What a Box Build Specification Document Should Include (And What Most Forget)

By Will Leverett | Connect on LinkedIn

A box build specification document is the single most important piece of communication between a client and a contract electronics manufacturer. It defines what you want built, to what standard, and with what acceptance criteria.

And yet, most box build specs we receive are incomplete.

They cover the obvious things: the PCB assembly, the enclosure, the connectors. But they miss the details that determine whether a build goes smoothly or stalls in week three with queries, rework, and scope issues.

This is what a complete box build specification should include including practical details that prevent problems before they start.

What a Box Build Actually Is

Before we get into the specification, it's worth defining the scope clearly. A box build is the complete assembly of an electronic product beyond just the PCB. It includes:

• PCB assembly (components soldered to the bare board)
• Mechanical assembly (enclosure, mounting hardware, heat sinks, fans)
• Cable and wire harness integration
• Final assembly and integration of sub-assemblies
• Functional testing of the complete unit
• Labelling, packaging, and kitting for shipment

The line between "PCB assembly" and "box build" can blur, but broadly: if it involves more than just populating a PCB — if there's an enclosure, cables, mechanical fasteners, or multi-stage integration — it's a box build.

When Should You Write This Specification?

The short answer: as early as possible, ideally before you request quotes from contract manufacturers.

A complete box build specification allows a CEM to quote accurately. Without it, you'll either get a vague estimate with caveats ("subject to final specification review"), or you'll get a quote based on assumptions that turn out to be wrong once the project starts.

The ideal timeline:

• During design phase — if you're involving a CEM early (see our guide on NPI programmes), they can flag specification gaps and DFM issues before your design is locked
• Before quoting — at minimum, have this specification complete before you approach manufacturers for pricing; incomplete specs lead to inaccurate quotes and scope creep later
• Definitely before NPI starts — once you've placed an order and the job pack is being reviewed, an incomplete spec causes delays while queries are raised and answered

If you're already mid-project and realising your spec has gaps, it's not too late — but expect queries, potential rework, and timeline adjustments as those gaps get filled.

Section 1: The PCB Assembly Specification

This is usually the most complete part of a box build spec, because it's the part clients are most familiar with. But even here, gaps appear.

What should be included:

• Gerber files (or ODB++ if your CEM supports it) — the PCB layout data
• Bill of Materials (BOM) with manufacturer part numbers for every component
• Assembly drawings showing component placement and orientation
• Pick-and-place files (if available) to speed up programming
• IPC class requirement — Class 2 (standard commercial) or Class 3 (high-reliability for medical/defence/aerospace)
• Conformal coating requirements, if applicable — what type, what coverage, what areas to mask

What gets forgotten:

• Test point locations and access requirements — if you need in-circuit test (ICT) or functional test access, specify which points need to be accessible and whether you're providing test fixtures
• Lead-free vs leaded solder — if you have legacy designs or specific reliability requirements, this needs to be explicit

Section 2: The Enclosure and Mechanical Assembly

This is where most box build specs fall apart. Clients assume the CEM will "figure it out," but mechanical tolerance, assembly sequence, and fastener specifications all affect cost, timeline, and quality.

What should be included:

• Enclosure specification — material (ABS, polycarbonate, aluminium), colour, finish, and supplier part number if it's a standard enclosure; or mechanical drawings if it's custom
• Mounting method for the PCB — standoffs, snap-fit, slide rails, screws — with fastener type and torque spec if critical
• Heat dissipation requirements — if the enclosure needs ventilation holes, fans, heat sinks, or thermal interface material, specify where and to what thermal resistance
• Cable entry and strain relief — how cables enter the enclosure, whether grommets or cable glands are required, and what IP rating (if any) needs to be maintained
• EMI/RFI shielding requirements — does the enclosure need conductive gaskets, shielded cable, or specific grounding points?
• Assembly sequence constraints — are there any parts that must be installed in a specific order (e.g., a display that can't be installed after the PCB is mounted)?

What gets forgotten:

• Enclosure tolerances and fit — if the enclosure is injection moulded or CNC machined, what are the acceptable dimensional tolerances? If the PCB is tight-fit, a 0.5mm variance can prevent assembly.
• Fastener accessibility — can all screws be reached with standard tools, or do some require custom drivers, offset screwdrivers, or specific torque wrenches?
• Labelling and branding position — where do product labels, compliance marks, and serial numbers go? Do they need to be applied before or after final assembly?

Preferred Supplier — If you have one, this allows us to purchase the correct enclosure from a source trusted by you.

Section 3: Cable and Wire Harness Integration

Cable harnesses are one of the most error-prone parts of box build assembly, and incomplete specs are the primary cause.

What should be included:

• Cable harness drawings showing every cable, its length, its routing path, and its termination points
• Connector specifications — manufacturer part numbers for every connector, including crimp terminals, housings, and any backshells or strain relief boots
• Wire gauge and type — AWG size, stranded vs solid core, insulation type, and colour coding
• Crimping and termination standards — IPC/WHMA-A-620 class requirement (Class 1, 2, or 3)
• Cable dressing and tie-down points — how cables should be routed and secured (cable ties, lacing cord, adhesive mounts), and whether there are bend radius restrictions
• Labelling requirements — do individual wires or cable bundles need labels? If so, what information and what labelling method (heat shrink, wrap-around tags, printed sleeves)?

What gets forgotten:

• Cable length tolerances — if a cable is specified as 150mm, is +/- 5mm acceptable, or does it need to be exact?
• Connector keying and orientation — if a connector can be inserted incorrectly, specify how to prevent it (keyed housings, colour coding, positional labels)

Test continuity requirements — should every cable be continuity tested before integration, or only after final assembly?

Section 4: Testing and Quality Acceptance Criteria

This is the section that determines whether a finished box build gets shipped or gets flagged for rework. And it's the section that's most often under-specified.

What should be included:

• Functional test procedure — step-by-step instructions for powering up the unit, what inputs to apply, what outputs to measure, and what constitutes a pass
• Test equipment required — if specific instruments are needed (oscilloscope, multimeter, signal generator), list them with required specifications
• Pass/fail criteria — numerical thresholds for voltage, current, frequency, signal quality, or any other measurable parameter
• Burn-in or soak test requirements — does the unit need to run for a specific duration at a specific temperature before it's considered tested?
• Environmental test requirements — temperature cycling, vibration, humidity, shock, or any other environmental qualification testing
• Visual inspection criteria — what cosmetic defects are acceptable (minor scratches, small marks) vs what triggers rejection (cracks, contamination, exposed conductors)

What gets forgotten:

• Who provides the test fixtures and test software? If functional test requires custom fixtures or code, that needs to be supplied, or the CEM needs a specification to build it (which adds cost and time)
• First article test vs production test — are first articles tested more rigorously than production units, or is the test procedure identical throughout?

Calibration requirements — does test equipment need to be calibrated to a specific standard (UKAS, NIST)?

Section 5: Packaging, Labelling, and Documentation

The box build might be complete, but if it's packaged incorrectly or labelled wrong, it's not shippable.

What should be included:

• Product labelling — what information goes on the product label (model number, serial number, compliance marks, voltage rating, manufacturing date)
• Packaging specification — ESD bags, foam inserts, cardboard boxes, and any specific anti-static or environmental protection required
• User documentation — does the unit ship with a manual, quick start guide, or compliance declaration? If so, who provides it?
• Serialisation and traceability — does each unit need a unique serial number? If so, what format, and does it need to be recorded against component batch numbers?
• Shipping markings — what goes on the outer carton (contents, quantity, handling instructions, shipping labels)

What gets forgotten:

• Accessories and cables — if the product ships with a power supply, USB cable, or other accessories, are they packed in the same box or separately? Who sources them?
• Configuration and programming — if the unit needs firmware loaded, network configuration, or calibration settings before shipment, specify what and how

Section 6: Compliance and Regulatory Requirements

If your product needs to meet specific regulatory standards, the box build process needs to support that — and the spec needs to state it explicitly.

What should be included:

• Applicable standards — CE, FCC, UL, RoHS, REACH, WEEE, or any other compliance requirements
• Declaration of Conformity requirements — does the CEM need to provide this, or does the client handle it?
• Material restrictions — restricted substances (RoHS, REACH), conflict minerals declarations, or specific material certifications required
• Traceability documentation — if the client or end customer requires full material traceability (common in medical, aerospace, and defence), specify what records are needed

What gets forgotten:

• Country of origin marking — does the product need to be marked with country of manufacture for import/export purposes?
• Compliance testing coordination — if the product needs to go to an EMC test lab or safety testing house, who coordinates that, and does the CEM need to provide pre-compliance testing?

What Happens When the Spec Is Incomplete

An incomplete box build specification doesn't stop the project. It just moves the decision-making from the design phase to the manufacturing phase, where changes are more expensive and delays are more costly.

The CEM will raise queries. Those queries will wait for answers. The job will sit on hold. And when answers do come back, they often require rework — either to the design, the assembly process, or the finished units.

A complete specification upfront prevents this. It means the CEM can quote accurately, plan the build process properly, and execute without ambiguity.

Using This Checklist

If you're preparing a box build specification for a contract electronics manufacturer, use this article as a checklist. The sections you can answer completely are the areas where the build will go smoothly. The sections where you're uncertain or missing detail are the areas where queries, delays, and cost overruns are most likely.

At ABL Circuits, we work with clients on box build assemblies across commercial, medical, defence, and EV applications. A well-specified box build is faster, cheaper, and higher quality than one where we're filling in gaps as we go.

If you're planning a box build project and want to talk through what your specification should include, get in touch.

Will Leverett is the director at ABL Circuits, a UK-based contract electronics manufacturer specialising in PCB assembly, box build, and NPI programmes across commercial, defence, medical, and EV sectors. Connect with Will on LinkedIn.