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4.4 Connector Families: The Mechanical Interface

The connector is the only part of the harness designed to be disconnected, making it the weakest link in the electrical chain. It is a complex electro-mechanical device that must maintain low contact resistance despite vibration, oxidation, and handling abuse. Selection is not merely about "fitting" the wire; it is about matching the contact physics to the signal type and ensuring the housing mechanics prevent accidental disconnects under stress.

4.4.1 Contact Physics: Plating and Fretting

The choice of contact plating is dictated by the voltage level, signal sensitivity, and the expected number of mating cycles. The two industry standards — Tin and Gold —a re not interchangeable.

The Galvanic Mandate

Never mate Tin to Gold.

Mixing these dissimilar metals creates a galvanic cell in the presence of humidity, leading to rapid corrosion at the interface. This oxide layer causes intermittent signal failure, often described as "No Fault Found" (NFF) because the act of unplugging and replugging temporarily wipes the oxide away.

Plating Selection Matrix

Feature

Tin Plating

Gold Plating

Primary Use

High Current / Power Circuits (> 10V, > 100mA).

Low Energy / Logic Signals / Data (Dry Circuits).

Physics

Soft metal; relies on high normal force (wiping action) to break through its natural oxide layer.

Hard, noble metal; does not oxidize. Relies on low normal force, preserving spring integrity.

Mating Cycles

Low durability (10 to 30 cycles). The soft tin wears through quickly.

High durability (100 to 1,000+ cycles).

Failure Mode

Fretting Corrosion: Micro-motion from vibration causes fresh tin to oxidize, eventually building an insulating barrier.

Pore Corrosion: If plating is too thin, base metal migrates through pores (prevented by Nickel underplate).

Design Rule: If the circuit voltage is too low to "punch through" an oxide layer (typically < 10V), Gold plating is mandatory. Tin is acceptable for power circuits where the arc/voltage can burn off minor oxidation.

4.4.2 Housing Selection: Locks and Keying

The connector housing provides mechanical protection and insulation. Its material and locking features determine whether the harness survives assembly and operation.

Material Science: Nylon vs. PBT

  • Nylon (PA66): The most common connector material. It is hygroscopic (absorbs moisture).
    Pros: Tough, resilient snap-fits
    Cons: Dimensions change with humidity; can become brittle if "dry as molded" before hydration.
  • PBT (Polybutylene Terephthalate): Used for precision automotive/industrial connectors.
    Pros: Dimensionally stable; does not absorb water.
    Cons: More brittle; snap-locks can break if over-flexed during assembly.

Locking Mechanisms (TPA & CPA)

For high-reliability (Class 3) and automotive applications, simple friction fits are insufficient. Secondary locks are mandatory.

  • Primary Lock: The plastic tang on the terminal or housing that clicks into place.
  • TPA (Terminal Position Assurance): A secondary plastic wedge or comb inserted after the terminals are seated.
    Function: It mechanically blocks the terminals from backing out. If the TPA will not seat, it indicates a terminal is not fully inserted.
    Mandate: TPA is required for all high-vibration connectors.
  • CPA (Connector Position Assurance): A locking tab on the outside of the mated pair.
    • Function: Ensures the two connector halves are fully mated and prevents accidental unlatching.

Keying and Polarization

Connectors must be keyed (shaped) to prevent:

  • Mis-mating: Plugging connector A into header B.
  • Reverse mating: Plugging it in upside down (180˚).

DFM Note: Use different color codes or physical keying options (Key A, Key B) for identical connectors located in the same harness area.

4.4.3 Sealing Mandates: Harsh Environments

Standard connectors are breathable. For harsh environments (automotive under-hood, outdoor, wash-down zones), the connector must be effectively sealed to meet IP67 (Dust tight / Immersion up to 1m) or IP68 ratings.

  • Interface Seals: A silicone ring that seals the mating face between the male and female housing.
  • Wire Sealing:
    Single Wire Seal (SWS): A discrete silicone plug crimped onto the insulation of each wire.
    Mandate: The insulation diameter must match the seal range exactly; too small leaks, too large tears the seal.
    Mat Seal (Block Seal): A single rubber block with holes for all wires. Used in high-density connectors.
  • Cavity Plugs: Mandatory for any unused pin position. Leaving a hole open in a sealed connector compromises the entire assembly.
  • Backshells: Rigid covers that screw onto circular connectors (e.g., MIL-DTL-38999). They provide:
    Strain Relief: Transferring bending force to the housing, not the crimps.
    EMI Shielding: 360˚ termination of the cable braid.

Final Checklist: Connector Selection Mandates

Mandate

Criteria

Verification Action

Plating Match

Never mate Tin to Gold.

Audit BOM to ensure mating pairs (Plug & Header) use identical contact plating.

Signal Integrity

Gold plating mandated for low-voltage/data circuits (< 10V).

Prevents "No Fault Found" failures caused by oxide buildup on Tin contacts.

Retention Security

TPA (Terminal Position Assurance) mandated for vibration environments.

Visual inspection confirms TPA is fully seated and locked.

Sealing Integrity

All unused cavities in sealed connectors must have Cavity Plugs.

Leak testing (vacuum decay) or visual check ensures the IP rating is maintained.

Mating Cycles

Connector durability rating must exceed the expected service life cycles.

Use Gold for high-cycle test ports; Tin is limited to approx. 30 cycles.


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