What is the maximum frequency at which microstrip on standard FR4 is practical?
FR4 Frequency Limitations
FR4 is a woven fiberglass reinforced epoxy laminate optimized for cost and mechanical reliability, not RF performance. Its electrical limitations stem from three sources: high dielectric loss (tan δ = 0.020-0.025), variable dielectric constant (εr = 4.2-4.6 across lots and 4.0-4.8 across frequency due to the glass weave structure), and high copper roughness on standard ED foil.
The dielectric loss tangent is the primary limitation. Loss per unit length in microstrip is proportional to f × tan δ × √εeff. At 1 GHz, this is tolerable. At 10 GHz, the dielectric loss alone contributes approximately 0.8 dB/inch, and total loss (including conductor and roughness) reaches 1.5-2 dB/inch. A 2-inch trace at 10 GHz loses 3-4 dB, which is unacceptable for most applications.
The glass weave structure of FR4 creates local variations in εr. Where the trace crosses a glass bundle, εr is higher (≈ 6-7 for glass) than where it crosses the resin-rich region (εr ≈ 3-3.5). This creates periodic impedance variations along the trace, causing phase ripple and reflection. For designs sensitive to these effects, spread-glass or NE-glass weave styles reduce the variation.
Frequently Asked Questions
Can I use FR4 for 5G mmWave?
Not for the signal path at 28 or 39 GHz. FR4 loss at 28 GHz exceeds 5 dB/inch. However, FR4 can be used for the DC bias, control, and low-frequency portions of the PCB while using low-loss laminate for the RF sections (hybrid stackup).
What about FR408 or IS680?
These are improved versions of FR4 with lower loss tangent (0.008-0.012) and more stable εr. They are suitable to 10-15 GHz at moderate cost. They use the same FR4 manufacturing infrastructure, keeping fabrication costs lower than PTFE-based laminates.
Is the glass weave pattern important?
Yes, especially above 10 GHz. Standard 1080 or 2116 glass styles have coarse weave patterns that create significant local εr variation. Spread glass styles (e.g., 1078 spread) minimize this variation. For the most critical RF lines, orient traces at an angle to the weave to average out the periodic εr modulation.