Embedded-cHow-ToIntermediate · 4 min read

How to Route Signals on Inner Layers in PCB Design

To route signals on inner layers in PCB design, first assign the inner layers as signal layers in your stack-up. Use vias to connect signals between layers, and carefully plan trace width and spacing to avoid interference and maintain signal integrity.

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Syntax

In PCB design software, routing signals on inner layers involves these key steps:

Define inner layers: Assign layers as signal layers in the stack-up manager.
Route traces: Use the routing tool to draw traces on inner layers.
Place vias: Use vias to connect signals between outer and inner layers.
Set design rules: Define trace width, spacing, and clearance for inner layers.

plaintext

Layer Stackup {
  Top Layer: Signal
  Inner Layer 1: Signal
  Inner Layer 2: Ground Plane
  Bottom Layer: Signal
}

Route Trace(layer="Inner Layer 1", width=0.15mm, net="Signal1")
Place Via(position=(x,y), diameter=0.3mm, connects=["Top Layer", "Inner Layer 1"])

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Example

This example shows routing a differential pair signal on an inner layer with vias connecting to the top layer.

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Layer Stackup {
  Top Layer: Signal
  Inner Layer 1: Signal (Differential Pair)
  Inner Layer 2: Ground Plane
  Bottom Layer: Signal
}

Route Trace(layer="Inner Layer 1", width=0.12mm, net="DiffPair+")
Route Trace(layer="Inner Layer 1", width=0.12mm, net="DiffPair-")
Place Via(position=(10,20), diameter=0.3mm, connects=["Top Layer", "Inner Layer 1"])
Place Via(position=(15,20), diameter=0.3mm, connects=["Top Layer", "Inner Layer 1"])

Output

Traces routed on Inner Layer 1 for DiffPair+ and DiffPair- with vias connecting to Top Layer at specified positions.

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Common Pitfalls

Ignoring impedance control: Inner layer traces often carry high-speed signals; not controlling impedance can cause signal loss.
Incorrect via placement: Placing too many vias or poorly positioned vias increases inductance and signal degradation.
Insufficient clearance: Not following spacing rules can cause shorts or crosstalk.
Routing on power/ground planes: Accidentally routing signals on plane layers can cause noise issues.

plaintext

/* Wrong: Routing signal on ground plane */
Route Trace(layer="Inner Layer 2", width=0.15mm, net="Signal1")

/* Right: Route signal on designated signal inner layer */
Route Trace(layer="Inner Layer 1", width=0.15mm, net="Signal1")

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Quick Reference

Step	Description	Tip
Define Inner Layers	Assign inner layers as signal layers in stack-up	Use dedicated layers for signals and planes
Route Traces	Draw traces on inner layers using routing tools	Maintain consistent trace width for impedance control
Place Vias	Connect signals between layers with vias	Minimize via count to reduce inductance
Set Design Rules	Define trace width, spacing, and clearance	Follow manufacturer specs for reliability
Check Integrity	Verify routing with DRC and signal integrity tools	Avoid routing on plane layers

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Key Takeaways

Assign inner layers as signal layers in your PCB stack-up before routing.

Use vias carefully to connect signals between inner and outer layers without adding noise.

Maintain proper trace width and spacing on inner layers to ensure signal integrity.

Avoid routing signals on power or ground plane layers to prevent interference.

Always run design rule checks to catch spacing and routing errors early.