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

IPC-A-630 covers theThe "Box Build"— is where precision electronics meet the integrationbrutal reality of PCBAsthe 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.

HardwareTorque InstallationSpecification Logic:

• Torque Verification:If Allfastening structuralmetal-to-metal (Chassis ground) → Then use star washers or thread-cutting screws to bite through oxidation/paint. Resistance must be torqued≤ to specified values.
  • Torque Striping: A line of "tamper paint" (Torque Seal) applied across the screw head and chassis. A broken line indicates loosening.
  0.1Ω.
• Thread Engagement:If Forfastening plastic bosses → Then use thread-forming screws into(PT nuts, the screw must protrude at least 1.5 threads past the nut body.
• Washers: Correct stack-up (Split washer -> Flat washer) is mandatorystyle) to prevent damagehoop 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 surfaceoperation is complete and ensurehasn't locking.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.

Chassis Cleanliness (FOD)

• 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: AnyPhysically looserotate/shake materialthe unit (wireif clippings,size solderpermits) balls,before washers)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 enclosurelast isunit ayou criticalbuilt defect.until proven otherwise.

• Shake Test: A mandatory audible check ("The Rattle Test") for loose hardware before closing the box.

Final Checklist

StepControl Point	ActionCritical Requirement	VerificationRisk Avoided
TorqueGrounding	ApplyEarth specifiedbond Nmresistance ≤ 0.1 Ω from inlet earth pin to furthest metal point.	Click-wrench or Electric DriverShock / Safety Compliance
MarkingFasteners	100% Torque Sealverification; Paintwitness marks applied.	VisualVibration (Unbrokenlooseness line)/ Short circuits
Gap CheckClearance	EnclosureMin Seams2.5 mm clearance between HV components and chassis (or per dielectric req).	FeelerArcing Gauge/ Dielectric Breakdown
Routing	Cables secured every 50-75 mm; No tension on connector headers.	Intermittent connection / Wire fatigue
Harness	Bend radius ≥ 3x wire diameter (ifstatic), spec'd)≥ 10x (dynamic).	Insulation crack / Conductor fracture
FOD	Shake/VisualShakedown test passed; no loose hardware audible.	NoCatastrophic looseshort debriscircuit