PCB Assembly Files Explanation

PCB Assembly Files Explanation

For PCB Assembly, 3 files are required at the very least to ensure we can assemble your boards. They are the PCB Gerber files, Bill of Materials (BOM) and centroid data.

1. What is a PCB Gerber file?

Gerber is the de-facto standard for representing PCB data for production. It is a 2D image format where each layer is separated into different files, for example, there are files for copper, solder mask, silkscreen and paste layers.  

99% of PCB CAD software supports the conversion of PCB design into ASCII Gerber file format, either RS-274D, RS274X or extended X (X2) format. Older software may export RS-274D format, but this is not recommended since many errors tend to appear with this format. NextPCB supports RS-274X and X2 formats.

While drill files are typically not Gerber files, when requesting Gerber files, we are implying that drill files are also needed as part of the production data required to produce your PCBs. Drill files are typically in Excellon NC drill format, and essentially tell the manufacturer where holes should be drilled and how big. Drill files in EIA binary format are not recommended, as they are not supported by all our equipment.

Generating drill files is often a separate process and missing drill files is one of the most common order problems, so it is wise to assume that drill files are also required when a manufacturer requests Gerber files and get used to exporting both.

Both Gerber files and drill files should be packaged into a single archive file (NextPCB supports .rar or .zip files). This can be uploaded on the NextPCB order page as part of the order process.

2. What is the Bill of Materials (BOM)

The Bill of Materials is a list of the raw materials that go into making a product or assembly. In the context of NextPCB's turnkey PCB Assembly service, the BOM file includes all parts that NextPCB should purchase and solder onto the boards. 

NextPCB's smart quotation platform can read BOM files and match part numbers with major distributors including Digikey, Mouser and HQ Online. By doing this, the platform can provide quotations in seconds rather than hours. In order to achieve fast quotations, the BOM file needs to be organized to NextPCB's format and must include the Manufacturer Part Number, Designator and Quantity. Please download the template for an example.

NextPCB BOM Sample
NextPCB BOM Template

HQ NextPCB supports customer-supplied parts. For this, the BOM file should include a 'Customer Supply' column and the relevant parts should be marked with a Y. The platform will not quote these parts but they will contribute to the assembly cost. For more information on customer-supplied parts and where to send them, please get in touch with your customer service representative.

For parts NextPCB does not need to purchase or solder, please remove them from the BOM or mark them with the letters 'DNP' in the 'Customer Supply' column.

3. What is the Centroid file?

Centroid files, (also known as pick & place file, XY file, component placement file, SMT file, etc.) tell us the x-y coordinates of every surface mount part in the design. It helps our engineers locate the parts during Design for Assembly (DFA) review, and set up the pick & place machine for machine assembly. Most PCB CAD software can export pick and place data, and they typically contain the designator, x-coordinate, y-coordinate and rotation value in degrees and more:

1. Designator - identifies the part. Unlike the BOM file, separate designators must have their own line and coordinates.
2. XY Coordinate - tells us the location of the part. X and Y coordinates should be in separate columns and the coordinate should center on the part origin if possible.
3. Rotation value - tells us the orientation of the part. This is important for polarized parts such as diodes and ICs, however, since the designer often does not know the orientation of the part as supplied (in the tape for example), this value may not be accurate and should be used as a reference only. 
4. Side - Some PCB CAD software includes a 'side' column in the centroid file for double-sided assembly, or export two files, one for the top and one for the bottom. NextPCB accepts either, and while not necessary, this can sometimes help with complex designs.

How to export centroid files (XY/pick and place files) for PCB assembly?

The centroid data should be packaged in a .rar or .zip archive file. Other files can be included with the centroid data that would help with assembly. This includes images, diagrams, 3D files and schematics.

Through-hole parts are typically not included in the centroid file since these are not placed by pick-and-place machines, and boards with only through-hole parts do not require centroid data, but please include some other information that could assist with assembly.

    • Related Articles

    • How to export centroid files (XY/pick and place files) for PCB assembly?

      Centroid files are also known as pick & place files, XY files, component placement files, SMT files. These are required for HQ NextPCB PCB Assembly orders except if all the parts on the board are through-hole parts. Here we made some gifs to show how ...
    • How to get extra fast PCB Assembly?

      HQ NextPCB's turnkey PCB assembly service can produce fully assembled PCBs from 5 working days by sourcing from HQ Online. For turnkey PCB assembly abroad, the slowest part of the production cycle, and the cause of most long waiting times is ...
    • How long is the lead time for PCB Assembly (PCBA) orders?

      After DFA review has been completed, the lead time for HQ NextPCB PCB assembly (PCBA) orders is around 7 business days if all the parts are from HQ Online and around 20 business days if not. The lead time may be affected by the PCB production time ...
    • How to place a PCBA order?

      Navigate to the NextPCB PCB Assembly order page from the top menu and upload your PCB Gerber files . The platform will try to read data from the Gerber files and extract some parameters from the files including PCB Size, Layer Count, Min. ...
    • Can I send components to NextPCB directly? (Consigned Parts)

      In addition to turnkey PCBA services, HQ NextPCB supports consigned components i.e. components that are provided by the customer instead of being procured by NextPCB. This is useful if certain parts are hard to source or have a particularly long lead ...