Introduction
The bill of materials (BOM) is a critical document that guides PCB assembly. It lists all the components, hardware, and materials required to build the board. Errors and omissions in the BOM will result in mistakes during manufacturing, adding delays and costs.
Creating an accurate, complete BOM is essential for smooth PCB production. But it's not the only factor. Design for manufacturability, component selection, file preparation, quality processes, and collaboration with your manufacturer also impact results.
This article provides best practices for maximizing manufacturing success. We'll cover:
- Creating optimized BOMs for assembly
- Designing boards for easier production
- Choosing components wisely
- Preparing complete manufacturing files
- Implementing quality checks
- Working effectively with your manufacturer
Follow these suggestions to achieve precision, efficiency, and avoid errors during PCB fabrication and assembly.
Creating Complete BOMs for PCB Manufacturing
The BOM informs assembly by listing all materials, parts and hardware needed to build the board. Accuracy is critical. Without a complete, error-free BOM, manufacturing problems are virtually guaranteed. Here are some best practices:
Include All Components
It may sound obvious, but double checking all components are present is crucial. Missing pieces stop production. Perform rigorous checks that passives, ICs, connectors, electromechanical parts, and all other PCB elements are listed.
Add Supplier/Manufacturer Details
Specify exact manufacturer parts numbers. Avoid vague descriptions like "1uF capacitor". Include manufacturer names so vendors can source each exact component.
Note Component Locations
Reference designator fields should indicate the reference designator (e.g. R1, C5, U3) for each component's placement location on the PCB. This aids assembly and diagnosing issues.
Specify Component Quantities
Indicate the total quantity needed for each component. This ensures sufficient materials. Shortages mid-production create delays.
Include Hardware
List any hardware like standoffs, fasteners, connectors, that mechanics will need to assemble the finished board. Omitting hardware stops complete assembly.
Add Materials
Include auxiliary materials required like solder paste, adhesives, thermal grease/pads, washers, nuts, etc. Don't assume the assembler will know to supply these.
Call out Test/Programming Equipment
Note any fixtures or other equipment needed for testing, calibration, or programming boards after assembly. The manufacturer needs to procure these.
Provide Lifecycle Information
Indicate component lifecycle status like active, EOL, obsolete. This allows proactive component sourcing if needed.
Add Revision History
Including revision history sets context for changes from previous board versions that alter the BOM. This helps manufacturers quickly see deltas.
Include Assembly Instructions
Providing special assembly instructions in the BOM saves time versus placing them in separate documentation. For example, note odd insertions, pre-soldering, gluing requirements, etc.
Choose a Standard Format
Using a standard BOM format like IPC-2581 or Excel makes the BOM consumable for manufacturers. Avoid custom formats that require conversion.
Perform Rigorous Reviews
Conduct exhaustive design team reviews of the BOM for accuracy before release. Leverage EDA/PLC tools to validate BOMs against PCB designs to catch issues.
By meticulously creating a complete BOM, you give your manufacturer the full component and material details needed for smooth, uninterrupted assembly. This prevents delays from missing pieces and keeps assembly lines moving.
Designing PCBs for Manufacturability
While the BOM guides assembly, the board design itself also impacts manufacturability. Following DFM guidelines when designing boards helps streamline fabrication and assembly:
Standardize Components
Use standard-sized and packaged parts that manufacturers can easily source, place, and process rather than custom components.
Provide Space for Placing and Routing
Give adequate space around components for routing, soldering, and rework. Tight spacing causes placement issues.
Avoid Tiny Passives
Tiny chip resistors and capacitors are difficult for robotic assembly systems to handle. Size 0402 and below may require specialized processes.
Watch High Pin Counts
Components with over 100 pins can be problematic for assembly. Break out functions across multiple smaller ICs if possible.
Minimize Board Layers
Stacking too many layers drives up fabrication costs. See if design requirements can be met with 2-6 layers before maxing out.
Include Test Points
Include test points to validate fabrication quality and aid diagnosing faults during prototyping. This reduces rework.
Allow for Flexible Assembly
Avoid requirements forcing sequential assembly. Parallel steps enable faster production and change accommodation.
Standardize Where Possible
Reusing familiar validated design modules, footprints, patterns, etc. lowers risk of errors versus overly custom designs.
Verify Manufacturing Capabilities
Consult with manufacturers early if planning advanced processes like HDI, flex/rigid designs, etc. Ensure capabilities align.
Keeping ease of fabrication and assembly in mind during board design reduces headaches transitioning from prototype to volume production.
Choosing the Right Components
Component selection also impacts manufacturing outcomes. Issues with sourcing, quality, or packaging can derail production:
Research Supply Chain Viability
Double check component availability in target volumes required before finalizing selection, especially for newer or specialty parts.
Select Parts with Qualified Packages
Opt for component packages proven reliable for assembly like SOICs vs leading-edge packages that may have technical risks.
Check Component Lifecycles
Avoid end of life or soon obsolete components that will require mid-production substitutions if demand persists longer than supply.
Standardize Parts Where Possible
Using common components across multiple designs smoothes purchasing and benefits from economies of scale.
Analyze Past Quality Issues
Review historical quality data to avoid recurring component issues known to affect yields or reliability.
Validate Technical Specifications
Thoroughly verify component technical specs meet requirements to prevent finding gaps only after design finalization.
Consider Moisture Sensitivity
Use caution with extremely moisture sensitive parts requiring special handing like baking during assembly to prevent damage.
Carefully selecting optimal, compatible components reduces delays, costs, and defects stemming from component-related issues during manufacturing processes.
Preparing Complete Manufacturing Files
In addition to the BOM, manufacturers need complete documentation and files to execute production without back and forth delays:
Submit Native Design Files
Provide board layout files in native format (e.g. .brd) to avoid conversion errors versus export formats like Gerber. Include all required layers.
Include Component Libraries
Supply component library files containing all parts used in the design so manufacturers can view full architectures.
Provide Readme Files
Usage notes in readme files clarify versioning, layer maps, and other nuances to prevent misinterpretations.
Submit Mechanical Drawings
Include drawings detailing board dimensions, any enclosure requirements, component positioning needs, etc. to smooth assembly.
Share Firmware/Software
Provide firmware code, software utilities, programming files, etc. required to test or program assembled boards.
Send Test Procedures/Specs
Detail manufacturing testing procedures, pass/fail criteria, test jigs/fixtures needed, and requirements for delivering test reports.
Create Documentation Sets
Consolidate design, assembly, test, and all other process documentation into standard sets for each major revision to avoid losing key information.
Utilize Collaboration Tools
Share native files directly instead of exchanging documents to avoid version confusion. Maintain single source of truth via tools like CAD collaboration platforms.
Submitting complete, consolidated documentation and files the first time through minimizes back and forth that slows manufacturing velocity.
Implementing Quality Processes
Robust quality processes during manufacturing help identify defects early when easier to correct. Here are some recommended steps:
Perform Incoming Inspections
Inspect received components for damage, fraudulent parts, loose leads, or other deficiencies that could impact assembly or performance.
Monitor Soldering Processes
Continually check solder joint quality during production for signs of cold joints, shorts, or incorrect fillets that signal issues.
Implement AOI
Use automated optical inspection (AOI) systems after soldering to catch defects like missing components, misalignments, etc.
Conduct X-rays
X-ray finished boards to detect issues like shorts, open vias, voids in solder joints, and other hidden flaws impacting reliability.
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Execute ICT Testing
Leverage in-circuit test (ICT) systems to validate connections between components and verify board operational health.
Perform Functional Testing
Exercise critical operating functions under environmental conditions to confirm boards deliver expected performance prior to shipment.
Manage Out-of-Tolerance Conditions
Follow defined processes to resolve, document, and prevent recurrence of units measuring outside of allowed tolerances.
Analyze Production Data
Aggregate quality data into reports highlighting trends to guide improvements of processes, equipment, components, designs, etc.
Embedding robust quality control throughout the assembly workflow identifies problems at the source for containment versus allowing defects to escape to customers.
Collaborating Closely with Your Manufacturer
Smooth PCB production requires close collaboration between designers and manufacturers. Here are tips for an effective partnership:
Engage Early in Design
Involve manufacturing representatives during design reviews to provide guidance improving manufacturability, component selection, DFM, etc.
Share Prototypes
Provide prototype boards and documentation to prove out manufacturability. Incorporate feedback into design revisions.
Clarify Requirements
Define not only product requirements but production expectations like volumes, change flexibility, quality standards, etc. to guide planning.
Align Tools
Where possible, utilize connected tools allowing real-time design visibility to synchronize activities.
Foster Open Communication
Encourage questions, ideas, and feedback between engineering and manufacturing teams rather than one-way document exchange. Develop relationships.
Conduct Production Reviews
Hold regular cross-functional project reviews to jointly assess progress, issues, and alignment between teams.
Share Lessons Learned
Analyze the project upon completion to identify what worked well and pain points. Apply these lessons to boost future success.
Visit Manufacturing Facilities
Engineers benefit tremendously from seeing firsthand the manufacturing environment, processes, challenges teams face. Tours build mutual understanding.
Co-creating solutions and plans rather than simply "tossing designs over the wall" results in a smoother path from prototype to production.
Conclusion
Achieving precision and efficiency during PCB fabrication and assembly requires diligence across multiple dimensions:
- Meticulously creating complete, accurate BOMs listing all required components, hardware, and materials
- Designing boards with manufacturing processes in mind
- Selecting optimal, qualified components
- Submitting comprehensive documentation and files needed to build the boards
- Embedding quality control throughout assembly workflows
- Collaborating closely across engineering and manufacturing teams
Paying attention to each area helps prevent defects, delays, and unnecessary costs when taking designs to volume production. But a rigorous focus on the BOM provides the critical foundation for PCB manufacturing success. Double and triple checking this artifact remains a wise investment before kicking off production.
With an unrelenting focus on quality, efficiency, and collaboration, engineering and manufacturing organizations can work together to achieve the defect-free PCB production essential for business success.
Frequently Asked Questions
What are the most common sources of errors in PCB manufacturing?
Some top sources of errors include incomplete/inaccurate BOMs, challenging component selections, poor design for manufacturability, lack of quality processes, insufficient collaboration between teams, and inadequate verification of requirements.
How can we reduce delays due to component shortages?
Perform rigorous supply chain checks during design to validate adequate component availability. Select parts with qualified packaging and lifecycle status. Standardize components where possible. Share potential shortages with manufacturer early to develop alternatives.
What are some design techniques that can improve manufacturability?
Allowing adequate component spacing, avoiding tiny passives, minimizing layers, including test points, standardizing on proven footprint patterns, and involving manufacturing input during design all improve fabrication and assembly outcomes.
How often should we share design file updates with the manufacturer?
Frequent design file exchanges help minimize misalignment as any changes occur. Configuring the manufacturer as a user in the engineering collaboration system enables real-time visibility as well.
What types of quality checks best help catch issues early?
Incoming component inspection, process monitoring, AOI, xray, ICT, and functional testing during production identify defects close to the source before they propagate further.
