How the Electronics Component List Can Make or Break PCB Development

When I was in college, my lab TA would always tell us that, “the devil is in the details.” I found out just what she meant during one lab when I connected an electrolytic capacitor backwards and popped it. This saying rings true for PCB designers as well as university students, especially when it comes to an electronics components list or bill of materials (BOM).

A thorough and well-planned BOM is one of the keys to a working circuit and maximizes your chances of an on-time delivery from your contract manufacturer (CM). If you forget even one or two details, you may end up releasing the magic smoke from more circuits than one, or worse, end up with boards that never leave manufacturing. So, let’s give the BOM the consideration it deserves as a critical part of your PCB development and clearly define the requirements for a good electronics components list.

What is a BOM?

First things first, an electronics components list is exactly what it sounds like: a collection of all the components you plan to use on your PCB. This includes both SMD and through-hole components, connectors, and everything else. It’s important that you provide detailed information on each and present the list in an easily accessible format.  Your design software can export a BOM in a preset or selectable format; however, you need to make sure the list is easily editable and readable by your CM. This will help facilitate component changes and minimize delays to your PCB development.

Things to Include in a BOM

The information available for your BOM depends on your PCB design software; however, there are some essentials that should be included to facilitate your product development.

• Manufacturer Part Number (MPN) - Even if you’re using a common component like a 555 timer, you need to include the MPN. The same IC can come in multiple types of packaging or have varying numbers of inputs or outputs. Your CM will need to know whether you want the SOIC-8 or PDIP-8 555 timer, for instance. Additionally, searching for a component may not yield a result or result in multiple options from different manufacturers. With the MPN, you can ensure that the correct component is being used.
• Reference Designator - It’s necessary to have an MPN, but the reference designator is also important as it specifies the component explicitly. This is critical for accurate placement of same type components during assembly.
• Description - A description field lets you input whatever you want about the part in question. This will help you readily identify the exact function of each part and enter comments about special considerations.
• Quantity - The quantity field helps ensure the number of components is accurate for assembly.

The essentials listing above can be augmented with additional component information; such as package type, side of the board where the component should mounted and alternate components in case of unavailability. Component location or mounting coordinates is required for assembly. For Gerber files, this given a specific separate file.

Consequences of a Poor Components List

When I put a capacitor in backwards, it explodes, so what happens when our BOM isn’t as accurate or thorough as it needs to be? If your electronics components list is substandard, you may experience errors in your manufacturing, mismatched parts, higher than expected costs, or delayed runs.

One of the primary concerns with an incomplete or poorly made BOM is the possibility of some components not making it onto the PCB or being placed in the wrong place. Sometimes a missing component can simply be added during rework. This will not only delay delivery but also cost extra. If your BOM doesn’t accurately tell the CM where each part goes on the board, some components may get swapped. In the best case, you may find out and your manufacturer fixes the batch or runs a new one. In the worst case, you receive your order and test your PCBs but experience premature board failure.

A well thought out and accurate electronics component list can help you avoid many issues that may delay your PCB development or increase the costs. To minimize the risk of these pitfalls, ensure that your list is up-to-date and contains all the relevant information for your CM.