Here's What You Need When Outsourcing Box Build Assembly

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Outsourcing an electronic printed circuit board assembly (PCBA) is usually a simple and well-defined process. Just hand over the Gerber files, CAD files, and bill of materials (BOM), and away you go.

Box build, or top-level assembly, on the other hand, can be less well-defined. A box build can mean many things—from a PCBA in a small enclosure to a large cabinet full of wires or a complex fully integrated electromechanical system with electronics and pneumatics.

So, what are the basics you will need to consider to get an accurate quote and help the build process go smoothly with your electronic manufacturing services (EMS) partner?


The first thing your EMS provider will ask for is a BOM. This should include all of the main components, clearly define what materials the EMS provider will source, and, where appropriate, what will be “free issued” from you.

You’ll also want to think about what to do with the smaller items: the nuts and bolts, washers, tie wraps, heat shrink, adhesives, etc. Are you going to define these or let your supplier decide? The same can go for wires and their identifiers. While these are often considered consumables, they still have a cost and need purchasing, so they must be defined somewhere to avoid unexpected cost increases and/or production delays.

Component drawings—particularly for “drawn” or bespoke items—should have tolerances and finishes clearly specified. Leaving these things open to interpretation could cause problems with assembly or quality control later, so it’s best to specify exactly what you need.


Where possible, provide 3D CAD models as this helps to visualize how the product goes together. Many CAD packages offer free drawing viewers. More advanced EMS providers are likely to have their own CAD packages to help convert drawings into build instructions (and to enable them to update the drawings if required and agreed by you).

A layout drawing showing where major components will go—routing of cables and so on—should also be included. This might be important to you for servicing, for example, or for design compliance reasons. Ideally, you will also be able to provide detailed build instructions, particularly in the case of an existing product that is already being manufactured. This may not always be so straightforward, however, if a product has been manufactured in-house.

For new products, some systems are so complex that it can be challenging to complete a design on paper or even in 3D CAD. Sometimes, an element of design and development has to happen as the first products are made.

When a large amount of labor, space, or specialist tools are required, it can make sense to outsource prototype builds rather than build them in-house. It also gives your assembly partner a chance to learn about the product and hit the ground running when full production starts. Naturally, you’ll need to choose an EMS supplier that can assist with this development rather than just “build to print.” For electrical systems, schematics (circuit diagrams) will be required.

Your manufacturing partner should decide on the best build method (for example, whether to opt for point-to-point wiring or pre-prepared cables/looms) and they will produce cutting lists accordingly. Again, try to provide these in an electronic format whenever possible.

A sample unit is always helpful and can often be the main source of information if the drawings are incomplete. In this case, though, you’ll definitely need a provider that can engineer and create the drawings for you to ensure consistent builds in the future.

Let your EMS provider know the size and weight of the unit. This is important not only for shipping but also for storage and handling through the build process. You also need to consider how you need the finished product packed and transported. Do you need special boxes, or a standard shrink wrap and pallet, for example?


Last, but certainly not least, think about test. For electrical systems, you should, at the very least, specify basic electrical safety testing (e.g., earth bond and flash tests). Consider whether you may want them to do some functionality testing as well, factory acceptance testing by your staff before shipment to an end customer, or perhaps a visual inspection would be sufficient.

Seek advice from your EMS provider if required, as they will have the knowledge and experience of what works best. Outsourcing box build assembly inevitably requires close cooperation between customer and suppliers. It can also tend to be an evolving process as a new product goes into manufacture. But by providing the right information to your EMS partner at the very start, you’ll have the peace of mind of knowing that everyone understands what is required.

Russell Poppe is the director of technology at JJS Manufacturing.



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