PCB & manufacturing
PCB Fabrication vs Assembly: Who Needs Which Files?
Fabrication and assembly are usually two different companies doing two different jobs. Which files each one needs, what they share, and where the handover fails.
You finish a board, zip up everything your ECAD tool produced, and email it to "the manufacturer". Two weeks later you get boards back with no components on them, because the company you wrote to makes bare PCBs and does not do assembly.
This happens often enough to be worth saying plainly: fabrication and assembly are usually two different companies doing two different jobs, and they need two different sets of files.
Who does what
A PCB fabricator makes the bare board. They etch copper, drill holes, apply soldermask and silkscreen, and apply the surface finish. They never see a component. Their entire world is the physical substrate.
A PCB assembler, often called an EMS (electronics manufacturing services) provider, takes bare boards and parts and puts them together. They print paste, place components, reflow, inspect, and often test. They do not make the board.
Some companies do both. A vertically integrated manufacturer, or a fabricator with an in-house assembly line, will quote the whole job. Many of the online "PCB and assembly" services work this way, which is convenient and is also why the distinction is easy to miss until you deal with a supplier who only does one half.
Even when one company does both, they will usually still want the two file sets kept separate, because internally they go to two different departments.
What the fabricator needs
Everything in this group describes the bare board.
| File | Purpose |
|---|---|
| Gerbers — copper layers | the conductive pattern on each layer |
| Gerbers — soldermask | where the green (or other colour) coating is absent |
| Gerbers — silkscreen | the printed legend |
| Board outline / profile | the shape to cut |
| Drill files | hole positions, sizes, plated and non-plated |
| Stackup | layer order, dielectric thicknesses, materials |
| Fabrication drawing | finish, final thickness, tolerances, hole table, notes |
| Impedance requirements | if any traces are controlled |
What they do not need: your BOM, your centroid file, your assembly drawing. Sending them is harmless, and they will ignore them.
What the assembler needs
Everything in this group describes what goes on the board.
| File | Purpose |
|---|---|
| BOM | which parts, how many, from whom |
| Centroid / pick-and-place | where each part goes, at what angle, which side |
| Assembly drawing | human-readable placement, polarity, pin 1 |
| Paste layer Gerber | to make the stencil — this one crosses over |
| Bare boards | either supplied by you or fabricated by them |
| Special instructions | conformal coating, selective soldering, hand-placed parts |
The paste layer is the interesting case. It is a Gerber, so it lives with the fabrication set, but it is used to cut the stencil for assembly. If the two suppliers are different companies, the assembler needs it even though everything around it goes to the fabricator.
The files that go to both
Three, and knowing which they are prevents most of the confusion:
Paste layer — stencil, as above. Board outline — the assembler needs it for tooling and fiducial reference. * The revision identifier — both need to know which version this is, and a mismatch between the boards and the parts list is a bad day for everyone.
Two scenarios
You buy boards from one supplier and assembly from another.
Send the fabricator the fabrication set. When the boards arrive, ship them to the assembler together with the assembly set and the paste layer. You own the interface between the two, which means you own the risk that the boards the assembler receives are not the revision the BOM describes. Label the physical boards.
One supplier does both (turnkey).
Send everything, clearly organised into two folders. You lose the labelling risk; you gain a dependency on one supplier reading your package correctly. Ask them to confirm the revision they are building before they start.
What goes wrong at the handover
The assembler receives boards from a different revision than the BOM describes. The most expensive failure in this article, and it only happens when fabrication and assembly are split. Prevention: the revision is on the silkscreen, and it is checked against the BOM on arrival. Not later.
Nobody sends the paste layer. The assembler cannot make a stencil, and the job stops until somebody notices.
The BOM arrives without the bare board count. Assemblers usually want a few spare boards for setup and for the inevitable rework. Ask how many.
Component supply split without saying who buys what. Consigned (you supply the parts) versus turnkey (they buy) changes everything about lead time and who is responsible when a part is unavailable. Agree it in writing before the first order.
A short sanity check before sending
- [ ] I know whether this supplier fabricates, assembles, or both.
- [ ] The fabrication set contains no assembly files I am expecting them to act on.
- [ ] The assembly set includes the paste layer.
- [ ] The board revision is on the silkscreen and in both sets.
- [ ] Consigned or turnkey is agreed in writing.
- [ ] The assembler has been asked how many spare bare boards they want.
- [ ] If the two are different companies, I have a plan for labelling the boards
in transit.
Does it matter if you send too much?
No. Suppliers are used to receiving a complete dump and extracting what they need. Sending a fabricator your BOM wastes nobody's time.
Sending too little costs a round trip, and on a typical lead time that is three to five days — longer if it crosses a weekend or a holiday. When in doubt, send the lot and label the folders.
Splitting the package correctly is a one-time piece of knowledge. The recurring problem is keeping the two sets in step: when a new revision changes the land pattern, the fabrication set and the assembly set both change, and they have to stay matched.
In XKEMA the manufacturing package belongs to a PCB revision, and an approved version is frozen so the sets cannot drift apart afterwards.
Hardware documentation PCB manufacturing package checklist * BOM, Gerber, CPL and drill files