Pre-order checklist before your first assembly run

The boards are laid out, DRC is clean, and the assembly order is one click away. Then the assembler comes back with a list: a part that is not available, a placement file that does not line up with the Gerbers, a diode that is backwards on the first ten boards. Almost none of this is exotic. It is the same short list of mistakes, and every item on it can be found before the order goes in.

Here is the list we would run on any first run. The details differ between assemblers, so where a claim comes from one assembler’s documentation, we say so.

1. Read the BOM as a buyer, not as a designer

A BOM that was fine for a prototype on your bench often fails at an assembler, because a bench build tolerates “a 10k resistor” and a line does not.

• Every line needs a manufacturer part number, not only a value. Eurocircuits describes its BOM as identifying parts by description, manufacturer part number (MPN) and supplier part number, mapped to reference designators. Its BOM and CPL guidance also advises defining as many generic parts as possible, so the buyer can substitute equivalent passives without asking.
• Say what “generic” means. For a resistor or capacitor, write the tolerance, voltage rating and dielectric you actually need. A generic line with no rating invites a purchase that is technically a 0402 and electrically wrong.
• Check lifecycle status for every active part. Manufacturers use stages like active, not recommended for new designs, last-time buy and obsolete. TI’s product life cycle page defines them, and notes that last-time buy means the product is being discontinued. A first run on a part already in last-time buy is a redesign waiting to happen. If you find one, our note on checking a chip’s longevity before you design it in covers what to look for.
• Find single-source parts. Mark every line that has one manufacturer and no drop-in alternate: the main MCU, a specific PMIC, a connector with a custom footprint. These are the parts that stall a build, so decide now whether you will buy spares or approve a second source.
• Mark do-not-populate parts clearly. Options fitted on some variants and not others should be marked in the BOM and excluded from the placement data. KiCad has per-footprint attributes for exactly this: the PCB Editor manual lists “do not populate” and “exclude from position files” and “bill of materials” options for footprints.

2. Make the BOM and placement file agree

The placement file (also called CPL, pick-and-place, centroid, XY or position file) tells the machine where each reference designator goes. Eurocircuits describes it as specifying the position, rotation and board side for each designator.

Two things go wrong repeatedly.

Designators do not match. JLCPCB’s checklist of assembly files says designators in the BOM must exactly match those in the placement file, and cautions against range shorthand like “R1-R8” in a BOM cell. Export from the same design state, at the same time, and diff the designator lists. Do not hand-edit one file and forget the other.

The origin differs. The same JLCPCB page calls a mismatch between the (0,0) point of the placement file and that of the Gerbers the most common error, and recommends a common origin for both. In KiCad this is the drill/place file origin, set from the Place menu in the PCB editor. Pick one, use it for Gerbers, drill files and the position file, and write it down.

Treat both of those as that assembler’s guidance. Another assembler may want a different layout of columns or a particular unit. Ask for their template before exporting, not after the first rejection.

3. Verify polarity and rotation by looking, not by trusting

Library footprints are drawn with a convention for what zero rotation means. A placement machine works from the orientation of the part on the reel. Those two conventions do not always agree, and the result is a diode, LED or IC placed 90 or 180 degrees off. JLCPCB’s page makes the same point: it attributes the error to a mismatch between the EDA library’s zero-degree orientation and what the machine expects, and says there is no substitute for visual verification.

So do a visual pass before the order:

1. Open the placement file in a viewer, or ask the assembler for their preview, and look at pin 1 on every polarized part: ICs, diodes, LEDs, tantalum and electrolytic capacitors, connectors, crystals with a marked pad.
2. Check the bottom side separately. Mirrored rotation is where most of the confusion lives.
3. Compare to the datasheet’s package drawing, not to the footprint. If a footprint came from a library, confirm pin 1 against the manufacturer drawing once.
4. Note any parts you know need a correction, and tell the assembler in writing, rather than hoping they spot it.

A rotation correction is a CAD or library convention fix. Whether the assembler applies a per-part offset on their side is their process, and you should ask rather than assume.

4. Decide how you will test the first boards

On a first run, the boards are also your test of the design. If nothing can be probed, a failed board is a mystery.

• Put test points on every power rail, reset, and the debug and programming interface. Our note on what test points and programming pads a board needs goes through which ones earn their space.
• Decide how firmware gets onto the boards. If it is not pre-programmed by the assembler, someone has to do it by hand, and a board with no accessible programming header will cost you time.
• Write down what “pass” means before the boards arrive: rail voltages within a window, the MCU responding on its debug port, one sensor reading plausible. Then you can say within minutes whether a board is good.
• Keep a few boards unpopulated, or specify a partial build, so a rework or a respin has something to start from.

5. Ask the panel and fiducial questions early

Small boards, odd outlines and parts near the edge all affect how a board is held and run. These are assembler-specific, so ask rather than guess.

• Do they want boards delivered as a panel, singly, or as a panel you design? Who designs the rails and tooling holes?
• Do they need fiducials on the board, the panel, or both, and in what style?
• Are there parts hanging over the edge, tall components on the bottom, or connectors that need support during reflow?
• Which side does each part sit on, and is there a second pass for through-hole parts or hand-fitted ones?

Eurocircuits keeps fiducial requirements on a separate guideline page, which is a good reminder that this is documented per assembler, not universal.

6. Know what you are sending, and what you are not

Before the order goes in, write one page that answers:

• Which parts are supplied by you, and which are bought by the assembler?
• Which parts are fitted, which are optional, and which are left off on purpose?
• What storage or moisture handling do any parts need? If a part has sat open, see when SMT parts that sat open need baking before reflow.
• What happens if a part is unavailable: stop and ask, or substitute within a stated rule?

That last line prevents the most expensive kind of surprise. A silent substitution on a first run can send you chasing a firmware bug that is really a different regulator.

Sources

• Eurocircuits: BOM and CPL data
• JLCPCB: files needed for PCB assembly
• KiCad 9.0 PCB Editor manual
• TI: product life cycle

We run this list as a read-through of your files before anything is ordered; send the BOM, the placement file and the board files, and tell us which parts you already hold.