3.6 Harnessing and Box Build

The transition from PCBA to Box Build (System Integration) is a shift from the microscopic precision of robots to the macroscopic variability of human hands. It is a common fallacy to treat the final assembly as a trivial "packaging" step. In reality, this is where tolerance stack-ups collide, where wires get pinched, and where valid components are destroyed by static electricity. Integration does not just add steps; it multiplies complexity. A perfect PCB inside a cracked enclosure is a failed product.

The Wire Harness: The Weakest Link

Cables and harnesses are the "veins" of the system, yet they are often designed as an afterthought. Unlike PCBs, harnesses are almost entirely handmade.

• The Engineering Reality: A crimp connection is not just squishing metal; it is a "cold weld" creating a gas-tight bond between the wire strands and the terminal.
• If the crimp height is incorrect by 0.1 mm → Then the resistance increases, causing the connector to overheat and melt during operation.
• If a cable lacks "Strain Relief" (slack) → Then vibration will eventually fatigue the copper strands, causing an intermittent connection that is impossible to diagnose.

Mechanical Fit: The Tolerance Stack-up

A PCB has tolerances of ±0.1 mm. A plastic injection molded case has tolerances of ±0.5 mm. When you screw them together, these variances accumulate.

• The Problem: The "Stack-up." If the PCB holes are drilled at the lower limit of their tolerance, and the case standoffs are molded at the upper limit, the board will not fit.
• If an operator forces a mismatched board onto the standoffs → Then the PCB bends, cracking ceramic capacitors (MLCCs) and creating latent electrical failures.
• Rule: Always design mounting holes with "slotted" or oversized diameters to accommodate mechanical variance.

Fastening: Torque is a Process Variable

A screw is a clamp, not a decoration. The clamping force is determined by torque.

• The Physics: Friction holds the screw in place. Too little torque, and it vibrates loose. Too much torque, and you strip the plastic threads or crack the enclosure (Stress Cracking).
• If you do not specify a torque value (e.g., 4.0 kgf-cm) → Then every operator will guess, resulting in loose rattles or broken cases.
• Control: Use calibrated electric screwdrivers that auto-shutoff at the specific torque limit. Never use manual wrist-force for production.

ESD (Electrostatic Discharge): The Invisible Assassin

The Box Build environment is dangerous. Operators are moving, generating static electricity, and handling exposed electronics.

• The Risk: A human can carry 3,000V of static charge without feeling it. A microchip dies at 100V.
• The Critical Moment: The instant before the case is closed, the PCBA is often handled by its edges.
• If an ungrounded operator touches the I/O port → Then the device dies instantly, or worse, suffers "latent damage" that causes it to fail a month later in the customer's hands.
• Rule: Wrist straps and ESD heel grounders are non-negotiable until the case is fully screwed shut.

Configuration Control: The "Last Mile" Error

The hardware is done. Now, what software goes on it? What label goes on the back?

• The Trap: Shipping the wrong firmware or the wrong power cord.
• If the "Gold Image" (firmware file) is not version-controlled → Then you might flash the Debug version instead of the Release version, exposing your source code to the customer.
• If the Serial Number label on the box does not match the Serial Number inside the firmware → Then traceability is broken, and warranty claims become a nightmare.

Final Checklist