Developing a new electronics product with a custom printed circuit board (PCB) involves extensive planning even before schematic entry begins. A systematic approach to planning PCB projects helps avoid costly mistakes and delays down the line.
This comprehensive guide covers how to effectively plan a PCB project end-to-end – starting from the product requirements all the way through fabrication, assembly, and testing. Read on to learn planning best practices that will set your next PCB project up for success.
Gathering Requirements
The critical first step is gathering detailed requirements that will drive the PCB design, manufacturing, and testing. Key requirements to obtain:
- Input/output interfaces – What types of wired or wireless interfaces are needed? Common standards like USB, Ethernet, WiFi determine layout constraints.
- Component selection – Will high power, high frequency, or space-grade components be used? Component sizes and pinouts affect PCB design.
- Functional modes – What modes of operation are needed? This defines use cases to design and test for.
- Performance criteria – What are the critical performance metrics like data rates, accuracy, response times, power consumption, etc?
- Environmental factors – What environments will the product operate in – indoor/outdoor, temperature range, vibration, etc? This influences design ruggedness.
- Compliance – What regulatory standards like UL, FCC, CE must be met? Compliance needs drive certain design and test criteria.
- Manufacturing volume – Will this be a short prototype run or mass production? Manufacturing and inspection processes depend on volumes.
- Budget constraints – What are cost targets for BOM, PCB fabrication, assembly, etc? Costs depend heavily on design choices.
Documenting all requirements creates a product requirements specification (PRS) that aligns cross-functional teams and ensures no gaps in planning the PCB project stages.
Selecting Components
With requirements defined, appropriate integrated circuits, discrete semiconductors, passives, connectors, and other components can be selected.
- Review datasheets for specifications like performance, operating ranges, packaging, pin counts, etc.
- Prefer use of standard components that are readily available and qualified. Custom or obsolete parts often delay projects.
- Consider environmental needs like temperature tolerance, radiation resistance, etc.
- Model board space needed for placements based on component dimensions.
- Check that packaging and pinouts are compatible with PCB assembly processes.
- Minimize different component types to reduce BOM cost and complexity.
Judicious component selection while meeting technical requirements optimizes downstream PCB design.
Schematic Design
The schematic captures the circuit architecture and connectivity on paper before physical implementation.
Schematic design tips:
- Use top-down hierarchical blocks for major subsystems. This simplifies complex designs.
- Adhere to logical grouping and naming conventions for nets and components.
- Include test points to facilitate hardware debugging and characterization.
- Design with manufacturability in mind. Schematics tightly constrain PCB layout options.
- Simulate circuit performance to verify operation before prototyping.
A complete schematic allows progressing to PCB layout with all interfaces, components, and nets logically defined.
PCB Layout
With the schematic complete, PCB layout can begin. This maps the logical schematic onto a physical board template.
Best practices for layout:
- Group related components together for efficient local routing.
- Ensure high speed paths have impedance control and isolation.
- Include modest spare board area for last minute layout changes.
- Follow all manufacturers’ design rules for trace spacing, hole sizes, etc.
- Design for thermal management with adequate copper heat sinks.
- Minimize trace lengths for critical signals like clocks and high speed buses.
- Review layouts regularly against requirements to avoid missing key features or constraints.
Careful schematic-driven PCB layout avoids costly respins due to functional or manufacturing issues.
Design Reviews
At multiple points during schematic capture and PCB layout, formal design reviews should be conducted.
Typical design reviews:
- Requirements Review – Verify all defined requirements are properly translated into the design.
- Schematic Review – Assess circuit correctness, manufacturability, testability.
- Layout Review – Check form fit, tolerances, serviceability, manufacturability.
- Safety Review – Ensure regulatory and safety standards compliance.
- Quality Review – Confirm adherence to quality processes and documentation.
Reviews help identify issues early when they can be readily addressed, reducing costly redesigns later.
Thermal Analysis
Thermal design is a critical aspect of PCB layouts. Excessive temperatures can cause failures.
Thermal analysis process:
- Power Maps – Estimate power dissipation for each component.
- Heat Spreading – Ensure power planes and thermal vias distribute heat.
- Thermal Simulation – Model temperature rises with finite element analysis.
- Heatsinking – Add copper pads, planes, and thermal reliefs to dissipate heat.
- Cooling – Evaluate need for fans, heat pipes, liquid cold plates, or other cooling.
Detailed thermal design improves reliability and prevents hot spots on the PCB.
Design for Manufacturing (DFM)
PCBs must be designed not just for function but also for manufacturability.
Key DFM considerations:
- Manufacturer Capabilities – Understand minimum trace widths, hole sizes, spacing, etc.
- Tolerances – Account for tolerances of ±10% or more in fabrication and assembly.
- Panel Utilization – Layout board shape and placement for efficient panel use.
- Split Planes – Avoid large copper fills which are prone to etching artifacts.
- Test Points – Include test pads connected to nets for characterization.
- Chamfers/Fillets – Use smooth corners on cutouts and pads for better fabrication.
Early DFM analysis avoids delays and costs of design re-spins to fix manufacturability issues.
Design for Assembly (DFA)
In parallel with DFM, assembly factors must be addressed:
- Component Placement – Ensure they are accessible by pick-and-place machines.
- Rotation/Orientation – Define component rotation to match assembly processes.
- Picker Access – Account for clearances required for vacuum picker heads.
- Adhesives – Plan locations of epoxies, tapes, staking compounds.
- Standoffs – Include standoff locations for securing components.
- Test Points – Provide adequate test points for in-circuit testing.
DFA techniques result in PCBs that transition smoothly from fabrication to automated assembly.
Cost Analysis
Performing cost analysis during the design process helps avoid cost overruns:
- BOM Cost – Calculate total component cost based on volumes, sourcing options.
- PCB Cost – Estimate board fabrication cost per units for chosen manufacturer.
- Assembly Cost – Project expenses for population based on complexity, lead times.
- Test Cost – Budget for test fixture development and unit testing time.
- Compliance Cost – Factor in costs to achieve regulatory certification.
- Tooling Cost – Account for any custom tooling like molds, jigs, fixtures.
Adjusting the design to align with target costs prevents budget surprises later.
Documentation
Maintaining up-to-date documentation is crucial for smooth PCB project execution:
- Requirements – PRS provides the foundation for development.
- Schematics – Fully detailed schematics essential for layout, BOMs, service.
- PCB Drawings – Manufacturing drawings with dimensions, notes, finishes.
- BOMs – Component lists with approved manufacturer parts and volumes.
- Assembly Drawings – Mechanical assembly diagrams, staking drawings.
- Test Procedures – Documented pass/fail criteria for every production test performed.
Complete documentation enables clear hand-off between teams and traceability.
Manufacturing and Assembly
With design complete, planning shifts to manufacturing and assembly.
Key planning activities:
- Fabrication Quotes – Get quotes from 2-3 manufacturers for cost/lead time comparison.
- Assembly Quotes – Same process for assembly houses to find best value.
- Contract Negotiation – Finalize volume pricing and terms with chosen vendors.
- Quality Agreements – Define defect limits, corrective actions, reliability metrics.
- Process Qualification – Audit and certify the PCB fabrication and assembly processes.
- Logistics – Determine shipment methods and schedules to synchronize supply chain.
Thorough planning with manufacturing partners results in quality product build at scale.
Test Strategy
Lastly, the overall test strategy must be defined:
- Inspection Testing – Optical, x-ray inspection to check assembled boards.
- Functional Testing – Validate operation against defined use cases.
- Environmental Stress Testing – Test under temperature, vibration, humidity extremes per requirements.
- Compliance Testing – Certify boards meet EMI, safety standards like CE, FCC.
- Quality Audits – Sampling based audits to confirm manufacturing quality.
- Test Jigs/Fixtures – Design any custom rigs needed to accurately stimulate and measure.
- Problem Reporting – Implement tracking for yield fallout and field failures.
- Test Documentation – Detail procedures, pass/fail criteria, equipment used.
Formal test planning reduces escaping defects and ensures robust, reliable products for customers.
Conclusion
Planning is the foundation of PCB project success. A structured approach spanning requirements gathering, design reviews, analysis, costing, documentation, manufacturing coordination, and test planning prevents problems downstream. Applying best practice project planning principles will lead to executing complex PCB programs on time and on budget.
Frequently Asked Questions
What are some key questions to ask PCB manufacturers during the planning stage?
Key questions are: capabilities and tolerances, lead times, change order rules, packaging methods, quality certifications, UL listing status, design for manufacturing (DFM) guidance, and available testing services.
What are some strategies to reduce costs during PCB projects?
Cost reduction strategies include: component standardization, dual sourcing components, value engineering design changes, negotiating vendor volume discounts, optimizing test coverage, and designing for automated assembly and inspection.
How early should compliance requirements like CE/FCC be considered?
Regulatory requirements should be analyzed right at project inception to assess needed design considerations, testing, and certification timelines which can impact schedules. Last minute certification fails can delay product release.
What are some pitfalls to avoid during requirements gathering?
Pitfalls include: unclear or conflicting requirements, leaving requirements unvalidated, excluding stakeholders, siloed teams lacking alignment, scope creep due to late additions, and failure to baseline requirements.
How can Fabrication and Assembly process capabilities be accounted for?
Engage vendors early for capabilities guidance. Perform process qualification audits. Review DFx guidelines. Define quality agreements clearly. Implement change control systems. Utilize manufacturability simulation tools during design.
