4.3 Wiring Harness and Cable Assembly Pack

Cable assemblies are the "nervous system" of the product, yet they are frequently the most under-documented component in the BOM. A vague cable drawing leads to intermittent connections, EMI failures, and potential fire hazards. Do not treat cables as simple commodities; they are custom electromechanical assemblies that require a strict manufacturing specification.

The Cable Drawing Package

A schematic line connecting two connectors is not a manufacturing drawing. The cable vendor requires a physical build map.

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

Create a dedicated table on the drawing that explicitly maps every connection.

• Format: Position A (Pin 1) ⭤ Wire Color/AWG ⭤ Position B (Pin 1).
• Twisted Pairs: If signal integrity requires differential pairs, explicitly group them in the drawing (e.g., "Twist Red/Black, 2 twists/inch").
• NC (No Connect): Explicitly state "No Connect" for unused pins. Leaving them blank invites questions or population errors.

2. Dimensional Tolerances

Cables are flexible, but their length tolerances are not infinite.

• Total Length: Define the measurement method (connector face-to-face or wire cut length). Apply realistic tolerances (e.g., 100 mm ± 5 mm, not ± 0.1 mm).
• Breakout Length: Define the distance from the cable jacket/shield exit to the connector.

3. Labeling and Orientation

• Labels: Define exact text, font, and distance from connector A (e.g., "Label 'P1' 50 mm from connector").
• Orientation: For right-angle connectors or keyed strain reliefs, define the clocking orientation relative to the other end.

Critical Assembly Specifications

The Crimp Quality

The crimp is a cold-weld process. If the compression is incorrect, the resistance increases, leading to heat and failure.

• Requirement: Specify the exact Terminal MPN and the matching Housing MPN.
• Validation: Demand Pull Force testing and Crimp Height analysis from the vendor during FAI.

Pro-Tip: Never mix contact vendors with housing vendors (e.g., Molex terminals in TE housings), even if they "fit." The retention barbs often differ slightly, leading to "push-back" failures during mating.

Shielding and Strain Relief

• Shield Termination: Define exactly how the shield connects. Is it a 360˚ crimp to the connector shell, or a "pigtail" drain wire soldered to a pin? Pigtails degrade EMI performance; 360˚ termination is preferred for high-speed signals.
• Strain Relief: Define the method (Overmold or Heat Shrink).
  • If Heat Shrink: Specify "Adhesive Lined" (Dual Wall) for rigorous environments to prevent the shrink tube from sliding off.

Workmanship Standard: IPC/WHMA-A-620

Stop arguing about "good enough." Adopt IPC/WHMA-A-620 (Requirements and Acceptance for Cable and Wire Harness Assemblies) as the governing standard.

• Define the Class:
  • Class 1 (General Electronic): Consumer toys, disposable devices.
  • Class 2 (Dedicated Service): Laptops, home appliances, general industrial. (Most common default).
  • Class 3 (High Performance): Medical, aerospace, life-support.
• Logic: If you do not state the class, the vendor will default to Class 1 or their internal "best effort."

Test Requirements

You cannot visually inspect a short circuit inside a connector. Electrical testing is mandatory.

• 100% Continuity Test: Every single cable produced must undergo an automated continuity check for Opens, Shorts, and Miswires.
• Hi-Pot (Dielectric Withstand):
  • If application is High Voltage (> 50V) ⭢ Then require Hi-Pot testing to check insulation integrity.
  • If application is Mains Power ⭢ Then Hi-Pot is mandatory for safety compliance.

Final Checklist