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How to Make a Prototype PCB Assembly?

Prototype PCB Assembly

Introduction to Prototype PCB Assembly

A prototype printed circuit board (PCB) assembly is a version of the PCB that is built before full production to test the design and functionality. Prototyping is an important step in the PCB design process as it allows you to verify that the board works correctly before committing to a full production run. A prototype build gives the designer a chance to test the PCB layout, component choice, assembly process, and overall functionality of the board.

Prototyping can help catch errors in the design before they become expensive mistakes down the road. It also provides an opportunity to test manufacturing processes and quality control procedures. Overall, building a prototype PCB assembly reduces risk and ensures that the final product meets specifications.

In this comprehensive guide, we will cover the key steps involved in prototyping a PCB assembly:

Steps to Build a Prototype PCB Assembly:

  • PCB Design Finalization
  • Generating Fabrication Files
  • Choosing a PCB Manufacturer
  • Selecting Components
  • Sourcing Components
  • Creating a Bill of Materials
  • Assembly Process
  • Testing and Verification
  • Design Changes and Re-spins

Thoroughly following these stages will help guarantee a successful prototype build. Let's look at each step in more detail...

PCB Design Finalization

The first step is having a tested, finalized PCB design file. This will typically be created in CAD software like Altium, Eagle, OrCAD, KiCad, etc. The designer should thoroughly verify the board layout, footprints, and connectivity before signing off on the files for prototype fabrication.

Some key design checks:

  • Confirm all traces and vias are properly connected
  • Verify component footprints match chosen part models
  • Check pin-to-pin clearance and trace width meets requirements
  • Review design rules like track spacing, mask expansion, etc.
  • Validate board shape and dimensions
  • Run electrical rule check (ERC) and design rule check (DRC)
  • Test critical paths, nets, power domains, and impedance requirements

Only release fabrication files after all design checks have passed and all errors corrected. It helps to have another engineer review the design before prototyping.

Generating Fabrication Files

Once the PCB design is finalized, you need to produce the fabrication data necessary to build the prototype PCB. This includes:

  • Gerber files - layered image files for each fabrication layer. Used to fabrication bare PCBs.
  • Drill files - NC drill files detailing hole sizes and locations.
  • Assembly drawings - PDF files with a visual drawing of the bare PCB for assembly.
  • Component centroid file - Location of every component for pick-and-place.
  • Bill of Materials (BOM) - List of all materials required.
  • Any other special instructions for PCB manufacturer.

Most PCB design software can generate the needed files automatically. Check with your board house for the optimal format and layer naming. Zip files together for upload to manufacturer.

Choosing a PCB Manufacturer

Selecting the right PCB prototype shop is crucial for quality results. Consider these factors when deciding on a fabrication partner:

  • Capability - Ensure they can manufacture the board layer count, material type, tolerances, and finish needed.
  • Quality - Evaluate their reputation, reviews, certifications, and inspection process.
  • Cost - Get quotes for desired order quantity and lead time. Factor in hidden costs like tooling and NRE fees.
  • Lead Time - Choose the fastest acceptable time frame for when you need prototypes.
  • Location - Domestic shops allow for easier communication and faster shipping.
  • Services - Some offer assembly, test, DFM, and other value-added options.
  • Previous Experience - Stick with a trusted partner if possible.

We recommend getting multiple quotes and asking for samples of previous work.

Selecting Components

The BOM dictates the types, models, and quantities of components needed to populate the PCB assembly. Carefully choose each part to meet electrical and physical requirements while optimizing cost, availability, and manufacturability.

Key considerations for selecting components:

  • Functionality - Ensure the parts meet all electrical performance specs.
  • Footprint - Double check footprint compatibility with selected models.
  • Tolerances - Verify the tolerance is acceptable for that application.
  • Packaging - Choose the optimal package size to fit board space and layout.
  • Availability - Check that parts are available and not end-of-life.
  • Lead Time - Prioritize shorter lead times for faster assembly.
  • Reliability - Select quality manufacturers with proven lifetimes.
  • Cost - Compare pricing between suppliers and quantities. Get samples if needed.
  • BOM Optimization - Reduce number of unique parts and suppliers.
  • Assembly Factors - Consider how parts will be placed and processed.

Get components from trusted vendors and order samples early to validate.

Sourcing Components

Once parts are selected, they need to be sourced and procured before PCB assembly can begin. There are several approaches to buying components:

  • Distributors - One-stop shop but potential cost premiums. Offers small quantities.
  • Manufacturer Direct - Lower cost but minimum order quantity (MOQ) required.
  • In-House Inventory - Use existing company stock if available. Fastest.
  • Brokers - Specialize in hard-to-find and obsolete components.
  • Substitutions - Replace unavailable parts with acceptable alternates.
  • Sample Kits - Useful for first prototypes before volume orders.

Make sure to order enough components to account for potential scrap and rework during assembly. Build in margin above actual BOM quantity.

Creating a Bill of Materials

The bill of materials contains all the information needed to accurately build the PCB prototype:

  • Designator - Unique ID for each component location.
  • Description - Part name or description.
  • Manufacturer - Supplier of the component.
  • MPN - Manufacturer part number.
  • Quantity - Number required for one board.
  • Reference(s) - Where used in the design files.
  • Packaging - Reel, tube, tray, etc.
  • Any special notes or instructions.

Example BOM Table:

Example BOM Table

Accurately populating every field helps avoid assembly errors and delays. Use your BOM generator's export function or create manually in a spreadsheet.

Assembly Process

Now that all the materials and data are prepared, PCB assembly can begin. Here is the typical workflow:

  1. Solder Paste Printing - Solder paste is printed on pads.
  2. SMT Pick-and-Place - Surface mount parts loaded by machine.
  3. Reflow Soldering - Solder paste reflowed to attach SMT parts.
  4. Cleaning - Flux residue removed.
  5. Soldering - Manual soldering of through-hole components.
  6. Cleaning - Final clean before inspection.
  7. Inspection - Optical and electrical testing.
  8. Conformal Coating - Optional protective material added.
  9. Packaging - Boards packaged and shipped.

Most prototype shops offer assembly service or you can assemble in-house if you have capabilities. Get assembly quotes for comparison.

Common Prototype Assembly Problems:

  • Incorrect or missing components
  • Wrong solder paste or reflow profile
  • Poor solder joints
  • Shifted or skewed parts
  • Board damage or scratches
  • ESD damage of sensitive components

Closely monitoring the build helps prevent issues. Do thorough incoming inspections of assembled boards.

Testing and Verification

The prototype stage is focused on confirmation - making sure the PCB functions as intended before committing to full production. All aspects of the assembly should be validated:

Typical prototype testing and verification:

  • Visual Inspection - Check for any errors in assembly, soldering, or physical defects.
  • Netlist Testing - Validate connectivity by probing test points on the board.
  • Functional Testing - Power up and test operation to spec at room temp.
  • Burn-in - Run boards for extended time periods under voltage and thermal stress.
  • Environmental Testing - Test functionality under range of temperature, humidity, vibration.
  • Compliance Testing - Run any required regulatory certification tests like EMI/EMC, safety, etc.
  • Test Engineering - Debugging or special test set ups to simulate real-world use.
  • Failure Analysis - Determine root cause of any prototype failures.

Document all test results. Work with the design and test teams to analyze failures and create a path forward. Additional prototypes may be needed to verify fixes.

Design Changes and Re-spins

It is common to discover needed improvements after prototyping reveals issues. Any changes should be made carefully to avoid unnecessary cost and delays:

  • Fix only confirmed bugs and minimize new features. Stick to bug fixes if possible.
  • Understand risks and impact of each change on the design, schedule, and budget.
  • Change components only if absolutely required.
  • Try to keep board layout adjustments to one area if possible.
  • Allow time for additional prototype runs to re-verify fixes.
  • Get agreement from management on proposed changes before re-spinning.

Finding the right balance of design improvement vs speed to market is key. The goal is shipping a quality product as fast as possible. With careful planning, any required changes can be managed smoothly.

Conclusion

Success in launching a new PCB product hinges on prototyping. Investing the time upfront to follow proven processes will ensure your prototype accurately reflects the production design. Testing thoroughly validates the product before large investments in tooling and production are made. This helps avoid costly late stage changes. By being diligent at each prototyping step, you can feel confident at production release and start generating revenue from your new product quickly.

Frequently Asked Questions

What are the benefits of building a prototype?

Prototyping offers many benefits including:

  • Validating overall design performance
  • Testing manufacturability before production
  • Catching bugs and flaws early when cheaper to fix
  • Reducing technical and business risk
  • Allowing testing of key functions
  • Demonstrating the product for marketing and funding
  • Gaining feedback from stakeholders
  • Optimizing assembly processes and test flows

What are the typical deliverables after prototyping?

Typical prototype deliverables include:

  • Fully assembled and tested boards
  • Comprehensive test reports
  • Documentation of any failures and fixes
  • BOM updates based on parts sourcing
  • Updated assembly drawings
  • Gerber design files for any modifications
  • Finalized test specifications
  • Updated cost estimate for production
  • Summary recommendation on readiness for production release

How many prototypes are typically built?

For simple boards, 10-25 units is common. For complex designs, 50-100+ prototypes may be needed. Key factors determining quantity include:

  • Complexity of the design
  • Critical nature of the product
  • Number of tests required
  • Budget available for prototyping
  • Time available for testing coverage
  • Number of team members involved
  • Requirements for field testing units
  • Quantity needed for regulatory certification

Evaluate these factors for each specific project. Build extra for potential re-spins.

What are the biggest mistakes during prototyping?

Common prototyping mistakes include:

  • Inadequate testing coverage
  • Accepting units that fail inspection
  • Under budgeting time and cost
  • Lack of quality process controls
  • Insufficient board quantity
  • Skipping design reviews and verification
  • Limited electrical and environmental testing
  • Accepting low quality assemblies
  • Not fully verifying BOM and documentation

Avoiding these pitfalls is key for prototyping success.

When a prototype is considered successful?

The prototype process can be considered successful if:

  • All tests pass with zero unresolved failures
  • The product meets requirements and specs
  • No significant design changes are needed
  • Manufacturing risks are identified and mitigated
  • Unit cost is close to target
  • Test coverage is comprehensive
  • All stakeholders are satisfied
  • The path to production is clear

Achieving this high confidence minimizes downstream risks.

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