Prerequisite files for a PCB assembly project

Article By : Suresh Patel

It is imperative that due attention be placed on the generation of files such Gerber, centroid, BOM, and assembly drawings, as several assembly stages rest on the accuracy of these documents.

The key to seamless PCB assembly, among other things, is the availability of certain files that offer information about the position of components, their orientation, and more. Some of these important files include: Gerber files, centroid, bill of materials (BOM), and assembly drawings.

As a PCB designer, you need to communicate this information to the contract manufacturer. In the absence of the above files, many errors may occur. Some of the likely errors include error in component quantity, absence of pin and polarity markings, and missing orientation indicators, to name a few.

The availability of the files, on the other hand, offers the following advantages:

• They ensure there is no information loss between the designer & the contract manufacturer.

• Ensures there aren’t costly errors/repairs to deal with at a later date.

• Data that is not gathered from the silkscreen is made available from these files.

• Ensures there are no discrepancies on account of the unavailability of important information such as polarity markings or position of through-hole components.

Having gone through the benefits of these files, let us look at these files a little in detail:

Gerber Files

These files are beneficial both for PCB manufacturing and PCB assembly. These files are important for every layer of the stack-up as well. Some of the important details contained in these files include layer descriptions, aperture definitions, and data for board etching.

Common file formats include: RS-274D, RS-274X, and Gerber X2.

Some of the common issues related to Gerber files include design file mismatch as well as multiple aperture lists. Therefore, it is very important to avoid Gerber issues, and if it occurs, it’s very important to fix Gerber files issues. It is important to pay attention to these to ensure that the files are accurate.

With the help of EDA tools, Gerber files can be generated. All you need to do is to select the number of layers required.

Centroid File

Centroid files are extremely helpful when it comes to:

  • X-Y location
  • Orientation
  • Reference designator
  • Layer details
  • Package information

Sometimes, these files are also known as X-Y data files as well as pick and place files.

The good news is that some design tools can automatically generate Centroid Files. Some tools, however, need you to modify existing data so that these files can be generated.

Bill of materials (BOM)

This is a very important file that offers information about the components that need to be assembled on the board. Some of the aspects to be included in a BOM include:

• Item#

• Reference Designator

• Package

• Quantity

• Description

• Manufacturer

• Notes (if necessary)

The level of accuracy of the BOM is directly proportional to the accuracy in component sourcing as well as assembly. It is therefore important that due attention be paid to the generation of the BOM.

Assembly Drawings

These are documents that describe the placement of components on the outer layers of the PCB. Reference to these documents will give the manufacturer & the inspection team a strong sense of accuracy as far as the placement of the components is concerned. The important elements of an assembly drawing include:

• A pictorial representation that is an outline of the board that shows the location of the components as well as the reference designators.

• Assembly details, which are required for densely packed regions.

• Assembly notes that have mounting instructions, soldering instructions, height restrictions, cable orientations, and more.

In creating assembly drawings, the following aspects also need to be taken care of:

• Any special mounting instructions such as those of connectors or heat sinks.

• Any additional hardware that needs to be mounted on the board. These could include stiffener bars, handles & more.

• It is also prudent that any critical areas of assembly are shown by way of an enlarged view.

• It is best to indicate the location of the assembly stickers on the drawing.

To sum up

The above information has a strong role to play whether it is to do with receiving accurate quotes, the actual assembly or inspection post-assembly that minimizes the chances of any errors. Several assembly stages rest on the accuracy of these documents. It is imperative, therefore, that due attention be placed on the generation of these files.


About the Author

Suresh Patel has worked as a Sales Engineer and held other management roles at Mermar Electronics. He brings 25 years of experience in printed-circuit-board sales and technical client service and managing business. You can connect with him on Twitter.


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