position B (Pin #). Define twisted pairs and all “No Connect” (NC) pins.	Mandatory on 2D manufacturing drawing.
Dimensional Tolerances	Defined measurement method and manufacturable limits.	e.g., 100 mm ± 5 mm. Specify total length (connector face-to-face) and breakout lengths.	Mandatory on 2D manufacturing drawing.
Crimp & Components	Validated crimp process and exact components.	Specify exact Terminal MPN and matching Housing MPN. Validate via Pull Force and Crimp Height reports.	Require in FAI (First Article Inspection).
Quality Standard	Governing workmanship and acceptance class.	Adopt IPC/WHMA-A-620. Define Class (1, 2, or 3). Class 2 is default for industrial equipment.	Reference standard and class on drawing.
Electrical Test	100% production testing for electrical integrity.	100% continuity test (opens, shorts, miswires). Hi-Pot test mandatory for >50V or AC Mains applications.	Specify as vendor requirement.