Wiring Overview and Principles

With the rapid development of high-speed circuit design, PCB routing has evolved beyond simple interconnection tasks. Engineers must analyze various distributed parameters using transmission line theory. Distributed parameter circuits account for spatial variations in voltage and current. Modern PCBs, with their complexity and density, include advanced features like microvias, buried/blind vias, and embedded components (e.g., resistors, capacitors). These advancements require PCB designers to understand production processes deeply and adapt their designs to manufacturing constraints.

Wiring can be classified into automatic wiring and manual wiring. Although automation offers speed, it often falls short of meeting the stringent requirements of hardware engineers, making manual routing the preferred approach.

DFM (Design for Manufacturing) Requirements in Wiring

1. Drilling

• Mechanical Drilling:
  • Recommended minimum size: 8 mils, with a limit of 6 mils.
  • Thickness-to-diameter ratio: Ideally 10:1 for manufacturability.
  • Device hole ring width: At least 8 mils per side.
  • Via ring width: At least 4 mils per side.
• Spacing Requirements:
  • Same-network vias: 6 mils.
  • Different-network vias: 275 μm.
  • Device hole spacing (different networks): 425 μm.
  • Non-plated holes to board edge: 150 μm.
  • Plated holes to board edge: Minimum 10 mils.
• Production Adjustments:
  Drill sizes increase by 150 μm during production to compensate for electroplating.

2. ETCH (Copper Traces)

• Copper thickness vs. minimum line width and spacing:
  • 0.5 oz: 3 mils line width, 2 mils spacing.
  • 1 oz: 3.5 mils line width, 4 mils spacing.
  • 2 oz: 4 mils line width, 5.5 mils spacing.
• Additional Guidelines:
  • Maintain at least 20 mils clearance for inner-layer copper.
  • Ensure routing symmetry for small discrete components.
  • Use thermal relief pads for SMT connections on large copper areas.
  • Distribute ETCH lines evenly to prevent post-processing warping.

Electrical Characteristics in Wiring

1. Impedance Control

• Avoid sharp angles and right-angle routing.
• Minimize the use of vias for key signal paths.
• For high-speed signals, consider arc routing for improved performance.

2. Interference Control (Crosstalk and EMC)

• Layer-based routing:
  • Separate high-speed and low-speed signals.
  • Partition digital and analog signals.
  • Isolate sensitive and noisy signals.
• Clock signals should be routed on inner layers for better shielding.
• Avoid routing beneath inductive devices like transformers to minimize parasitic capacitance and resonance issues.

3. Signal Integrity

• Maintain impedance continuity using controlled-length routing (3W principle).
• Key signals should be routed with a ground reference plane.
• Wrap key signals with ground where necessary.
• Avoid forming loops in return paths to adhere to EMC principles.

Timing and Topology Considerations

Meeting timing requirements is essential for system stability. Routing delays should be minimized and equal length routing should be applied to ensure timing integrity. Understanding the underlying timing constraints helps in designing effective equal-length routes.

Power Supply and Signal Routing

• Protect and filter the power inlet circuit.
• Use short and wide traces for chip pins and filter capacitors.
• Increase clearance for high-voltage networks.
• Avoid placing metal beneath inductors to reduce parasitic effects.

Thermal Considerations in Wiring

Thermal management is critical with increasing power consumption.

• Ensure trace widths meet current-carrying capacity requirements.
• Plan vias to enhance heat dissipation.
• Add copper fills under high-heat areas like ICs, and connect them with ground vias.
• Keep high-speed and sensitive signals away from heat-generating areas.

Wiring Summary

PCB wiring is a systematic and multidisciplinary effort requiring:

• Technical expertise in design and manufacturing.
• Strong analytical skills to balance competing requirements.
• Continuous learning to adapt to evolving technologies.

PCB design isn’t magic—it’s rooted in real-world principles and processes. By thinking critically, asking questions, and experimenting, you can master the art of PCB wiring.