When a customer sends a PCB assembly project to our quote team, the file package contains the Gerber, the drill files, the centroid (X + Y) file, and the BOM. Our engineers run DFM checks on every file in the package, including the BOM, before the project enters production planning.
This post covers what a PCB assembly BOM contains, how our engineers verify it against the PCB, and how we source the parts on it. The screenshots come from a customer BOM for a BLE gamepad PCB assembly project that passed through our quote process.
The Files with BOM
A PCBA project has three production stages, and DFM applies to all of them.
- The Gerber files. They define the copper layers, the solder mask, and the silkscreen of the PCB. Our CAM engineers check trace widths, spacings, and annular rings against our production capability.
- The drill files. They list every hole with its diameter and position. Plated and non-plated holes are checked separately, because the two enter the process at different stages.
- The centroid file. It maps every component position with its X and Y coordinates and its rotation. The pick-and-place programs build directly from it.
- The BOM. It describes all the information about the components soldered and mounted on the PCBA and for the enclosure, including the names, values, manufacturers, manufacturer models, quantity, datasheet, and certificates.
Our engineers run the DFM check across all files together, because most defects sit in the gaps between files. A footprint in the Gerber that no BOM line covers, or a designator in the centroid with no part number behind it, surfaces only when the files are read as one package.
What Is a BOM in PCB Assembly
BOM stands for bill of materials. In a PCB assembly project, the BOM lists every component that gets mounted onto the PCB.
In a BOM, each line carries a part number, a manufacturer, a quantity, and the reference designators that tie the part mounted on the PCB.
The pick-and-place machines load reels according to the BOM and the centroid file, and the finished PCBA is checked back against the same BOM during the automatic optical inspection (AOI) and first-article inspection.
A BOM Example
The example BOM in this post covers two variants of the same BLE gamepad. Both variants run on the Nordic nRF52832 SoC and share the accelerometer, the LEDs, and the switches. Only the pressure sensor line differs, with a quantity of 1 for one variant and 0 for the other.
What a PCB Assembly BOM Contains
The screenshot shows the header row and the first component lines of the gamepad BOM. The columns fall into a few groups describing the part.
Reference designators. IC4, IC5, D1, IC2. Each designator marks one component position on the PCB. During a BOM check, we match every designator in the BOM against the centroid file and the PCB artwork. A designator present in one file but missing from the other stops the project until the customer confirms which side is correct.
Part numbers. The orderable part number, the manufacturer, and the manufacturer model. Many BOMs also list a distributor part number, such as the Mouser number next to the nRF52832 in this BOM. A complete part number lets procurement order the exact part.
Technical description. One column in the example BOM carries the key specs, such as 512KB memory and 64KB RAM for the nRF52832. The description lets our engineers check the part against the circuit design as well as the footprint.
Quantity columns. The example BOM carries one quantity column per gamepad variant. The pressure sensor line shows 1 for the first variant and 0 for the second. The assembly plan reads the right column for the right batch, so both products build from the same file.
Price columns. The BOM lists the price at 1 piece and at 1000 pieces. Distributors set their prices by volume break, so the same reel costs far more at single-piece quantity than at production quantity.
Datasheet, RoHS, and certificates. Each line carries a datasheet link, a RoHS compliance entry, and certificate documents. These links support the compliance file for the finished product, and they give our engineers the package and interface details needed for the DFM check.
The example BOM also groups the lines under category headers, including ICs, sensors, switches, and LEDs. Grouping keeps a long BOM readable for both the customer and the assembler.
How Our Engineers Check Your BOM
Every customer file package goes through DFM before we give a quote. For the BOM, the check runs on three levels.
The information is complete and correct. Each part number exists in the manufacturer catalog, with no typos, and each line names both the manufacturer and the model. Ambiguous lines go back to the customer as questions before quoting.
The BOM matches the PCB. Every reference designator in the BOM appears on the PCB, and every component position on the PCB appears in the BOM. The package in the BOM also has to match the footprint on the PCB.
The parts are available. We check current stock at the distributors and the lifecycle status of every part. A part marked EOL, or not recommended for new designs, needs a replacement plan before the project starts. We also record the market price of each line, because a part can be in stock and still be priced far above its normal level.
If a part is out of stock worldwide, on a long lead time, or priced several times above its usual level, the next step is an alternative component search.
Alternative Components When a Part Runs Short
When a part is unavailable or too expensive, we search for a replacement. The alternative component must provide the same function as the original and carry the same certificates. Its pinout, package, and operating range must match the design intent.
We send the customer a comparison of the original part and the proposed replacement, with both datasheets attached. No substitution proceeds without the customer's written approval. The customer reviews the documents and makes the decision.
BOM Component Sourcing at PCBONLINE
Beyond the DFM check and alternative parts, we provide BOM component sourcing for PCB assembly projects.
The Hong Kong procurement team. Our component procurement team is headquartered in Hong Kong, with a supply network that reaches authorized distributors and agents across regions. Hong Kong sits at the center of the Asia component trade, and our buyers work that market daily.
Regional price differences. The same manufacturer part number can carry different prices in different regions. Our global network allows us to buy the tapes and reels at their lowest regional price, and the difference shows up in the quote.
Volume built since 2005. With two advanced PCB manufacturing bases and an advanced PCB assembly factory, PCBONLINE has accumulated customers since 2005. The combined orders give us steady volume across many BOMs. We consolidate demand across projects and order in bulk. Bulk orders reach the lower volume breaks on the distributor price list. A small project therefore gets production-quantity prices on every line of its BOM.
Component cost saving. Components are the largest cost share in a PCB assembly project. A lower unit price on each line of the BOM is the largest single lever on the total project cost. For a full production run, the reduction on the component cost is the largest saving.
Send Your BOM for a Free Check
PCBONLINE provides free DFM checks, BOM verification, alternative component proposals, and component sourcing for PCB assembly projects. Our engineers review the Gerber, the drill files, the centroid file, and the BOM as one package, and the quote comes back with a component price breakdown.
As a turnkey OEM PCB manufacturer, PCBONLINE provides not only PCB manufacturing and assembly from prototypes to bulk production, but also component sourcing, custom jigs and molds, conformal coating, aging testing, IC programming, custom enclosures, wiring harnesses, and box-build assembly.
Founded in 2005, PCBONLINE has two large advanced PCB manufacturing bases in Jiangsu and Jiangxi Provinces in China, one turnkey PCB assembly factory in Shenzhen, and a smart OEM center and headquarters also in Shenzhen.
Source factory manufacturer providing one-stop PCB manufacturing and assembly from prototype to bulk production.
Each SMT line at PCBONLINE is equipped with three advanced pick-and-place machines, supporting up to 376 SMD placements per PCB, enabling us to handle all types of SMT assembly, including high-density and large-format PCBs.
PCBONLINE has expertise in hardware and software R&D from the project's early development phase to the field application engineering (FAE) stage.
By BOM component sourcing and in-stock warehousing, as well as alternate search and PCB optimization advice, we reduce PCB assembly costs without sacrificing function or quality.
Our experienced R&D and engineering teams provide one-on-one support, solving technical challenges while offering optimization suggestions to ensure your project runs smoothly from start to finish.
High-quality OEM PCBA manufacturing certified with ISO 9001:2015, ISO 14001:2015, IATF 16949:2016, RoHS, REACH, UL, and IPC-A-610 Class 2/3.
When your project enters the bulk production stage, PCBONLINE refunds the fees of prototyping, including its value-added services.
Send your Gerber and BOM, and we will return a quote, a lead time, and engineering feedback. To get a quote for your PCB/box-build project from prototype to bulk production, email us at info@pcbonline.com.
Frequently Asked Questions About PCB Assembly BOM
Q: What is a BOM in PCB assembly?A BOM, or bill of materials, lists every component that gets mounted onto the PCB, with part numbers, manufacturers, quantities, and reference designators. The assembler buys the parts, builds the placement programs, and inspects the finished PCBA against the same BOM.
Q: What files should I send with my BOM?Send the Gerber files, the drill files, the centroid file, and the BOM as one package. The Gerber defines the PCB, the drill files list the holes, the centroid file maps each component position, and the BOM names the parts and quantities.
Q: What does a BOM check include?Our engineers verify that every part number is complete and correct, that every reference designator in the BOM matches the PCB and the centroid file, and that every part is in stock and reasonably priced. Ambiguous lines go back to the customer before quoting.
Q: What happens if a component in my BOM is out of stock?We search for a replacement with the same function, package, and certificates, send you a comparison with both datasheets, and wait for your written approval. No substitution happens without the customer's confirmation.
Q: Can one BOM cover two product variants?Yes. A BOM can carry one quantity column per variant, with shared parts listed once. The example BOM in this post builds two BLE gamepad variants from a single file, with only the pressure sensor line differing between them.
Q: How does component sourcing through a manufacturer reduce cost?Components are the largest cost share in a PCB assembly project. A manufacturer with a global sourcing network buys each reel where its price sits lowest, and consolidated volume across many customers reaches the lower price breaks on every line of the BOM.
PCB assembly at PCBONLINE.pdf
