4.11 Form Board and Routing Design

The Form Board (or Jig Board) is the physical template that transforms a 2D engineering drawing into a 3D wire harness. It is not merely a piece of plywood with nails; it is a precision calibration instrument. If the board is wrong, every harness built on it will be wrong.

Designing the board and managing the physical routing requires a deep understanding of material behavior. Wires have mass, stiffness, and bend limits. Bundling materials (ties, tape, lace) exert compressive forces that can crush insulation. The goal of this phase is to ensure the harness fits the chassis perfectly every time, without forcing, stretching, or kinking.

4.1.11.1 Fixture Design: The layout Mandate

The form board acts as the "Third Hand" for the operator and the "Go/No-Go Gauge" for the harness dimensions.

A) Layout Geometry and Tolerance

A CAD drawing represents a wire as a zero-width line. Real wire bundles have diameter.

• The Centerline Problem: If you place routing nails exactly on the CAD centerline, a thick bundle will grow outward, making the final harness too short.
• Offset Design: Nails and jigs must be offset by half the bundle diameter to ensure the centerline of the physical harness matches the drawing.
• Dimensional Tolerance: The board must hold the harness to the drawing tolerance (typically ± 10 mm for overall length, ± 5 mm for breakout positions).

B) Hard Fixturing (Jigs vs. Nails)

• Routing Nails: Used to define the path.
  • Mandate: Nails must be straight, rigid, and sleeved (covered with tubing). Bare metal nails can nick wire insulation during tensioning.
  • Head Height: Nails must be tall enough to contain the full bundle stack-up without wires spilling over.
• Connector Jigs: Connectors should not dangle. They must be held in fixed mating jigs screwed to the board.
  • Function: Holds the connector rigid for pin insertion and guarantees the breakout length (distance from bundle to connector) is correct.
  • Keying: Jigs should verify physical keying (e.g., preventing a "Key A" connector from being built as "Key B").

C) Visual Aids and Ergonomics

• Full Scale Plot: The engineering drawing should be plotted 1:1 and glued to the board surface.
• Color Coding: Use color-coded zones on the board to indicate:
  • Red: No-Go zones / Keep-outs (e.g., where a chassis bracket will be).
  • Blue: Tie-wrap locations.
  • Yellow: Label placement zones.
• Ergonomics: The board is often tilted (easel style) to reduce operator back strain. Tools (tie guns, tape dispensers) must be tethered or staged to prevent damage to the board surface.

4.1.11.2 Routing Mechanics: Managing Stress

Routing is the sequence of laying wires onto the board. The sequence determines the neatness and flexibility of the final product.

A) Lay-Up Sequence

1. Heavy Gauge First: Large power cables (e.g., 10 AWG) are stiff and define the backbone. They go on the bottom.
2. Signal Wires: Lighter gauge wires are routed on top or alongside the power core.
3. Twisted Pairs: Must be laid flat and not crushed by the heavier wires.

B) Bend Radius Rules

Forcing a wire into a sharp corner puts the conductor under permanent stress and thins the insulation (dielectric degradation).

• The 3x / 5x Rule:
  • Static Bend: Radius must be ± 3x the outer diameter (OD) of the bundle.
  • Dynamic Bend (Flexing): Radius must be ≥ 5x to 10x the OD.
• Board Design: Routing nails at corners must be spaced to force this radius. Never use a single nail for a 90-degree turn; use a three-nail arc or a radiused block.

C) Service Loops (Maintenance Loops)

Wires should rarely enter a connector under tension.

• Mandate: Design a Service Loop (a small amount of slack) immediately before the connector.
• Purpose: Allows for future disconnect/reconnect cycles and re-termination (cutting off a damaged contact) without making the harness too short.

4.1.11.3 Bundling Mechanics: Containment and Cold Flow

Once routed, the wires must be secured into a unified bundle. The choice of restraint (Tie-Wrap vs. Lacing) dictates the reliability.

A) Tie-Wraps (Cable Ties): The "Cold Flow" Risk

Cable ties are fast but dangerous. They apply constant compressive force.

• Cold Flow: Plastic insulation moves (flows) away from pressure over time. A tight cable tie can eventually cut through the insulation, causing a short circuit.
• Tension Guns: Never tighten ties by hand. Use a calibrated Cable Tie Gun with an adjustable tension setting.
  • Setting: Set the gun to the minimum tension required to hold the bundle round, not to crush it. The bundle should be able to rotate slightly under the tie if twisted.
• Flush Cutting: The tail of the tie wrap must be cut flush with the head, leaving no sharp protrusion.
  • Defect: A protruding sharp tail cuts operator hands and abrades adjacent wires in the chassis.
• Spacing: Standard spacing is spot tying every 50-75 mm to keep the bundle organized without losing flexibility.

B) Lacing Cord: The Aerospace Standard

Lacing cord (waxed nylon or polyester string) is used in Class 3 aerospace and military harnesses.

• Advantages:
  • Zero Cold Flow: The broader surface area and lower tension do not cut into insulation.
  • Low Profile: The knot is smaller than a tie-wrap head (critical for tight wire ducts).
  • Temperature: Nylon ties become brittle in heat/cold; lacing cord remains stable.
• Technique: Uses continuous lacing (Marlinespike hitch) or spot ties (Surgeon's Knot).
• Mandate: Knots must be tight and locked. Loose lacing is a Foreign Object Debris (FOD) hazard.

C) Spot Taping

Used for lower-cost industrial harnesses.

• Friction Tape: Used for high abrasion areas.
• PVC Tape: Used for spot bundling. Defect: Tape must be wrapped with a 50% overlap. Using too much tape makes the harness stiff; using too little allows it to unravel (flagging).

4.1.11.4 Breakouts: Managing Stiffness Transitions

A "Breakout" is where a single wire or small group exits the main bundle to reach a connector. This is the most common point of mechanical failure due to Stiffness Transition.

The Stress Riser Effect

The main bundle is rigid (many wires + taping). The breakout wire is flexible. Vibration concentrates exactly at the transition point where rigid meets flexible.

DFM Mandates for Breakouts

1. The "Y" Transition: Avoid 90-degree breakouts if possible. A "Y" shape flows stress more evenly.
2. Strain Relief Distance: The bundle tie/tape must stop at least 25 mm (1 inch) before the connector body.
   • Rationale: This "free length" absorbs vibration. If you tie the bundle right up to the connector backshell, any movement transfers directly to the crimp terminal, causing fatigue breakage.
3. Support at the Breakout: Place a tie-wrap or lacing knot immediately before and immediately after the wire exits the main bundle. This locks the geometry so the breakout wire doesn't pull back into the bundle under tension.

Final Checklist: Routing and Form Board Design

Mandate	Criteria	Verification Action
Fixture Accuracy	Board geometry accounts for bundle diameter offset, not just the centerline.	Measure the physical harness length against the drawing tolerance (± 10 mm).
Nail Protection	All routing nails must be sleeved or radiused.	Visual check: No bare metal nails touching wire insulation.
Bend Radius	Minimum bend radius ≥3x bundle diameter maintained at all corners.	Audit board layout; ensure no single-pin 90-degree turns.
Tie-Wrap Tension	Ties are secure but can be rotated slightly; insulation is not crushed.	Calibrated Tie Gun used; tension settings verified daily.
Flush Cutting	No sharp edges on tie-wrap tails.	Tactile check (run finger over the tie head); reject if sharp.
Breakout Relief	Strain relief distance (slack) exists between the bundle exit and the connector.	Visual check: Wires must not be under tension entering the connector housing.
Connector Jigs	Connectors are held in rigid mating jigs during assembly.	Ensures consistent breakout length and prevents pin damage during loading.