5.3 Cable & Harness Assembly (IPC/WHMA-A-620)

Cable assemblies are the nervous system of the product. They are often built by hand, making them the most variable and failure-prone component in the BOM. IPC/WHMA-A-620 is the standard that separates a reliable connection from an intermittent field failure. If a crimp is bad, the resistance increases, heat generates, and the connector melts.

Crimping Criteria (The Gas-Tight Seal)

A crimp is not a fold; it is a cold-weld. The goal is to compress the copper strands into a solid mass, eliminating oxygen to prevent corrosion.

Critical Visual Attributes:

• Bellmouth: The flared trumpet shape at the entry of the crimp.
  • Requirement: Must be visible at the wire-entry end. It prevents the sharp metal edge from slicing the wire strands during vibration.
• Conductor Brush: The wire strands extending past the crimp barrel.
  • Target: Visible strands (flush to 1x wire diameter).
  • Defect: No strands visible (risk of "short crimp" or empty barrel).
• Insulation Crimp: The strain relief grip.
  • Target: Fully supports the insulation without piercing it.
  • Defect: Insulation crimped into the conductor barrel (conductivity risk).

Pro-Tip: Crimp height is a proxy for compression. A "Go/No-Go" gauge is useless if the tool wears out. Measure Crimp Height with a micrometer at the start of every shift.

IDC (Insulation Displacement Connection)

Used for ribbon cables. The blades slice through insulation to contact the core.

Alignment Rules:

• IF the connector is misaligned (cocked) -> THEN Reject. Uneven pressure leads to open circuits.
• IF the wire is cut or strands are broken -> THEN Reject.

Strain Relief & Routing

A harness is a dynamic structure. It moves, vibrates, and expands.

Mechanical Logic:

• Cable Ties: Must secure the bundle without crushing the insulation.
  • Rule: If the tie leaves a deep impression, it is too tight.
  • Safety: Tails must be cut flush. A protruding sharp tail is a "blood draw" hazard for the installer.
• Bend Radius:
  • Static: Min 5x cable diameter.
  • Dynamic (Flexing): Min 10x cable diameter. Violating this guarantees internal conductor fatigue and breakage.

Final Checklist