PCB Routing

INTRODUCTION

Routing is the act of connecting traces among each connection point (e.g., IC pins, connector pins, and electronic component pads).

BACKGROUND

• RAT's NEST: It'a term commonly used in the PCB layout tool to show un-routed connections among component pads.

DESIGN STEPS

1. High speed signal breakout and routing
   • High speed requires tightly coupled impedance control and timing requirements for data to clock length matching. They need to be routed first.
2. Power plane copper shape sizing and routing
   • low power AC impedance from voltage regulator to digital supply pins needs wide and short power planes to reduce loop inductance.
   • low power path impedance from the voltage regulator to high current loads reduce IR drop and PCB copper temperature.
3. Low speed signal break out and routing
   • Low speed signal generally are more robust at rejecting noise due to higher voltage swings
   • Avoid running parallel with high speed interfaces or static traces to reduce cross talk.
4. GPIO breakout and routing.
   • Avoid running parallel with a high speed interface to reduce cross talk.

BEST PRACTICES

• Avoid running any trace underneath a power regulator switching node, especially the inductor.
• Isolate high speed interface at least 2x of its trace width from any other trace.
• Run adjacent signal layers in orthogonal directions to reduce capacitive coupling cross talk.
• Use Power planes or shapes for power routing
  • A power shape or plane should have direct access underneath or next to the IC or BGA pins, and they need to placed adjacent to ground planes for better power noise filtering.
• Use ground planes to create low impedance current return.
• Use ground short pads for high switching power grounds for isolation
  • generally a ground plane is sufficient due the natural tendency of high speed current path wanting to return to path of least impedance.
• Stitch all ground layers together for a meshed low impedance grounding.
• Stitch PCB perimeters with vias fencing to reduce electromagnetic interference.
• Ground pour on unfilled spots on PCB to improve rigidity of the PCB.
  • Thieving could also be used to achieve rigidity for large and thin PCBs.

SUMMARY & CONCLUSION

• Routing refers to connecting wires on the PCB among components pads
• Route in the order of high speed, power, low speed, and static trace.
• 2x trace spacing is recommended for high speed trace to any trace
• Use planes for ground and copper shapes for power planes.

Further Reading

N/A