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.