PCB Design BOM Planning: A Balancing Act
The balancing act of knowing how much or little info is needed when designing your BOM is often convoluted and muddied. If done incorrectly, you could be stuck in a communication loop with a confused manufacturer, frustrated engineer, or a designer. This will inevitably cost you money, time and effort that otherwise would have been avoided.
In order to assist us, today we’ll assume that you’ve begun to wrap up the final stages of your design, and are beginning to send out your feelers to third party suppliers, manufacturers, or verification engineers (or any other third party entities that tickle your fancy).
Since the BOM is one of our common languages we use and send to others (especially when working internationally), it’s crucial that we maintain the correct amount of info throughout the BOM design, while allowing for enough wiggle room when things inevitably get tight. In order to manage this balance effectively, we’ve compiled a list of tips and tricks in order to keep your PCB BOM design stage running smooth and error free.
Who Is Your Audience
I went on the most beautiful hike yesterday in the Central Cascades just outside of Seattle. It was cold but amazingly beautiful. If I could have a recording of the sounds I heard, alone, it would be incredibly rewarding.
Most of you won’t give two farts as to yesterday's’ hike, though, since you’re obviously after proper design of your PCB BOM. This theory is similar to BOM design. As a writer, I have to consider the audience I am writing to just as you’ll have to consider the audience that your BOM is going to.
Who will be reading your current BOM? Engineers? Manufacturers? Designers? Each of these segments will absolutely require a varied level of detail in the BOM. It’s then up to you to determine what your balance level should be for each audience.
Engineering individuals (or teams) may only need to know the components involved in a design so they, perhaps, could manipulate or understand the functionality of the PCB. A manufacturer, on the other hand, may require every single part on the board required for manufacturing. This is quite obvious, but very much worth noting.
Knowing the audience you are presenting your to will greatly assist in knowing what information to include and exclude
Considering Part Variability
Once you get an idea of the audience that you will be presenting your BOM to, you’ll then have to consider the variability of each part you plan to incorporate. As an example, say you are submitting an RFQ for a few prototypes from a handful of manufacturers.
The current BOM you are floating around is defined to the T and leaves very little room to wiggle. Often, each manufacturer will take your BOM design at face value. They’ll think that if you’ve sent a fully defined BOM then obviously you’ve thought through and know exactly what you want, so there are no needs for suggestions as far as part variations go.
Suddenly, each quote that gets returned to you has skyrocketed due to a single-sourced passive resistor that is in high demand. Dangit! In the future, you’ll look to steer clear of single-source parts, especially passive ones. They’ll just serve to complicate things, especially when floating your BOM around to a variety of manufacturers (who often have different networks of sources).
To mitigate this price jump in the present example, you loosen your definition slightly by undefining a few sources and notice that not only are quotes lowering, manufacturers are beginning to suggest even better alternatives. Noticing now that it took twice as many communication touch points to get back each quote than it should’ve, you’ll now consider the variability of parts on your BOM design to cater to each audience segment.
Single-Level or Multi-Level
Running in parallel to the aforementioned considerations, you’ll additionally benefit from determining the number of levels you wish to create within your BOM. You may have a simple PCB without a terribly complex function; this may be appropriate to only run a single leveled BOM giving a broad overview of the design in an easy to digest format.
But you also could be running through the gauntlet of a 24 layered HDI PCB that incorporates a 36 pin input and a 128 pin output with a turbocharger that draws 12 gigawatts of power… hmm. Maybe a little exaggerated, however, the fact remains that the use of a single level BOM likely will be immense. Leveling up your BOM now, you can split up each little sub-assembly, or sub-function, into an easy to understand format.
All the while we should still be considering the audience and variability of the BOM. Does your engineer need each multi-level BOM or will their work be more efficient when working with a single level? Is there a certain assembly that you’d like kept under wraps from certain manufacturers? Multi-leveled BOM designs can allow you to hide or show assemblies at your will.
Knowing how many levels are appropriate for your specific audience segment can help in avoiding information overload
Taking a Step Back
Your PCB BOM design efforts will go much smoother if you simply take a great big step backward and asses all the moving parts: Who will be dealing with my BOM? What info do they need? What info don’t they need? Where is my bottom line? Where is their bottom line? Where do our lines meet? Where don’t they meet?
It’s not rocket science except when it is, but we can usually be confident in that a poorly designed BOM will simply result in a failed attempt at communication and will only serve to slow things down. Of course, this can be traumatic for a company that relies on its speed and reliability in communication, but at least you won’t be catching the building on fire with your faulty BOM. Keep the balance to keep the efficiency.
Additionally, utilizing fantastic PCB design software, such as Altium, can greatly assist in your BOM design throughout the life of the design. Features like an ActiveBOM built-in, and smart manufacturing output file generation will ensure a proper BOM design.