Tips to Reduce BOM Errors and Ensure Compliance

One of the most common reasons for delays in PCB assembly, as well as extra costs, is errors in parts information in a BOM. There are plenty of reasons errors can arise in a BOM, and these errors can range from a simple part number mistake to missing environmental compliance data or marking parts as DNP. Omitting this data creates new liabilities for manufacturers and can lead to incorrect part ordering by the procurement team.

Staying on schedule is all about having a process and tools to catch these errors early. Here, we’ll give you some tips on how to implement such a process and how you can use your design tools to catch these problems in your BOM.

Catch These BOM Errors Before You Order Parts

Although BOM errors are costly to designers and create schedule delays, they very often require simple changes to part data to avoid. The key is to catch these errors early in the design process, rather than waiting until the PCB layout is finished.

With that in mind, here are some of the most annoying (and costly) BOM errors, along with steps you can take to prevent them.

Part Number and Package Mismatch

The problem: The package and footprints placed in the PCB layout do not match the part number in the BOM.

This problem is almost always caught in PCB assembly, and by then you've put yourself in a real predicament. Do you scrap your PCBs and start the project over? Or do you try to find an alternative part that would fit into the existing land pattern on your PCB?

Sometimes there are not a lot of good options here, but the common solution is to locate another part in the same part number family but with a different packaging option. In the worst case, you may have to build a custom interposer PCB or possibly scrap the PCBs.

The solution? Designers need to have a parts creation and library review process that can catch this problem before a PCB enters production. Some subscription CAM tools are also capable of catching this problem during a DFM/DFA review in a semi-automated process. Large companies usually have someone responsible for librarian tasks, but smaller companies should rely on a trusted parts source to find CAD data for their components.

DNP Parts Called Out Incorrectly

The problem: A DNP part is called out on the same line as populated parts, or is not called out at all.

Ideally, DNP parts should not appear in the BOM or the pick-and-place file. If a DNP part does appear in the BOM, the assembler will need to check that it will get manually removed when programming the pick-and-place machine. This all happens when the assembler does a manual review of the BOM and checks it for consistency across your deliverables.

Things go haywire when the BOM is exported without properly calling out the DNP parts, or at least without using a consistent approach to calling out DNP parts. For example, in the image below, there is a DNP part on a single line which is highlighted yellow. The very next line also lists a DNP part, but not in the same column as the previous part. The assembler will naturally wonder what is going on, and you will need to consult your project documentation to verify that this part is supposed to be marked DNP.

The solution? Use a feature such as Variants in Altium Designer to define variants, apply schematic markup to callout DNP parts, and check this against DNP callouts in a BOM manager. Unless you can implement a process that checks for the correct presence of DNP markings on your parts, don’t do this and instead rely on assembly variants to control placements in the BOM and pick-and-place simultaneously.

Unknown Part Numbers For Passives

The problem: The designer only specifies package sizes and values for passives, but nothing else.

Contrary to popular belief among beginner designers, the part number used to specify a passive component does matter. This is very common in turnkey assembly BOMs or in a BOM destined for an overseas manufacturer. The designer relies on the manufacturer to do the hard work for them and select a part number based on a simple description.

The reality is that many applications require more specific parts specifications than you get from just a part description. That’s why it’s much more preferable to select a specific manufacturer part number rather than leaving it up to your PCB assembler. You might find that they make a selection that doesn’t work for your desired electrical functionality.

The solution? Be willing to do the hard work in your schematic design and select part numbers for passives. This takes a bit of extra work earlier in the design, 

Incompatible Alternates

The problem: The selected alternates don’t match the specs and/or package of the preferred part number.

In many instances, it’s a good idea to select alternate parts that can be drop-in replacements for the desired parts in a PCBA. This is very important for the most valuable components in your design, which could include specialized ASICs, digital processors, and some RF components. For these important parts, designers are advised to select and plan for their own alternates and even create variant designs with these alternates.

For other parts, designers usually rely on recommendation engines to determine alternates. These recommendations could come from distributor websites or be based on finding parts with matching descriptions. Unfortunately, if the alternate part number is incompatible, you may not realize this until the PCBA is finished and you are testing the device. Make sure your recommended alternates are being pulled from the industry’s best available supply chain data set, and don’t just rely on matching part descriptions or packages.

The solution? As always, do a side-by-side comparison of the major specifications for an alternate to ensure it is acceptable in your design. You can bring up the datasheets on the main specifications and place them next to each other on your screen, and this will help you see the main specifications directly.

Environmental Compliance

The problem: REACH/RoHS compliance is the wrong standard or is unknown.

When we talk about REACH and RoHS, most commonly we're referring to the US or EU versions of the RoHS directive. However, there are other versions, such as the China RoHS directive. A component that is US or EU RoHS compliant does not guarantee China RoHS compliance, or vice versa.

The solution? This is also where more granular data is needed to see the RoHS compliance status for a part in different regions of the world. Compliance with the US, EU, and China directives will ensure compliance in most of the rest of the world, so focus on these three as well as the REACH directive. With higher-quality data inside your supply chain management tools, you can see RoHS and REACH compliance status for each line item in your BOM.

Packaging With High MOQ

The problem: Component packaging information leads to higher-than-expected prices on a parts order.

When creating a component order, the packaging matters as it will impact price and MOQ. For example, tape-and-reel packaging sometimes gets its own supplier part number, and quantity one of a tape-and-reel part number might really mean one reel of thousands of components.

The solution: This is one aspect that is easily caught in a BOM review. Free web applications like the Octopart BOM tool or the BOM portal in Altium 365 can catch these simple part number mistakes. If you prefer to review in the design tool, the supplier links feature in Altium Designer can also show you MOQ values and will give you a clue that you have selected a reel for your order rather than an individual part.

Quantities Don’t Match Reference Designators

The problem: The number of reference designators on a BOM line item does not match the quantity of parts listed on the line item.

I’ve seen this in BOMs where line items are copied between different designs. This is a very bad practice, but someone may do it as a time saver, usually because they do not want to create or modify library components. Sometimes these modifications are created by someone other than the designer, and it is almost always due to manual changes applied to the BOM. The result will be a mismatch between the pick-and-place file and the BOM file, and the assembler will delay the production of your project while you resolve the problem.

The solution? Never let anyone copy and paste information into the line items in your BOM. Modifications should be made in the design or in the libraries, then regenerate the BOM inside your PCB design software.

Instead of forcing users to manually update component libraries with sourcing information, Altium is taking a different approach with a set of tools that allow supply chain information to be automatically imported into your PCB libraries and subsequently into a BOM. If you're ready to learn more, open up your Altium 365 workspace and give BOM Portal a try. Experience a seamless transition to a new workflow that expands your reach and capabilities.