PCB Stackup Design Considerations for Intel® FPGAs

ID 683883
Date 6/28/2017
Public
Document Table of Contents

1.2.1.4. Skin Effect

In addition to dielectric absorption, signal attenuation can also occur because of resistive losses from the channel. Channel resistance is a function of frequency. For low frequencies, you can calculate the DC channel resistance from below equation.

DC Channel Resistance

Where:
  • is the DC channel resistance in ohms (Ω)
  • is the resistivity of copper 6.787x10-7 ohm-in)
  • is the trace length in inches
  • is the cross-sectional area of the trace in square inches

However, as the frequency increases, the resistive channel loss increases because current flows toward the surface of the copper trace. The surface penetration of this current flow is referred to as the skin depth (δ). This skin effect reduces the cross-sectional area of the channel, increasing the channel resistance. Countering this effect typically requires widening the trace width to increase the effective surface area. Increasing the copper weight of the traces does little, because most of the current is limited by the skin depth. The extent of trace width required usually entails calculating the skin depth.

Skin Depth

Where:
  • is skin depth in inches
  • is frequency in MHz

Channel Resistance for a Copper Trace

Where:
  • is the frequency dependent channel resistance for a copper trace
  • is the resistivity of copper (6.787x10-7 ohm-in)
  • is the trace length in inches
  • is skin depth in inches
  • is the trace width in inches

Based on the results of the skin depth and channel resistance, signal traces must be sized appropriately to reduce resistive losses because of the skin effect.