1.2 Wire Selection and Management

Wire and component selection is a critical Design for Manufacturing (DFM) mandate, trading off cost, flexibility, and current-carrying capacity. Managing these material choices correctly prevents overheating, signal loss, and mechanical failure in the final harness.

1.2.1 Wire Gauge (AWG) and Current Derating

Wire size, measured in American Wire Gauge (AWG), is inversely related to its diameter; a lower AWG number means a thicker wire and higher current capacity. Selection must balance the required electrical performance with the mechanical limitations of the assembly space and connectors.

• Current Capacity: The primary concern is preventing wire overheating, which degrades insulation and risks fire. Wires must be sized based on the maximum required continuous current.
• Derating: When multiple wires are bundled tightly (harnesses), the heat dissipation is reduced. Therefore, the current capacity of individual wires must be derated (reduced) based on the size of the bundle and the ambient operating temperature. Failing to derate is a serious safety and reliability risk.
• Voltage Drop: For long harnesses or power delivery circuits, the wire gauge must be large enough (low AWG) to ensure the voltage drop (∆V) remains below the required system tolerance (e.g., 5% maximum).

1.2.2 Insulation Types and Environmental Mandates

The insulation material dictates the wire's resistance to heat, chemicals, and abrasion.

Insulation Type	Key Characteristic	Typical Application	Mandate
PVC (Polyvinyl Chloride)	Highly flexible, low cost, good dielectric strength.	General purpose, internal chassis wiring (low heat).	Avoid high-temperature areas or where chemical resistance is critical.
Polyethylene (PE)	Excellent low-temperature performance, low dielectric loss (good for RF/High Speed).	Outdoor, low-temperature, or data cables.	Susceptible to cracking and melting at high temperatures.
PTFE (Teflon)	Superior resistance to high heat, solvents, and abrasion.	Aerospace, automotive engine compartments, harsh chemical environments.	Mandatory for high-temperature or solvent exposure; higher cost.
Silicone	Extremely flexible, wide temperature range.	Applications requiring constant flexing or tight bending radii.	Low abrasion resistance; requires mechanical protection.

1.2.3 Connector Family Selection

Selecting the correct connector is a system-level DFM decision based on environmental exposure, mating requirements, and mechanical durability.

• Contact Plating: Plating dictates contact resistance and durability. Gold plating is mandatory for low-voltage/low-current signal integrity and high mating cycles (≥100). Tin plating is sufficient for high-current power applications and low mating cycles. Never mix tin and gold contacts in the same mated pair due to corrosion risk.
• Durability (Mating Cycles): The connector must be rated for the expected number of connect/disconnect cycles over the product's life. Standard industrial connectors may be rated for 50 cycles, while high-reliability test connectors may exceed 5,000 cycles.
• Sealing and Retention: For harsh environments, IP-rated (Ingress Protection) sealed connectors are mandatory to prevent moisture and dust ingress. Connectors must also feature locking or latching mechanisms to guarantee mechanical retention under vibration and shock.

1.2.4 Managing Wire and Component Traceability

In high-reliability manufacturing (IPC Class 3), traceability of raw materials is mandatory.

• Lot Control: Every reel of wire, batch of terminals, and lot of connectors must be tracked by its Manufacturer's Lot Number. This information must be logged against the final harness Serial Number (SN) in the MES.
• Purpose: If a defect occurs (e.g., terminal fatigue or wire insulation degradation), traceability allows immediate quarantine and root cause analysis against the specific raw material batch used, preventing widespread field failures.
• Wire Marking: The wire itself must be clearly identified with permanent printing (e.g., wire type, gauge, and supplier code) to prevent incorrect gauge insertion during manual assembly.

Final Checklist: Wire Selection Mandates

Mandate	Criteria	Rationale
Current Derating	Wire gauge is derated based on harness bundle size and ambient temperature.	Prevents insulation degradation and fire risk due to overheating in bundled harnesses.
Connector Plating	Gold plating is mandatory for signal integrity and high mating cycles. Tin for power and low cycles.	Ensures stable contact resistance and required mating durability.
Insulation Match	Select insulation based on chemical exposure and ∆T range (e.g., PTFE for high heat/solvents).	Protects wire integrity against the operating environment.
Traceability Link	Every wire and terminal lot is linked to the final harness SN in the MES.	Allows rapid quarantine and root cause analysis of material-related failures (IPC Class 3 mandate).
Voltage Drop	Wire gauge is selected to maintain the voltage drop (∆V) below the system's required tolerance (e.g., ≤ 5%).	Guarantees stable voltage supply to distant loads.