Gerber & BOM Files for Assembly: A Pre-flight Checklist

Gerber & BOM Files for Assembly: A Pre-flight Checklist

By Evie Zhang, Project Manager at Faradine. Updated June 14, 2026.

Key Takeaways

• A complete assembly package is three things: Gerbers (or ODB++/IPC-2581) for the board, a BOM that names exact parts, and a pick-and-place file that locates and rotates every component.
• The single most common cause of quoting delays is an ambiguous BOM. List the manufacturer part number (MPN), not just a value, for every line.
• Your pick-and-place file (centroid/XY file) must use the board origin, units, and rotation convention your assembler expects, or every part lands wrong.
• Include a fab drawing and assembly drawing. They resolve the questions a netlist and Gerbers can't answer, such as polarity, special handling, and stackup.
• Run a design rule check and a manufacturing review before you export. Catching a footprint error on your screen costs minutes; catching it on the line costs a re-spin.

A clean file package separates a PCB assembly order that ships on schedule from one that stalls in engineering review for days. To prepare Gerber and BOM files for assembly, you need three core deliverables: fabrication data (Gerbers, ODB++, or IPC-2581), a fully specified bill of materials, and a pick-and-place file. This checklist is for hardware engineers and procurement managers sending boards to a contract manufacturer, and it covers what each file must contain so your order clears review the first time.

The Three Files a PCB Assembler Actually Needs

A turnkey assembler needs three things to build your board: fabrication data that tells the factory how to make the bare PCB, a bill of materials that tells them what to put on it, and a pick-and-place file that tells the machine where each part goes. Everything else supports those three. Get them right and the rest is detail.

Here's the standard package most assemblers expect:

The diagram below shows how those files map to the build: fabrication data makes the bare board, the BOM names the parts, and the pick-and-place file tells the machine where each one goes.

Fabrication data Gerber / ODB++ / IPC-2581 makes the bare board

If you're new to overseas production, our guide on how to outsource pcb assembly china walks through the full handoff, of which file prep is the first step.

How Do I Export Gerber Files Correctly?

Export your Gerbers in extended Gerber (RS-274X, the format maintained by Ucamco) with embedded apertures, or skip Gerbers entirely and send ODB++ or IPC-2581, which bundle copper, drill, and netlist in one intelligent file. The intelligent formats are harder to misread, so a growing number of fab houses prefer them. If you do send Gerbers, send a complete set.

A complete Gerber set includes every copper layer, top and bottom solder mask, top and bottom silkscreen, top and bottom paste, board outline, and an Excellon drill file. Two things trip people up most. A missing or ambiguous board outline forces the fab to guess your edges, so put the outline on its own mechanical layer. And a drill file with wrong units turns clean holes into chaos, so confirm units (mm vs. inches) and that plated and non-plated holes are distinguished.

Before you zip the folder, open the exported files in a free Gerber viewer and look at them as the factory will. Check that the paste layer matches your pads, that no silkscreen sits on bare pads, and that the layer count matches your stackup.

How Do I Build a BOM the Factory Won't Question?

A factory-ready BOM lists one part per line with an unambiguous manufacturer part number, the reference designators that use it, and the quantity. The MPN is what makes a BOM buildable. A line that says "100nF 0402" could be any of hundreds of real parts with different voltage ratings, dielectrics, and tolerances, and the assembler can't source it without guessing.

Every BOM line should carry these columns:

1. (C1, C2, R5) so each part maps to a footprint.
1. per assembly.
1. and.
1. for a sanity cross-check.
1. , including critical specs like voltage or tolerance.
1. Aor "Do Not Populate" flag for unstuffed positions.

For passives, an MPN plus an approved-alternates note keeps the line buildable when a part goes short. For anything polarized, semiconductors, or connectors, pin the exact MPN. Sourcing genuine, traceable parts against your MPNs, rather than gray-market substitutes, is the difference between a board that works and one that fails in the field, which is why a serious assembler verifies component authenticity before placement. Loose BOMs are a recurring theme among the dfm mistakes pcb that quietly add days to a build.

What Goes in the Pick-and-Place File

A pick-and-place file (also called a centroid, XY, or insertion file) lists every surface-mount part with its reference designator, X and Y coordinates, rotation angle, and which side of the board it sits on. The SMT machine reads this file to position each component, so an error here means parts land in the wrong place or at the wrong angle.

Three details matter for a usable pick-and-place file:

• Coordinates reference the board origin in millimeters or inches. State which, and keep it consistent with your fab drawing.
• Zero-degree orientation and rotation direction must match what the assembler expects.defines a fixed zero-component orientation so CAD libraries and pick-and-place data stay consistent, but always confirm, because mismatched rotation on polarized parts is a classic first-article failure.
• Each line marks Top or Bottom so double-sided boards build correctly.

You don't have to fix every footprint rotation yourself; a good assembler normalizes your data during DFM. But the cleaner your export, the fewer assumptions anyone has to make. A clean footprint library is one reason a thorough pcb dfm review pays for itself before files ever leave your desk.

Why Send Fab and Assembly Drawings If I Have Gerbers?

Drawings answer the questions Gerbers and a BOM physically cannot. A fab drawing specifies your layer stackup, copper weight, board thickness, surface finish (ENIG, HASL), and impedance requirements. An assembly drawing shows component polarity, special placement, conformal coating zones, and DNP parts. These are the human-readable instructions behind your machine-readable data.

This is also where acceptance class lives. IPC-A-610 Revision J (published March 2024) defines three classes of workmanship: Class 1 for general products, Class 2 for dedicated service, and Class 3 for high-reliability and harsh-environment hardware. State your class on the drawing. It changes how an AOI or X-ray inspector judges every solder joint, from fillet height to BGA void percentage, and it changes the price. Say nothing and you're usually built and inspected to Class 2 by default.

Your Pre-flight Checklist Before You Hit Send

Before you send anything, run one final pass against this list. It takes ten minutes and prevents the most common round-trips that delay a quote. Treat it as the gate between "done designing" and "ready to build."

• [ ] DRC clean, no unresolved errors
• [ ] Complete Gerber set (or ODB++/IPC-2581), verified in a viewer
• [ ] Board outline on its own layer
• [ ] Drill file with correct units, plated vs. non-plated marked
• [ ] BOM with an MPN on every populated line
• [ ] DNP positions flagged in both BOM and assembly drawing
• [ ] Pick-and-place file with origin, units, rotation, and side
• [ ] Fab drawing with stackup, finish, and IPC class
• [ ] Assembly drawing with polarity and placement notes
• [ ] A short README naming the design, revision, and quantity

A full pcb dfm review at the design stage will have caught the structural issues; this checklist catches the packaging ones. When you're ready for pricing, the pcb assembly quote checklist covers what else to include so the number you get back is real and not a placeholder.

Frequently Asked Questions

What file formats do PCB assemblers accept?

Most assemblers accept extended Gerber (RS-274X) plus an Excellon drill file, and increasingly prefer intelligent formats like ODB++ or IPC-2581 that bundle copper, drill, and netlist together. For the BOM, send CSV or XLSX. For pick-and-place, send a CSV or TXT centroid file. Drawings should be PDF.

Do I need a pick-and-place file for a small prototype run?

Yes, even for one board. The pick-and-place file is what positions each surface-mount part on the SMT line, so without it the assembler either places parts by hand from your drawings or asks you to generate one, both of which add delay. Export it from your EDA tool alongside your Gerbers.

What is the most common reason a PCB assembly order gets delayed?

An incomplete or ambiguous BOM is the most frequent cause. A line that lists only a value and package, with no manufacturer part number, forces the assembler to stop and ask which exact part you mean. Every populated line should carry an MPN so the team can source it without guessing.

Should I send Gerbers or ODB++ for assembly?

Either works, but ODB++ and IPC-2581 reduce errors because they carry copper, drill, netlist, and component data in one validated file, leaving less room for a missing layer or mismatched drill. Gerbers are still universally accepted. If your EDA tool exports a clean intelligent file, send that; otherwise send a complete, verified Gerber set.

How do I mark "Do Not Populate" parts?

Flag DNP parts in two places: add a clear DNP or "Do Not Populate" status in the BOM, and mark those positions on the assembly drawing. Listing them in only one place is risky, because a part flagged in the drawing but priced in the BOM, or the reverse, leads to a board that's either over-built or under-built.

What does IPC class mean for my assembly?

IPC-A-610 class sets the workmanship and inspection criteria your board is judged against. Class 1 is general-purpose, Class 2 is the default for most commercial products, and Class 3 covers high-reliability hardware like medical or aerospace. State your class on the fab drawing, since it directly affects inspection rigor and cost.

Ready to Send Your Files?

A tight file package is the cheapest insurance you can buy against a delayed build. If you'd like a second set of eyes before production, send your Gerbers and BOM and we'll run a full DFM and manufacturability review, with a dedicated English-speaking project manager as your single point of contact. Get in touch with the Faradine team and we'll flag anything that would stall your order.