5.12 Connector Types and Mating Sequences

Connector mating is the final electrical step in Box Build assembly. Unlike crimping (which is permanent), mating is a high-risk, repeatable process. Failure to execute the correct mating sequence — or using the wrong connector type — leads to bent pins, damaged sockets, poor conductivity, and field failure. The process must guarantee that mechanical alignment, electrical contact, and final locking are executed in the sequence defined by the manufacturer.

3.2.5.12.1 Connector Classification and Selection

Connectors are categorized by their function and mechanical engagement method. Selection is governed by the required durability, vibration resistance, and environmental sealing.

A) Common Connector Families

Family	Typical Use	Keying/Latch	Notes for Assembly
Wire-to-Board	Power & I/O (Mini-Fit/Micro-Fit)	Polarized housing + latch; TPA/CPA.	Push on the housing (not wires); verify TPA/CPA is fully engaged; partial mates are common—use tug test.
FFC/FPC	Displays, compact links (ZIF/LIF).	Latch (flip/slide).	Open latch fully; insert to depth mark; close evenly; no crease at exit.
Circular Sealed	Rugged/IP links (M8/M12).	Keyed shells; coupling nut.	Align key; hand-tight then torque to spec; check O-ring/gasket.
D-sub / Micro-D	Legacy serial/parallel.	Jackscrews.	Start threads by hand; torque evenly; avoid twisting the shell.
Coax (U.FL/SMA)	RF / High frequency.	Snap-on or threaded.	Align carefully; click/torque to spec; cap unused ports.
Blade/Power	High current.	Polarized; staged pins.	Seat square; confirm full seating so staging (ground first) works correctly.

B) Keying and Polarity

Mandate: Mechanical keys and asymmetric shrouds must be used to make wrong insertion impossible.
Pin-1 Marking: Pin-1 triangles must be marked on the PCB silkscreen, the connector, and the harness label. Alignment of the Pin-1 mark to the triangle is a mandatory final check.
Secondary Lock: CPA/TPA (Connector/Terminal Position Assurance) devices must be used wherever available; the Work Instruction (WI) must include a mandatory sign-off step: "CPA engaged ✓".

3.2.5.12.2 Mating and Unmating Sequences

The order of engagement is mandatory to prevent physical damage (bent pins) and preserve connector durability.

A) General Mating Protocol

Alignment: Visually align the connector body using the keying features. If using screw-lock mechanisms, engage the lead screw by hand (2–3 turns).
Seating: Apply force axially and uniformly along the connector centerline. Support the PCB behind the header to prevent board bowing.
Locking: Confirm the primary mechanical lock engages. Listen/feel for the latch click; engage TPA/CPA where present.
Assurance: Apply final torque to jackscrews or coupling nuts as listed in the WI.

B) Flexible Circuit Mandates (FFC/FPC)

Flat Flexible Cables (FFC) and Flexible Printed Circuits (FPC) rely on Zero/Low Insertion Force (ZIF/LIF) connectors.

Latch Open: The locking bar must be opened fully.
Insertion: The cable must be inserted straight, flat, and axially until the tail is fully seated at the depth line (silk mark).
Latch Close: The latch must be closed gently and evenly until it locks level with the housing.
Tug Test: Apply a light tug test to confirm capture; re-insert if it moves. Prohibited: Folding the FFC tail immediately at the latch.

Alignment: Use guide pins and funnels. Prohibited: Relying on screws to pull parts into alignment.
Force: Press straight down near the connector body; verify the join line is parallel (no corner high).

3.2.5.12.3 Final Verification and Integrity Checks

Verification must confirm both mechanical retention and electrical potential.

A) Verification Test

Click & Flush: Latch clicked, housings flush all around (no daylight at the join).
TPA/CPA: Fully seated (visual line flush).
Tug Test: A light, firm pull on the housing confirms the lock holds and prevents movement.
Label Check: Verify the harness Pin-1 mark aligns with the Pin-1 triangle on the PCB.

B) Coax and Grounding Specifics

Coax: For snap-on (U.FL), align perfectly and press until click. For threaded (SMA/TNC), finger-start the threads to avoid cross-threading, then torque to spec (WI lists N·m). Prohibited: Twisting the coax jacket.
Ground Lug: For ring lugs on the chassis, verify the connection has a low-ohm bond (< 0.1 Ω) after final torque, ensuring the serrated washer has successfully bitten through the finish.

Final Checklist

Mandate	Criteria	Verification Action
Lock Engagement	Latch clicked; shells fully home; TPA/CPA flush and engaged.	Tug Test and visual check (no partial mates).
FFC/FPC Seating	Cable seated to the depth line; latch level; no crease at exit.	Visual confirmation that the latch is closed evenly across the width.
Threaded Mates	Jackscrews/coupling nuts torqued to spec N·m; threads finger-started.	Calibrated torque tool used for final engagement.
Alignment Protection	Blind mate systems secured with guide pins/funnels.	Operator supports the PCB behind the header during seating.
Coax/RF Integrity	Connector aligned perfectly; either click or torque confirmed; no twist in the cable.	Use a nylon tool for unmating snap-on coax; never use fingernails.
Strain Relief	First clamp secured ≤80 mm from backshell; no bend at the cup.	Visual confirmation that the wire is not levered at the termination point.