Why can't FR-4 be used above 3 GHz?

FR-4 has a loss tangent of 0.020 to 0.025 at 1 GHz, which increases with frequency. At 5 GHz, the dielectric loss of a 50-ohm microstrip line on 0.8 mm FR-4 is approximately 0.35 dB per centimeter. A 10 cm filter or antenna feed network would lose 3.5 dB in the substrate alone, which is unacceptable for most RF applications. Additionally, the dielectric constant of FR-4 varies by plus or minus 10 percent between manufacturing lots and changes with frequency (from 4.7 at 100 MHz to 4.2 at 5 GHz), making accurate impedance control impossible for narrowband circuits. The moisture absorption of FR-4 (0.10 to 0.15 percent) further shifts the dielectric constant in humid environments. Rogers and PTFE laminates solve all three problems: low and stable loss tangent (0.002 to 0.004), tight Er tolerance (plus or minus 1.5 percent), and low moisture absorption (0.02 percent).

How does substrate choice affect microstrip impedance?

Microstrip impedance depends on the ratio of trace width to substrate thickness (w/h) and the dielectric constant (Er). For a 50-ohm line: on FR-4 (Er = 4.4, h = 0.8 mm), w = 1.53 mm. On RO4003C (Er = 3.55, h = 0.508 mm), w = 1.11 mm. On RT/duroid 5880 (Er = 2.2, h = 0.787 mm), w = 2.41 mm. Lower Er requires wider traces, which is advantageous for manufacturing tolerance (a 0.05 mm etching error on a 2.4 mm trace is 2 percent impedance change, versus 3.3 percent on a 1.5 mm trace). However, wider traces mean larger circuits and more board area. The dielectric constant tolerance of the substrate directly affects impedance accuracy: a plus or minus 5 percent Er variation causes approximately plus or minus 2.5 percent impedance variation, which translates to a return loss floor of about 30 dB.

When should I use PTFE versus hydrocarbon ceramic substrates?

PTFE substrates (like Rogers RT/duroid 5880 and 5870) offer the lowest loss tangent (0.0009 to 0.002) and lowest dielectric constant (2.2 to 2.33) available in planar laminates. They are the best choice for loss-critical applications above 10 GHz: satellite receiver front ends, radiometer feeds, and millimeter-wave antenna arrays. The drawback: PTFE is soft, dimensionally unstable during PCB processing, and difficult to plate through-holes in. It costs 5 to 10 times more than FR-4 and requires specialized fabrication. Hydrocarbon ceramic substrates like Rogers RO4003C (Er = 3.55, tan delta = 0.0027) are processed using standard FR-4 fabrication equipment, accept plated through-holes reliably, and cost 2 to 3 times FR-4. They are the default choice for 1 to 10 GHz applications where the loss of FR-4 is too high but the cost and processing complexity of PTFE are not justified.

PCB Design

Substrate

Two engineers design the same 5 GHz bandpass filter. One uses FR-4; the other uses Rogers RO4003C. The FR-4 filter has 2.5 dB insertion loss (most of it in the substrate), center frequency shifted 3% from the target (because the dielectric constant drifted between the datasheet and the actual panel), and 18 dB return loss (because impedance control is poor with ±10% Er tolerance). The RO4003C filter has 0.8 dB insertion loss, center frequency within 0.5% of target, and 25 dB return loss. Same layout, same copper, same fab house. The only difference is the substrate material underneath the copper. In RF design, the substrate is not a commodity; it is a precision component that determines whether the circuit works or does not.

Category: PCB Design

Key Specs: ε_r, tanδ, thickness tolerance

Choice Drives: Loss, impedance, frequency accuracy

RF Substrate Material Comparison

Material	ε_r	tanδ	ε_r Tol.	Cost (vs FR-4)	Freq. Range
FR-4	4.2 to 4.7	0.020 to 0.025	±10%	1×	DC to 2 GHz
Rogers RO4003C	3.55	0.0027	±1.5%	2 to 3×	1 to 10 GHz
Rogers RO4350B	3.66	0.0037	±1.5%	2 to 3×	1 to 10 GHz
RT/duroid 5880	2.20	0.0009	±2%	5 to 10×	5 to 77 GHz
RT/duroid 6010	10.2	0.0023	±2.5%	5 to 10×	1 to 40 GHz
Alumina (ceramic)	9.8	0.0001	±1%	10 to 20×	1 to 100 GHz

Dielectric loss per unit length:
α_d = (27.3 × ε_r × tanδ) / (λ₀ × √ε_eff) dB/m
RO4003C at 5 GHz: 0.04 dB/cm. FR-4: 0.35 dB/cm

Impedance sensitivity to ε_r:
ΔZ/Z ≈ −Δε_r/(2ε_r)
±10% ε_r (FR-4): ±5% Z. ±1.5% (RO4003C): ±0.75% Z

50 Ω trace width (h = 0.8 mm):
FR-4: w = 1.53 mm. RO4003C: w = 1.69 mm. 5880: w = 2.41 mm

Common Questions

Frequently Asked Questions

Why not FR-4 above 3 GHz?

tanδ = 0.025 at 5 GHz: 0.35 dB/cm loss. 10 cm line = 3.5 dB gone. ε_r varies ±10% between lots and with frequency (4.7 at 100 MHz to 4.2 at 5 GHz). Moisture absorption shifts ε_r further. Rogers laminates: tanδ = 0.003, ε_r ±1.5%.

Impedance impact?

50 Ω microstrip width depends on w/h and ε_r. Lower ε_r = wider trace = better manufacturing tolerance. ±5% ε_r = ±2.5% impedance = 30 dB return loss floor. Tight ε_r tolerance is essential for narrowband circuits.

PTFE vs. hydrocarbon ceramic?

PTFE (5880): lowest loss (0.0009 tanδ), but soft, dimensionally unstable, hard to plate vias, 5 to 10× cost. RO4003C: standard FR-4 processing, reliable PTH, 2 to 3× cost. RO4003C is the default for 1 to 10 GHz. PTFE for >10 GHz or loss-critical.

Related Terms

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Material Selection

Substrate Comparison Tool

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