TECHNICAL

Introduction

Printed circuit boards (PCBs) serve as the core platform for mounting and interconnecting electronic components in a product. Designing your own custom PCB provides immense flexibility and cost savings compared to using off-the-shelf boards.

This comprehensive guide covers the end-to-end process of DIY PCB design – from preparing the schematic, layout considerations, routing guidelines, generating fabrication and assembly files, to final artwork submission. Follow these tips to successfully design your custom PCB from concept to reality.

Step 1 - Prepare the Circuit Schematic

The PCB layout is based on the circuit schematic diagram which shows the electrical connectivity.

• Use EDA software like Eagle, KiCad, OrCAD, Altium etc. for drawing schematics.

• Arrange components neatly with digital parts on left and analog parts on right.

• Name each component, wire, and net as per a systematic naming convention.

• Group related components together with supply rails to make areas of the schematic.

• Add test points, debug LEDs, pull-up/pull-down resistors as needed.

• Connect any off-board components, connectors, cables in the schematic.

• Simulate and verify the schematic for functionality before PCB layout.

Step 2 - Select the Right PCB Materials

• Choose substrate material (FR-4, Rogers, polyimide, etc.) based on mechanical, electrical, thermal needs.

• Determine copper thickness – 1 oz, 2 oz or thicker copper based on current requirements.

• Calculate number of copper layers needed – 2 layer, 4 layer, 6 layer etc. in line with complexity.

• Select surface finish – HASL, immersion silver, ENIG, OSP etc. per soldering method.

• Choose solder mask color, finish – matte, glossy etc. Green and red common colors.

• Pick legend printing – negative or positive printing, font size, color.

• Specify board thickness, connector types, special coatings.

Step 3 - Placement Outline and Placement

A. Board Outline

• Draw board profile aligned with fabrication panel sizes – 12″x18″, 18″x24″, etc.

• Decide mounting holes – quantity, diameter, location suiting enclosure.

• Add any edge connectors, cable cutouts, handles, guidefins.

• Account for panelization tabs for assembly, breakoff rails.

B. Component Placement

• Place components starting with largest mechanically fixed parts.

• Arrange parts with consideration for accessibility and serviceability.

• Group components according to high speed and low speed circuits.

• Place connectors along edge for accessibility.

• Ensure clearance of parts from mounting holes and edges.

• Add inspection points, test lands, fiducials for assembly.

• Optimize placement for manufacturability and ease of assembly.

Step 4 - Floor Planning and Routing

A. Power Planes

• Define power net shapes on inner layers adjacent to signal layers.

• Allow clearance between planes for routing signals through vias.

• Add voids in planes for vias and thermal isolation needs.

• Connect planes to supplies with multiple vias for redundancy.

B. Signal Routing

• Route critical high speed traces first with controlled impedance.

• Minimize crosstalk by spacing out parallel traces based on signals.

• Use 45° and 90° angles instead of arcs for controlled impedance.

• Limit acute angles with maximum right angles for manufacturability.

• Route traces on a single layer first before changing layers.

• Verify trace widths and clearances as per fabrication rules.

Step 5 – Final Checks and Finishing Touches

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• Run design rules check and electrical rules check on completed layout.

• Review manufacturability guidelines like minimum hole size, trace spacing etc.

• Examine board for any missing connections in the schematic.

• Check for unrouted nets, unintended shorts, clearance violations.

• Confirm via stitching, back drilling for high speed signals.

• Examine decoupling capacitor placement near each IC.

• Review any redundant vias, traces overlapping pads.

• Rename nets, components as needed for readability.

Step 5 - Final Checks and Finishing Touches

• Run design rules check and electrical rules check on completed layout.

• Review manufacturability guidelines like minimum hole size, trace spacing etc.

• Examine board for any missing connections in the schematic.

• Check for unrouted nets, unintended shorts, clearance violations.

• Confirm via stitching, back drilling for high speed signals.

• Examine decoupling capacitor placement near each IC.

• Review any redundant vias, traces overlapping pads.

• Rename nets, components as needed for readability.

Step 6 - Generate Fabrication and Assembly Files

A. Gerber Files

• Export individual Gerber files – top and bottom layers, inner layers, drill file, solder mask, legend.

• Generate additional files like drill drawing, testpoints file, paste mask.

• Confirm files are formatted correctly and not mirrored or inverted.

B. Assembly Files

• Output BOM in csv format for components, reference designators, quantities.

• Export centroid or placement file from CAD software.

• Create pick and place file with rotation and side details.

• Generate stencil file for solder paste application.

Step 7 - Panelization for Board House

• Arrange multiple PCBs together within the panel for fabrication.

• Add mouse bites, breakaway tabs, fiducials, test coupons.

• Ensure adequate spacing between boards for cutting and routing.

• Check for exposed copper or traces along panel edges.

• Confirm panel dimensions align with board house requirements.

• Adjust layout to avoid any exposed board edges post breakoff.

Step 8 - Final Design Review and Artwork Submission

• Complete design review with PCB designer’s checklist as signoff.

• Have your layout reviewed by an experienced PCB designer.

• Submit Gerber files zipped in a folder along with readme documentation.

• Share BOM, centroid and placement files for assembly separately.

• Keep a backup copy of all files before submission.

Tips for Advanced PCBs

For high complexity boards:

• Model layout in 3D for visualizing fit and assembly.

• Perform signal and power integrity analysis through simulation.

• Account for controlled impedance traces and length matching.

• Incorporate thermal analysis of critical hot components.

• Implement EMI/EMC techniques – stitching vias, shielding etc.

• Design for reliability using redundancy, derating, and testing points.

Common Design Mistakes to Avoid

Schematic Errors

• Unconnected pins, missing power connections.

• Net name mismatches between schematic and PCB.

• Simulation not capturing actual circuit behavior.

Layout Errors

• Inadequate clearance between traces and pads.

• Acute trace angles. Long, unrouted traces.

• Too many vias in pads reducing solderable area.

• No provision for mounting or enclosure.

Manufacturability Issues

• Trace/space smaller than fabrication capability.

• Annular rings around pads insufficient.

• Placing via under BGA causing assembly issues.

• No provision for thermal relief, stitching vias.

Documentation Errors

• Incorrect layer order in fabrication drawings.

-BOM not matching with reference designators.

• Important instructions missing from readme.

Frequently Asked Questions

Q1. What are the essential supplies needed for hand soldering PCB assembly?

• Soldering iron, solder wire, flux, desoldering braid, tweezers, flush cutters, screwdrivers, adhesive, magnifying glass.

Q2. What are some methods used for double sided PCB alignment?

• Using alignment pins on jig, optical alignment with microscopes, using fiducials and markers on the board.

Q3. How can vias be tenting be avoided during assembly?

By specifying via fill, requesting cap on vias from board house, plugging by hand before assembly.

Q4. What is the typical thickness of soldermask over bare copper pads?

Around 0.5 mils of solder mask is usual over hang over copper pads. It provides insulation besides solderability.

Q5. How to check if PCB trace length matches circuit requirements?

By comparing electrical length in layout versus calculated wavelength of critical traces to ensure matching.