What is a BOM in PCB Design?

 A bill of materials (BOM) is an essential documentation tool for any electronics hardware product development. Specifically within printed circuit board (PCB) design, intelligent use of BOMs throughout the development process helps coordinate many cross-functional teams toward manufacturability and assembly success.

This guide will cover key roles BOMs play across PCB workflow stages - from component selection through fabrication, assembly, test and beyond.

BOM Basics

At the most basic level, a BOM lists out all the ingredients constituents that compose an electronic product. These ingredients primarily include:

• Electronic components - ICs, resistors, capacitors etc
• Electromechanical parts - connectors, switches, sensors etc
• PCB materials - soldermask, silkscreen, copper finishes etc
• Mechanical pieces - brackets, standoffs, fasteners etc

Grouped together into a single tabular listing, BOM provides vital transparency for product realization stakeholders across planning, procurement, fabrication, assembly, test and support.

Item	Reference(s)	Description	Part Number	Quantity	Supplier
1	U1	Microcontroller	ATMEGA328P	1	Digi-Key
2	C1 C5 C7	0.1uF Capacitor	99999-ND	3	Mouser
3	J1	USB Connector	XXX-0000-0	1	Mill-Max

This baseline tabular listing begins connecting part identities to attributes useful throughout product lifecycles.

BOMs Across PCB Design Stage

As PCB development progresses, BOM utility evolves across planning, design and release phases:

Conceptual BOM

Early product scoping begins linking proposed functionality to candidate components. This conceptual BOM nails down key integrated circuits and devices under consideration along requirements like:

• Target performance specs - speed, power etc
• Peripheral interfaces - USB, Ethernet etc
• Software environments - bare-metal, RTOS etc
• Physical ecosystem - connectors, sensors etc
• Budgetary or availability constraints

Refinement weighs alternatives against non-functional needs around certification, lifetimes, environments, QA processes and other enterprise concerns.

Settling on component selections scopes purchasing requests, hardware prototyping and preliminary scheduling.

Engineering BOM

As schematic design converges, the engineering BOM transitions component names/numbers to specific parts used. Additional details add clarity on exact models meeting functional needs:

• Target manufacturer/vendor stock numbers
• Lifecycle status - active, planned obsolescence etc
• Data sheet links documenting operating conditions
• Pricing across quantity levels
• Procurement lead times for budgeting

Version control maintains continuity between schematic symbols, PCB footprints and BOM listings as refinements occur. Enterprise processes like JPL Space Parts linking allow unified component definition reuse.

Manufacturing BOM

Finalizing the PCB layout synthesizes an interconnected manufacturing BOM per netlist connectivity. This shifts focus toward fabrication and assembly requirements:

• Unique part quantities by design instance
• Layer stackup materials - dielectric types, copper weights etc
• Board and assembly tolerances - soldermask expansion, paste layers etc
• Approved vendor lists - preferred fab shops or assembly houses
• Detailed operations notes - test procedures, programming sequences etc

Syncing the components placed in layout back to schematic BOMs ensures continuity heading into prototyping.

Here is an example showing color-coded manufacturing callouts overlayed on schematic symbols and linked PCB layout nets:

As-Built BOM

After hardware delivery, comparisons between as-built BOM listings and prior engineering intentions help maintain transparency should issues emerge. Configuration management discrepancies might uncover:

• Approved substitute component revisions
• Alternate manufacturer selections based on lead times/availability
• Mounting adjustments from original footprint design
• Modified test or programming sequences

While allowing flexibility to account for real-world variability, change processes must control alterations with considerations to qualification impacts.

As the source of product truth, the latest as-built BOM can guide troubleshooting, repair and obsolescence management throughout service lifetimes. Integrations to enterprise resource planning (ERP), manufacturing execution (MES) and product lifecycle management (PLM) tools provide continuity across business ecosystems.

BOM Software Integrations

PCB design tools connect BOM processes through several built-in and extended capabilities:

Native Features

Most PCB editors like Altium Designer provide embedded BOM generation tools pulling directly from placed schematic components and layout connectivity nets. Custom configurations allow some tailoring of columns and filtering for typical outputs needed.

Native outputs handle reasonable standard needs, but lack scalability for complex parsing or integration with external databases.

Extended Capabilities

For advanced needs, expanded BOM generation software integrates more harmoniously with design workflows. These tools access native data models while adding capabilities like:

• Multi-BOM documentation - fabrication, assembly, test etc
• Interlinked component data through centralized databases
• Custom parsing, sorting, formatting for ERP consumption
• Microsoft Excel integration
• Comparison reports assessing changes

Seamless interoperability reduces friction during handoff between engineering BOM expectations and manufacturing build realities.

Here is an example advanced BOM software generating multi-purpose outputs parsed from native design files:

PCB BOM Design Standards

Given BOM importance spanning product life cycles, standards around creation, consumption and maintenance help mitigate realization risks:

Component naming conventions

Unique and consistent naming of components on schematics and PCB layout ensures accurate parsing of parts between engineers and automated BOM processes.

Reference designators

Group components into logical reference designator sequences - U for ICs, C for caps, J for connectors etc. Maintain symmetry between schematic and layout for BOM linkages.

Cross-checking

Leverage capabilities like schematic-to-layout back annotation to validate netlist connectivity matches engineering intentions for BOM accuracy.

Revision change processes

Control component replacements/substitutions through Engineering Change Orders (ECOs) to assess qualification impacts with oversight authority.

Data continuity

Unify component selection/definitions between schematic libraries, PCB footprints and BOM databases for consistency as designs iterate.

Standardizing these foundations helps industrialize BOM interoperability between design engineers and the extended supply chain realizing production.

BOM Templates

For executions requiring rigidity, BOM templates document required deliverable structure covering:

• Column headers - component class grouping, attributes like reference designators, descriptions, quantities etc
• Sort orders - group by component type typically
• Formatting - font specifications, border styles etc catering to production needs
• Output targets - PDF, Excel, CSV etc

Templates might be consumed by ERP systems requiring specificity for automated import.

Here is an example BOM template from an EMS provider conveying requirements:

Strict BOM documentation standards demonstrate attention to detail in component management - an indicator of engineering meticulousness often assumed by production personnel.

In Summary

A BOM provides critical continuity between engineering design efforts and downstream manufacturing processes realizing products. Evolving clarity around component identities, sourcing, assembly and test flows mitigate realization risk as designs progress from concept to production.

Synchronization between tools, naming conventions and revision control processes enables transparency between teams interacting through different development phases. Well-crafted BOM infrastructure demonstrates commitment to design precision - establishing trust in reliability across stakeholders.

Frequently Asked Questions

Q: At what point in the design process should I start compiling a formal BOM?

A: An early conceptual BOM helps scope manufacturing expectations, budgetary estimates and component availability timelines to guide decisions. Formalize in greater detail during latter design stages as exact parts and configurations solidify.

Q: What are some key BOM rules we should adopt for our engineering team?

A: Some ideas would be - set BOM change authority processes, implement component naming guidelines between schematics and layout, control revision histories, require reference designator symmetry in placement etc.

Q: What types of data should I include for items in our fabrication and assembly BOM requirements?

A: Important columns cover - supplier identifiers, manufacturer parts numbers, component values, descriptions, reference designators, comment fields for notes, quantities, units of measure, lifecycles, datasheet links etc.