How to Prioritize Your PCB Design Variants

It’s common for design teams to create PCB design variants when preparing prototypes, experimenting with new functionality, or just replacing an obsolete component. Design variants aren’t a new concept, and most of the top-end CAD applications support design variants as a standard feature. Variants are even stored as project-level data, so they can be entered into a version control system and tracked just like any other set of design data.

PCB design variants provide another important function that is sometimes not considered early in the design phase. They can be used to help you overcome supply chain problems that result from inventory and supply fluctuations. It is often the case that desired parts go out-of-stock, go EOL without notice, or simply become too expensive to hit price targets. When this occurs, the design team needs to have a variant of the design that can perform the same (or nearly the same) functionality of the original design.

Variants mostly involve alternative parts, so how can you design variants that prioritize your highest-value alternative components? It takes a mix of data, guesswork, and bookkeeping.

Most Important Parts in PCB Design Variants

Prioritizing design variants for your PCBA requires prioritizing the alternative parts you will include in each variant. Any PCBA could include hundreds or thousands of parts, and millions of BOM variations are possible. If you want to have variants pre-prepared and ready to put into production at any moment, you will need to prioritize the most important substitute parts that will appear in each design variant.

The list below shows one possible prioritization that would apply to a broad range of digital systems. In this author’s opinion, the prioritization should proceed as follows:

1. Primary processor/compute elements
2. Specialty ASICs with no pin-compatible or functional replacements
3. Memories
4. Common ASICs
5. Common active ICs
6. Passives and standardized connectors

The list above is not exhaustive, but it covers many categories of designs. If a part in one of these categories goes out of stock, then you will need to brave the broker market, or you will need to quickly prepare a variant for production. The top categories are most important as they create the most value and can be most difficult to replace on short notice.

Embedded and ASICs at the Top

The list shown here places embedded systems components at the top because these are such huge drivers of important functionality in modern electronics. The top two part categories (processors and ASICs) can have specialized functionality, specific package types, specific pinouts, capabilities, and programming requirements that cannot be found in just any other component. Changing to a different part number family or a different part vendor could force major changes in your system capabilities, embedded application codebase, or both.

Since these are some of the highest value parts, and they can carry significant risk, what can smaller companies do to ensure they can access their required parts volume? If you aren’t big enough to receive allocation from the major semiconductor manufacturers, take these steps:

• Prioritize interface/pin compatible parts
• Prioritize parts in the same part number family
• Use an over-designed part and source lower-capability parts for variants

Common ASICs

The list of common ASICs is long, spanning across linear regulators to amplifiers. For these parts, you can often prioritize package-compatible replacements rather than worrying about finding alternatives with identical pinouts. The main reason for this is that it is typically more important to prioritize the functional capabilities in a design variant, and some ASICs can vary greatly in their capabilities.

The other reason we can often accept alternative packages for common ASICs and ICs is because they tend to have much lower pin counts than processors and specialty ASICs. This translates into lower design time required to complete a design variant.

What About Passives?

Finally, passives have been put at the bottom of the list, and for good reason. Passive components like resistors and capacitors come in very standardized case sizes, tolerances, and values. It is a very simple matter to swap out part numbers in a BOM, and no work would be needed in the corresponding CAD data.

There are exceptions to this, which includes some of the following:

• Certain diodes, such as in DO-214 packages
• Some circuit protection components (resettable fuses, TVS diodes, etc.)
• Specialty RF inductors with SRF ratings in the GHz range
• Precision passives (less than 1%)
• Capacitors with specific dielectrics (such as C0G)

This list of passives should definitely have higher priority in terms of creating design variants. They tend to have exacting specifications that are not easily found in other components, or they might come in unique packages. Therefore, these parts might require more design effort to include in a variant, including footprint and routing changes. They also provide much more specific functionality that cannot be easily replaced, and so they create more value than simpler passives.

Cofactr gives electronic component buyers instant visibility into component specifications so that buyers can manage sourcing for their design variants. Design teams and buyers use Cofactr to quote, purchase, manage inventory, and manage logistics for their electronic components. Cofactr also provides warehousing and logistics management services through its online