Bill of Materials: The Building Block of PCBAs

Key Takeaways

• A bill of materials provides all the relevant information for PCB design and supports netlisting of the project.

• Designers can expect to find categories in a bill of materials, including land patterns and reference designators.

• Much thought goes into component selection, including price, availability, and the impact on the manufacturing process.

The bill of materials includes all of the components of the assemblies and sub-assemblies necessary to achieve the desired functionality.

An electronic design relies on a curated list of components and a meticulously designed board to deliver performance and functionality. Layout designers tend to focus more on the latter as they generally have more control over that process. However, as the layout is most often downstream design-wise of the bill of materials (BOM), it’s worthwhile to understand the motivation and methods for populating the list.

Bill of Materials Categories

The Bill of Materials Influences Board Layout

The BOM is one of the controlling documents of PCBA design. It evolves in tandem with the schematic after modeling to determine the components necessary to realize the design intent of the circuit. Abstractly, circuit design tends to think of the function or parameters of the components and less so the actual physical integration – is the part the right size and fit for operating conditions? Ordering the wrong components is an acceptable mistake during proof of concept or prototyping when designs are highly malleable but a cataclysmic error for any appreciable production quantities.

In addition to supporting procurement, a BOM collates information from multiple sources for easy integration into the PCB design. It can prepare a schematic for netlisting by associating all reference designators (the unique identifier for schematic symbols sorted by general circuit function) with the corresponding land pattern.

A component’s land pattern affects DFM in the following ways:

1. The land pattern's size, shape, and pinout will affect placement and routing strategies. 
2. An incorrectly designed land pattern or poor observation of best layout practices can hinder the solderability and total reliability of the board.

The BOM spreadsheet should contain all the necessary information for populating the schematic/board file and sourcing/procurement data. Some common categories will include:

• Quantity - A particular component may appear dozens or even hundreds of times within the circuit, especially simple passives like resistors and capacitors. The quantity gives procurement teams an easy way to build up orders when production lots increase.
• Land pattern - Every electrical component needs a land pattern to represent its space requirements and support routing and soldering. Purely mechanical onboard components will also require a land pattern (although this may be as simple as a pad) to prevent place bound collisions. Land patterns verification for accuracy is necessary before use; this verification could originate from a prior project or require a check from another layout/library team member. Pre-verified land patterns direct from the manufacturer or through a third-party vendor are also an option, but ensure reputable sources before incorporation into the design.
• Reference designator - This cell will contain all the reference designators tied to a particular footprint and will be the basis for schematic symbol/part association. The designator is a crucial link between the schematic and the board file; the standard path for updating this association will go from the schematic to the board file (forward annotation). However, there may be times when designers want to incorporate changes in the board file to the schematic (backward annotation).

Criteria for BOM Component Selection

The BOM is a living design document that will evolve throughout prototyping and into full production. Generally, designers can expect the design to flow through simulation first to ballpark signal performance and component values as a framework.

A few significant factors designers must juggle when finalizing a BOM at any design stage:

• Cost - This is the premier factor of component selection, especially for mass production quantities. However, procurement cost may only tell part of the story: cheaper components may require more costly manufacturing techniques to integrate into the board. When deciding upon components, it’s necessary to consider the full scope of production and its impact on DFM rules.

• Package type - Components come in many shapes and sizes to support the needs of designers and manufacturers. As packages shrink or pin counts become more multitudinous, the price per component escalates. Additionally, components typically group into one of two primary assembly incorporation methods: through-hole and surface mount technology (SMT). The choice of either (or the combination across a design) will limit the available automated techniques during assembly. As with all DFM, designers must feasibly balance design and manufacturing constraints.

• Availability - Demand and supply remain out-of-sync due to component manufacturer backlogs due to supply chain disruptions from 2020. Even simple passives have become more difficult to procure in the current market. While efforts to strengthen global manufacturing are in progress, they will not likely begin paying off for several years. 

For all these reasons, BOMs should include a reasonable amount of uncertainty. A great example is do-not-install (DNI) components, which express a need for land pattern accommodation in the layout without a matching component in the assembly.

Why would a parts list include items not to be included? The reason is that the design may only fully settle once assembly is complete and the exact parameters are confirmable. Furthermore, designers may want to reserve board space for future component upgrades or the addition of features as the design process matures.

Cadence ECAD Solutions Accelerate BOM Design

Building a bill of materials requires tradeoffs between performance, producibility, and cost; knowing where a board can comfortably trade one aspect for another is invaluable. BOM population should always begin with simulation to build the framework for a design and verify functionality is as expected. 

Cadence’s PCB Design and Analysis Software suite avails product development with a rich and interlinked environment to promote seamless workflow from one stage of electronics design to the next. After the BOM design is complete and land patterns are ready for netlisting, OrCAD PCB Designer gives layout engineers all the tools necessary to place, route, and prepare manufacturing files for accurate and rapid production turnaround times.

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