RF PCB Circuit Layout and Routing Guidelines

1. Overview

2. Layout Guidelines

2.1 Mechanical Structure Requirements

• Typical outer cavity thickness: 4 mm
• Inner cavity width: 3 mm (2 mm for dispensing process)
• Corner radius: 2.5 mm
• Positioning: Use grid multiples of 0.5 mm (minimum 0.1 mm) for precision machining.

Structural outline drawing of finished PCB design

2.2 General Layout Rules

• Prioritize the RF signal chain layout, then other circuits.
• Follow the schematic sequence (input → output).
• Arrange components in straight line or L‑shape.
• Keep tight spacing for networks such as π‑networks.

Straight‑line RF chain layout example (left‑to‑right signal flow)

U‑shaped layout with isolation cavity

• Sensitive circuits
• High‑gain paths (to prevent self‑oscillation)

Straight layout with transverse isolation barrier for self‑oscillation prevention

2.3 Peripheral Circuit Layout

• RF ICs: Follow the datasheet strictly; place components as close as possible.
• If a metal baseplate exists: Avoid placing components on the contact area; do not slot the baseplate.

LMX2531LQ2080 chip layout example

3. Routing Guidelines

• Use 50 Ω impedance trace width with inner‑layer reference.
• Route from pad centers, keep straight, and prefer top layer.
• Bends: Use 45° or arc; avoid right angles.
• Use resistor/capacitor pads as turning points.
• Follow datasheet length and shape for matching traces.

Routing example: center‑fed straight trace with pad‑based turning points

General High‑Frequency Routing Rules

1. Layer selection: Use inner planes for GND/power to reduce parasitics and interference.
2. Bends: 45° or arc to minimize radiation, coupling, and reflection.
3. Length: Keep traces as short as possible.
4. Vias: Minimize via count.
5. Direction: Route layers perpendicularly (top horizontal, bottom vertical).
6. Copper pour: Add GND copper to reduce crosstalk.
7. Guard tracing: Guard critical signals with GND.
8. Loop avoidance: Use daisy‑chain routing; eliminate loops.

4. Grounding Rules

4.1 RF Chain Grounding

• Use multi‑point grounding.
• Copper gap: 20–40 mil.
• Place GND vias on both sides at uniform spacing.
• Place GND vias near decoupling caps/resistors and component ground pads.
• Recommended via pitch: 60–80 mil.

RF chain multi‑point grounding diagram

4.2 Cavity Shell Ground Vias

• Use two staggered rows of vias for better contact.
• Solder mask opening at PCB–cavity contact areas (not on signal traces).

Grounding at PCB partition positions; bottom copper exposure for baseplate contact

4.3 Screw Placement

• PCB–cavity: Place screws at cavity intersections; four corners mandatory.
  
• Screw positions for cavity fastening
• PCB–baseplate: At least one screw per cavity; add at corners; place near SMA/connectors to prevent board flex.
  
• Screw layout inside cavities

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