A board that passes functional test is not yet a product. The unit that ships also has an enclosure, a harness, a thermal path, labels, accessories and a test record, and every one of those interfaces is a place where a programme can stall. Box build is the step that closes them, and it works best when it is planned with the boards rather than added at the end.
This article covers what changes when the deliverable becomes a unit, the scope decisions that have to be settled before quotation, the interface problems that only appear at unit level, and how SUPERB Assembly supports the build from board to shipped unit.
What changes when the deliverable becomes a unit
| Area | What it involves | What goes wrong when it is split |
|---|---|---|
| Enclosure and mechanics | Housing, brackets, panels, fasteners, torque values | Fit found late, cosmetic rejects, rework on a finished unit |
| Interconnection | Harness, connectors, keying, routing, strain relief | Pinout errors, wrong keying, damage found only at final test |
| Thermal path | Interface material, gap control, airflow clearance | Hot spots that pass bench test and fail in the field |
| Configuration and identity | Options, serial numbers, labels, accessories | Mixed configurations shipped, warranty claims that cannot be traced |
| Test and records | Unit level functional test, safety checks, unit record | Board passed, unit failed, nobody owns the gap |
The pattern is the same on every programme: the risks sit between the parts, not inside them. A harness that was never verified against the board drawing is a unit level problem, and it appears after the boards are already built.
Settle the scope boundary before the quote
Box build can mean a subassembly in a frame or a configured unit in retail packaging. The price and the schedule depend on which one is meant.
- What the deliverable is. An open subassembly, a closed unit, or a configured unit ready for the customer's own channel.
- Who supplies what. Enclosures, harnesses, mechanical parts, connectors and accessories can come from the customer, from us, or from a mix held to the same revision control.
- Test scope. Board level only, unit level functional test, or unit level test plus safety and environmental screening.
- Unit identity. Serial numbers, option codes, label content and where each record is stored.
- Delivery condition. Protective packing, accessories, documentation and freight terms.
Written down, these five items remove most of the arguments that otherwise appear during the first build. Left open, they are discovered in a first article meeting when the units are already assembled.
Interface problems that only appear at unit level
- Harness against the board drawing. Connector part number, pinout, wire gauge, keying and shield termination have to be checked against the released assembly drawing, not against a photograph of an earlier build.
- Mating and torque. Connectors and threaded fasteners have a specified torque window. Above it, housings crack, below it, contact resistance drifts with vibration.
- Coating boundaries. Conformal coating protects the board but must stay off connectors, test points and press fit areas. The coating plan is a unit level decision, not a board level one.
- Thermal and mechanical clearance. Interface pads, stand offs and airflow gaps have tolerances that stack. A stack up that works on one unit is not a specification.
- Handling and static control. Units move further than boards do, and every extra handling step is a chance for ESD damage or cosmetic marking.
What unit level test and records look like
Board level test proves the circuit. Unit test proves the product, and it is the record that a customer or an auditor will ask for later.
- Functional test on the assembled unit. Run through the same interface the end user sees, with limits taken from the released specification.
- Safety and insulation checks. Insulation resistance and dielectric strength where the product has mains or high voltage sections, applied to the finished assembly.
- Thermal soak or burn in. Where the product requires it, a loaded run at temperature to catch joints and interface material problems before shipment.
- Serial based record. Unit serial linked to board serials, harness lot, coating batch, software revision and the test data of that specific unit.
How SUPERB Assembly supports box build
Superb Automation has supplied components and PCB assembly since 2000, and box build runs in the same flow as the board, which removes the handover where responsibility usually gets lost. Board support covers up to 68 layers with controlled impedance to five percent, assembly runs across 7 surface mount lines in an ESD controlled area with 3D paste inspection, 3D automated optical inspection, X-ray with void control and in-circuit and functional test, and the unit stage adds harness assembly, selective conformal coating, mechanical integration, labelling and packing to the customer's released drawing.
Unit level test fixtures are built for the product, so the functional test a first article sees is the test every later unit receives. Workmanship follows IPC-A-610 Class 2 or Class 3 for the board as the drawing states, harness work is held to the same controlled standard, and the quality system is ISO 9001 with medical programmes handled against ISO 13485 aligned requirements. Where the product needs it, units are packed to the delivery condition agreed at quotation, including accessories, documentation and unit identity.
- Box build and final assembly: the full unit level workflow.
- Cable and wire harness assembly: connectors, routing and continuity checks.
- Conformal coating selection: protecting the board without blocking the interfaces.
- Labelling and packaging: unit identity and the delivery condition.
Frequently asked questions
Can box build start before the board is fully released?
Planning can start early, since scope and interface decisions do not need finished artwork. Assembly needs the released board revision, because the harness and the enclosure are checked against it.
Do you accept customer supplied enclosures and harnesses?
Yes. Customer supplied parts are received against the released revision, inspected for damage and identity, and recorded in the unit file. Where a part arrives without a usable marking, that is raised before the build rather than during it.
Is unit level test included as standard?
The test scope is agreed at quotation. A fixture based functional test at unit level is the usual baseline for box build, and safety, insulation or burn in stages are added where the product specification calls for them.
Next step
Send the assembly drawing, bill of materials, harness and enclosure drawings, plus the unit test requirement and the delivery condition. The reply separates what is confirmed from what is still open, quotes the board and unit stages together, and lists the first article items that have to be approved before the build is released.
