PCB Fabrification

Introduction

PCB Fabrication: This is the multi-step process of converting the electronic schematic and PCB layout into a physical board. It involves a series of complex procedures such as photolithography, chemical etching, and electroplating.

Background

• Line Trace and Spacing:

  • Trace Width: Affects the current-carrying capacity and impedance of the trace. Calculated based on the current requirements and temperature rise.

  • Trace Spacing: Important for minimizing cross-talk and ensuring signal integrity. For high-frequency signals, the "20H rule" often applies, where the space between traces should be at least 20 times the height of the trace above the ground plane.

• Material Selection:

  • FR-4 is the most commonly used material, known for its good dielectric properties and decent thermal performance. Alternative materials like Polyimide are used for flex circuits.

  • Dielectric Constant (ϵr): Affects the speed of signal propagation.

  • Thermal Conductivity (k): Important for heat dissipation.

• Gerber Files:

  • Comprises layers for copper traces, solder mask, and silkscreen.

  • Should include drill files and board outline files.

• Surface Finish:

  • HASL (Hot Air Solder Leveling), ENIG (Electroless Nickel Immersion Gold), and OSP (Organic Solderability Preservatives) each have specific pros and cons in terms of cost, shelf life, and signal integrity.

• Panelization:

  • Utilizes V-grooves or tab routing to combine multiple PCBs in a single panel, optimizing manufacturing efficiency.

• Quality Control:

  • AOI (Automated Optical Inspection) and ICT (In-Circuit Test) are standard.

  • X-ray inspection for BGAs and hidden connections.

Fabrication Steps

• Material Cutting:

  • The substrate is cut using a CNC machine to the dimensions specified in the Gerber files.

• Copper Pre-Bond:

  • Copper foil is thermally bonded to the substrate, usually both sides.

• Photoresist Application:

  • A layer of photo-sensitive film is applied to the copper layers.

• UV Exposure and Development:

  • Photolithography techniques are used. The mask aligner ensures accurate pattern transfer.

• Chemical Etching:

  • Ferric chloride or ammonium persulfate solutions are common etchants that remove unprotected copper.

• Drilling:

  • Mechanical Drilling: Tolerances as low as ±0.005” are achievable for larger via holes and component mounts.

  • Laser Drilling: Used for microvias in HDI boards, allowing for via diameters as small as 50 microns. Typically used in blind and buried vias.

• Electroplating:

  • Copper is electrochemically deposited into the holes to form vias.

• Solder Mask and Silkscreen:

  • LPI (Liquid Photo-Imaging) for solder mask and epoxy-based ink for silkscreen are industry standards.

• Component Assembly:

  • SMT (Surface Mount Technology) and THT (Through-Hole Technology) are the two primary methods. Pick-and-place machines and reflow ovens are commonly used.

• Surface Finish Choice:

  • Consider corrosion resistance and wire-bonding needs.

• Conformal Coating:

  • Types: Varnish, epoxy, and polyurethane coatings are used to protect the board from environmental factors like moisture.

  • Application Methods: Spraying, dipping, and selective coating.

• Testing and Inspection:

  • Flying probe tests for short circuits and open circuits. Thermal cycling for stress testing.

Best Practices:

• Cost Considerations

• Layer Count: More layers generally mean higher costs.

• Materials: Specialty materials can significantly increase the cost.

• Turnaround Time: Rush orders will be more expensive.

• Via Choice: Consider the use of laser-drilled microvias for high-density layouts. Mechanical vias are generally more cost-effective for standard designs, but laser drilling can offer higher precision and smaller sizes, which might be crucial for high-frequency or compact designs.

• Trace and Spacing Guidelines:

  • Use wider traces for power delivery and narrower traces for signal lines.

  • Follow manufacturer guidelines for minimum trace width and spacing to ensure manufacturability.

  • For RF and high-speed digital signals, consult relevant standards like IPC-2221 for specific guidelines on trace width and spacing.

• DFM Checks:

  • Check for solder bridge risks, acid traps, and minimum spacing violations.

  • Design for Test (DFT)

• Test Points: Ensure that the design includes accessible test points for crucial signals and power rails.

• Programer and Debug header: Implementing test access ports for complex boards to facilitate testing.

• Impedance Control

  • Controlled Impedance Lines: These are crucial for signal integrity in high-speed designs.

  • Impedance Testing: TDR (Time-Domain Reflectometry) is used to validate the impedance of the fabricated lines. typically on coupon traces.

• Environmental Compliance

  • RoHS: Restriction of Hazardous Substances in the EU.

  • WEEE: Waste Electrical and Electronic Equipment regulations.

• Post-Fabrication Processes

  • Depaneling: Methods include laser cutting, milling, and breakaway tab routing.

  • Final Inspection: May include thermal cycling tests, burn-in tests, and functional tests.

Summary & Conclusion

• Understanding the Fabrication Steps: A deep understanding of each step can help designers make more informed decisions during the design phase to ensure manufacturability.

• Quality Control is Crucial: A multi-layered quality assurance process involving both automated and manual inspection is key to high-reliability PCBs.