5.4 Box Build & Mechanical Assembly (IPC-A-630)

The "Box Build" is where precision electronics meet the brutal reality of the physical world. While a PCBA is fragile and static, the final enclosure is dynamic; it must survive drops, vibration, thermal cycles, and user abuse. This chapter governs the "Macro" assembly—integrating boards, harnesses, and sub-assemblies into a functional chassis. The goal is structural integrity and thermal management, not just electrical continuity. If the enclosure rattles, overheats, or allows ingress, the perfect solder joints inside are irrelevant.

Mechanical Fastening & Torque Control

Loose screws are the silent killers of field reliability. A fastener is not just a "holder"; it is a tensioned spring that maintains clamping force.

Torque Specification Logic:

• If fastening metal-to-metal (Chassis ground) → Then use star washers or thread-cutting screws to bite through oxidation/paint. Resistance must be ≤ 0.1Ω.
• If fastening plastic bosses → Then use thread-forming screws (PT style) to prevent hoop stress cracking. Do not use standard machine screws.
• If high-vibration environment → Then apply thread-locker (Loctite Blue) or use Nyloc nuts.

Pro-Tip: Never rely on "hand-tight." Defining a torque value (e.g., 0.6 N·m) is useless without a calibrated driver. Mark torqued fasteners with a "torque seal" (tamper-evident varnish) to visually confirm the operation is complete and hasn't vibrated loose.

Cable Routing & Thermal Management

A "Rat's Nest" of cables is a functional failure, not just a cosmetic one. Poor routing blocks airflow, induces crosstalk, and creates pinch points.

Routing Logic:

• If cable crosses a hinge or moving part → Then add a "service loop" (slack) to prevent fatigue failure.
• If cable runs near high-frequency lines → Then route at 90˚ angles or maintain separation distance to minimize EMI coupling.
• If securing bundles → Then use flush-cut cable ties. Sharp tie-ends cut operator hands and insulation.

Airflow Management:

verify that cable bundles do not obstruct intake/exhaust vents or heatsink fins. A 10% reduction in airflow can lead to a 5-10˚C rise in junction temperature ($T_j$), halving component life.

Cosmetic Acceptance Criteria

Do not waste money rejecting flaws that the customer will never see. We inspect based on Visibility Classes:

• Class A (Primary Surface): Front panels, screens, top user-facing surfaces. Zero scratches, dents, or molding flash visible at arm's length (600mm).
• Class B (Secondary Surface): Sides, back panels. Minor scratches allowed if they do not penetrate the coating/paint.
• Class C (Internal/Hidden): Inside the chassis, bottom. Scratches, tooling marks, and minor discoloration are Acceptable provided they do not compromise structural integrity or corrosion resistance.

Foreign Object Debris (FOD)

A loose screw or conductive clipping inside a box build is a ticking time bomb. It will eventually bridge a live circuit.

FOD Protocol:

1. Inversion Test: Physically rotate/shake the unit (if size permits) before final close. Listen for rattles.
2. Blind Mate Check: Inspect connectors for bent pins before mating. Forcing a blind mate often bends back pins, causing intermittent failures.

Pro-Tip: Use a "shadow board" for assembly tools. If a screwdriver bit is missing at the end of the shift, assume it is inside the last unit you built until proven otherwise.

Final Checklist