Complete engineer’s reference on PCB dielectric constant (Dk) and dissipation factor (Df): definitions, formulas, frequency effects, material selection, and design tips.

Every PCB engineer has had this moment: you build a board against a simulation that predicted clean 25 Gbps eye diagrams, and what comes back on the scope is closed by 40% with enough jitter to close a third of the timing window. You check the layout, the components, the fab. Eventually you dig into the material datasheet and realize the laminate’s PCB dielectric constant and loss tangent at 12.5 GHz are nowhere near what you used in your impedance calculations. That’s the scenario this reference is written to prevent.

This guide covers Dk and Df from a working engineer’s angle — what they actually mean physically, how to use them in transmission-line math, how they shift with frequency and temperature, how to select materials against a loss budget, and what to actually ask your material supplier before the fab order goes out.

What Is PCB Dielectric Constant (Dk)?

The PCB dielectric constant — also called relative permittivity, denoted Dk or εr — is a dimensionless number describing how much a material stores electric energy relative to a vacuum. Rogers Corporation’s practical definition: it’s the ratio of a capacitor’s capacitance with a dielectric between the plates to that same capacitor’s capacitance with only air (or vacuum) between them. Air has a relative permittivity of ~1.0006. Everything else has a higher Dk, with common PCB materials running from 2.1 to 10.2.

That one number does more than any other material parameter to shape how your board behaves electrically. Dk controls:

• Characteristic impedance of every trace on the board.
• Signal propagation velocity — higher Dk means slower signals.
• Wavelength inside the dielectric — higher Dk means shorter wavelength, which shrinks resonant antenna elements.
• Trace geometry requirements for a given impedance target.

The Propagation Velocity Relationship

Signal velocity through a PCB dielectric is related to Dk by:

v = c / √Dk

Where c is the speed of light in vacuum (~3 × 10⁸ m/s). On FR-4 with Dk = 4.3, signals propagate at roughly 145 mm/ns. On PTFE with Dk = 2.1, the same signal moves at about 207 mm/ns — a 43% speed improvement. For multi-gigabit signaling, that velocity difference directly affects propagation delay, skew between traces of different lengths, and timing budget across a high-speed digital channel.

Dk and Impedance Control

Effective dielectric constant for a microstrip trace follows approximately:

εeff = [(εr + 1)/2] + [(εr − 1)/2] × [1 + (12h/W)]^(-½)

Where h is dielectric thickness and W is trace width. This matters because impedance depends on εeff, trace geometry, and copper thickness. A Dk shift of 0.05 on a 50 Ω microstrip moves impedance roughly 0.3 Ω. That’s within typical build tolerance. A Dk shift of 0.3 (common between datasheet-typical and lot-measured values on inexpensive laminates) moves impedance enough to bust tight spec-limits on differential pairs.

What Is Dissipation Factor (Df)?

The dissipation factor, loss tangent, or tan δ — they’re all the same thing — describes how much of the signal energy passing through a dielectric is converted to heat. Mathematically, Df is the ratio of the energy dissipated per AC cycle to the energy stored per cycle. Physically, it’s a measure of how “lossy” the material is.

The crucial point: Df is what kills RF signals. Dk determines where the energy goes; Df determines how much of it arrives.

Df and Dielectric Loss

A practical approximation for dielectric loss on a transmission line:

Dielectric Loss (dB/inch) ≈ 2.3 × f × √Dk × Df

Where f is frequency in GHz. Read this carefully: loss scales linearly with frequency. A Df of 0.020 (standard FR-4) at 1 GHz is annoying but workable. The same Df at 25 GHz is catastrophic — your signal is down 3-6 dB over a few inches of trace before it reaches the connector.

This is why high-frequency designs migrate to low-loss materials. At low frequencies, Df is a nice-to-have. At mmWave and multi-gigabit rates, it’s the difference between a product that works and one that doesn’t.

PCB Dielectric Constant and Df Across Common Materials

Here’s a consolidated table of typical Dk and Df values at 10 GHz for the materials you’re most likely to encounter, organized by material class:

Material Class	Example	Dk @ 10 GHz	Df @ 10 GHz
Standard FR-4	Various	4.3 – 4.8	0.020 – 0.025
Mid-loss FR-4	Isola 370HR	4.04	0.021
Low-loss FR-4	Isola FR408HR	3.68	0.0092
Very low-loss	Panasonic Megtron 6	3.61	0.004
Ultra-low-loss	Panasonic Megtron 7	3.40	0.002
Hydrocarbon ceramic	Rogers RO4350B	3.48	0.0037
Hydrocarbon ceramic	Rogers RO4003C	3.38	0.0027
Ceramic PTFE	Rogers RO3003	3.00	0.0010
Glass-reinforced PTFE	Taconic TLY-5	2.20	0.0009
Random-fiber PTFE	Rogers RT/Duroid 5880	2.20	0.0009
Ceramic-filled PTFE	Taconic RF-35	3.50	0.0025
Ceramic-filled PTFE	Taconic TSM-30	3.00	0.0013
High-Dk ceramic	Rogers RO3010	10.2	0.0022

A few practical observations from this table:

Df spans more than an order of magnitude across common materials. Standard FR-4 sits at 0.020+; premium PTFE hits 0.0009. That’s a 20× difference that translates directly to insertion loss at high frequency.

Low Dk doesn’t guarantee low Df. Rogers RT/Duroid 5880 and Taconic TLY-5 both have Dk 2.2 and Df ~0.0009 — but some low-Dk hydrocarbon laminates have much higher Df. Always check both numbers.

High Dk isn’t always bad. Ceramic-filled laminates like RO3010 reach Dk 10.2 with Df of only 0.0022 — excellent for impedance-matched power amplifier circuits and for shrinking antenna element size at lower frequencies.

For the full Taconic product catalog — TLY, TLX, TLC, TLE, RF, TSM, TLF, and TRF series — our reference guide to Taconic PCB material selection walks through every grade by Dk and Df.

How PCB Dielectric Constant Varies with Frequency

One of the biggest mistakes first-time RF engineers make is assuming Dk is a single number. It isn’t. Dk is frequency-dependent and typically decreases slightly as frequency rises:

Frequency	Typical FR-4 Dk	Typical RO4350B Dk
100 MHz	4.50 – 4.80	3.66
1 GHz	4.30 – 4.50	3.54
10 GHz	4.00 – 4.30	3.48
25 GHz	3.80 – 4.10	~3.45

For high-frequency designs, what matters isn’t just the absolute Dk — it’s how stable Dk is across your operating frequency range. Premium RF laminates are engineered for Dk stability, which is why they command the price premium. FR-4 Dk drift across frequency is one of the main reasons FR-4 signal integrity falls apart above 3-5 GHz.

How Df Varies with Frequency

Df generally increases slightly with frequency — the opposite direction from Dk. This is because at higher frequencies, the dielectric’s molecular polarization can’t keep up with the oscillating field, and more energy is dissipated per cycle. For critical applications, always match the Df measurement frequency to your design’s operating frequency. A Df spec measured at 1 MHz tells you almost nothing about how the material behaves at 10 GHz.

Dk and Df in High-Speed Digital Design

High-speed digital is where the PCB dielectric constant and Df conversation gets interesting for engineers who don’t think of themselves as “RF designers.” Modern SerDes rates — 25 Gbps NRZ, 56 Gbps PAM4, 112 Gbps PAM4 — push Nyquist frequencies into the 5-28 GHz range. At those frequencies, the PCB substrate is no longer a passive carrier; it’s part of the transmission channel.

Two practical consequences:

Loss budget drives material selection. A 56 Gbps PAM4 channel typically has an insertion-loss budget of 28-35 dB at Nyquist. If your trace is long — say, a 12-inch backplane trace — you need a material with Df ≤ 0.003 at Nyquist to fit within that budget. FR-4 is simply not an option for long high-speed channels.

Glass-weave effect matters. Woven glass fabrics have periodic variation in local Dk — resin-rich spots between fiber bundles vs. glass-rich spots at crossovers. At high frequency, a differential pair sitting on alternating resin and glass regions accumulates timing skew, which closes eye diagrams. Spread glass, mechanically spread glass (MSG), or non-reinforced laminates solve this, which is why premium high-speed materials like Megtron 7 and TSM-DS3 pay attention to glass construction.

Loss Reduction from Lower Dk

Signal Integrity Journal published an interesting analysis showing that for the same impedance target, a 20% reduction in Dk (from 4.0 to 3.2) lets the designer either:

• Reduce dielectric thickness by 18.5%, or
• Widen the trace (for the same impedance), reducing conductor loss by up to 13.2%, or
• Split the benefit between the two.

That’s before any Df improvement. Lower Dk alone gives you design margin for thinner stackups or lower-loss traces. Combined with lower Df, the improvement compounds.

Factors That Affect PCB Dk and Df in Practice

The datasheet is the starting point, not the end of the story. Several real-world factors shift the actual Dk and Df you see on a built board:

Resin content. PCB laminates are composites of resin and glass. Higher resin content means lower effective Dk (resin Dk is typically 3.0-3.5; glass Dk is 6.0-6.4). Thicker cores or prepregs with higher resin content measure lower Dk than thinner, glass-rich laminates of the same grade. Isola datasheets make this explicit by publishing Dk for different glass styles and resin contents.

Moisture absorption. Water has a Dk of ~78. Even small amounts of absorbed moisture shift a laminate’s Dk noticeably and increase Df. PTFE absorbs 0.02-0.10%; FR-4 absorbs 0.1-0.25%; polyimide can hit 0.5%. For outdoor or humid environments, specify low-moisture materials and store panels in controlled humidity.

Temperature. Most PCB materials have a temperature coefficient of dielectric constant (TCDk) in the 40-200 ppm/°C range. For narrow-band filters and phase-critical circuits, TCDk must be factored into the design margin across temperature extremes.

Copper roughness. This is a subtle one. Surface roughness interacts with skin-effect currents at high frequency, effectively increasing the path length and raising measured insertion loss. The material datasheet doesn’t include this, but at frequencies above 5-10 GHz, copper roughness dominates total loss. Specify low-profile (Rms < 1 µm) or very-low-profile (Rms < 0.4 µm) reverse-treated foils for anything above 6 GHz.

Anisotropy. PCB laminates aren’t uniform in all directions. The Dk measured through the thickness (z-axis) can differ from the in-plane (x-y) Dk by 0.1-0.2 in some materials. If your design involves both stripline (z-axis propagation components) and microstrip (in-plane), this matters.

Measurement Methods and IPC Standards

When you see a Dk or Df number on a datasheet, you should know how it was measured. Different methods give slightly different numbers, and direct comparison across vendors requires matching methods. The main industry-standard methods fall under IPC-TM-650:

IPC Method	Technique	Typical Use
IPC-TM-650 2.5.5.5	Stripline resonator at 10 GHz	Most widely reported RF Dk/Df
IPC-TM-650 2.5.5.5.1	Modified stripline resonator	Taconic-style measurements
IPC-TM-650 2.5.5.6	Full-sheet resonator	TCDk measurements
IPC-TM-650 2.5.5.13	Split post dielectric resonator	Higher-accuracy Dk at specific frequencies
ASTM D150	Parallel plate	Low-frequency Dk/Df
IEC 61189-2-721	Ring resonator	Wideband characterization

Engineering tip: When comparing two materials, always compare values measured by the same method at the same frequency. A material listed as Dk = 3.50 measured by stripline resonator at 10 GHz is not directly comparable to a material listed as Dk = 3.50 measured by parallel plate at 1 MHz. Most modern high-frequency datasheets specify the method and frequency explicitly; if yours doesn’t, ask.

Dk and Df Selection Guide by Application

Here’s a practical decision framework for selecting material Dk and Df based on application:

Application	Typical Frequency	Recommended Dk	Recommended Df
Consumer electronics, low-speed logic	DC – 100 MHz	Any (FR-4 fine)	< 0.025
Wi-Fi, Bluetooth, cellular	2.4 – 5.8 GHz	3.5 – 4.5	< 0.015
Mid-speed digital (10-25 Gbps)	5 – 12.5 GHz	3.0 – 4.0	< 0.005
High-speed digital (56-112 Gbps)	14 – 28 GHz	3.0 – 3.5	< 0.003
Sub-6 GHz 5G, cellular infrastructure	2 – 6 GHz	3.0 – 3.66	< 0.003
mmWave 5G, automotive radar	24 – 80 GHz	2.9 – 3.5	< 0.002
Satellite radio, GPS antennas	1 – 3 GHz	3.5 – 4.5	< 0.0035
Phased-array radar	8 – 40 GHz	2.9 – 3.5	< 0.0015
MMIC packaging, mmWave modules	40 – 100 GHz	2.1 – 2.5	< 0.001

Two important caveats:

Cost scales inversely with Df. The lower the required Df, the higher the raw-material cost. Materials at Df ≤ 0.0015 run 5-10× the cost of standard FR-4.

Tight tolerance matters more than absolute value. A material with Dk 3.50 ± 0.05 is much more useful than a material with Dk 3.38 ± 0.20 for controlled-impedance design, even if the second number is nominally lower.

Design Best Practices Using Dk and Df

A few practical rules from the bench:

Use the design Dk value, not the datasheet-typical. Reputable fabs can measure your actual lot’s Dk using Test Method 2.5.5.5 on coupons from your production panels. For critical impedance work, use the lot-measured value in your field solver, not the nominal datasheet number.

Stack-up planning matters more than material choice. A well-designed stackup on a mid-grade material often outperforms a poorly-designed stackup on a premium material. Work with your fabricator on stack-up definition before finalizing the design.

Budget for Df drift at high frequency. If your material datasheet reports Df at 10 GHz but you’re running 28 GHz channels, Df at your actual operating frequency will be higher. Ask for measured values across your frequency range or use conservative estimates.

Factor in copper roughness. Modern field solvers (Ansys HFSS, Keysight Momentum, Polar Si9000) include copper-roughness models (Hammerstad, Groisse, Huray). Use them above 5 GHz. Without them, simulations will predict lower insertion loss than you’ll actually measure.

Use simulation to validate material choices. Tools like Polar’s Si9000, HyperLynx, or HFSS can take datasheet Dk and Df values and predict insertion loss across your design. Run the numbers before you commit to a stack-up.

Useful Resources and Data References

Worth bookmarking for material selection and Dk/Df analysis:

• San Francisco Circuits PCB Material Reference Guide — comprehensive Dk, Df, Tg, and CTE comparison across major laminate manufacturers (Rogers, Isola, Panasonic, Taconic, Arlon, Nelco).
• Sierra Circuits Material Selector Tool — interactive online tool comparing 50+ rigid and 10+ flex materials by Dk, Df, thickness, Tg, and IPC compliance.
• Microwaves101 Laminates Comparison Chart (PDF) — downloadable cross-reference of every major RF laminate sorted by Dk, with Df, CTE, and construction details.
• Rogers Corporation Technical Documentation — processing guides and Dk frequency charts for every RT/Duroid and RO laminate product.
• Isola Materials Selector — lists Dk and Df values at 100 MHz, 1 GHz, 2 GHz, and 10 GHz for the same laminate grades with different glass styles and resin contents.
• Panasonic Megtron Datasheets — Megtron 4, 6, 7, and 8 are industry-standard benchmarks for high-speed digital; datasheets include frequency-dependent Dk/Df curves up to 40 GHz.
• IPC-TM-650 Test Methods — free from the IPC website; the measurement standards referenced throughout all PCB laminate datasheets.
• Signal Integrity Journal — ongoing technical articles on material characterization for high-speed design, including frequency-dependent Dk/Df and measurement variance.

FAQs

What is a good PCB dielectric constant value?

There’s no universally “good” Dk — the right value depends on your design. For controlled-impedance traces, what matters most is Dk stability (tight tolerance, stable over frequency and temperature), not the absolute value. For RF antennas, Dk determines element dimensions. For high-speed digital, lower Dk means faster signal propagation and wider trace options for the same impedance. General targets: Dk 3.0-3.5 for most RF and high-speed digital; Dk 2.1-2.5 for mmWave and ultra-low-loss applications; Dk 4.3-4.8 for low-cost digital under 1 GHz.

What is the difference between dielectric constant and dissipation factor?

Dk (dielectric constant) measures how much electric energy a material stores; Df (dissipation factor, loss tangent, tan δ) measures how much energy the material dissipates as heat. Dk controls signal velocity and impedance; Df controls insertion loss. Both matter for high-frequency design, but they drive different design decisions. Dk affects stack-up geometry and timing; Df affects loss budgets and reach.

How does frequency affect PCB dielectric constant and Df?

For most PCB materials, Dk decreases slightly with increasing frequency (a few percent from 1 GHz to 10 GHz). Df, in contrast, generally increases with frequency because molecular polarization can’t keep up with the faster-oscillating field. Premium high-frequency materials are engineered for stable Dk and low Df across wide frequency ranges — that stability is what justifies their premium price vs. standard FR-4.

What Dk and Df do I need for 5G or mmWave design?

For sub-6 GHz 5G infrastructure, target Dk 3.0-3.66 with Df ≤ 0.003 (materials like Rogers RO4350B or RO3003 are common picks). For mmWave 5G at 24-40 GHz and automotive radar at 77 GHz, target Dk 2.9-3.5 with Df ≤ 0.002 (Rogers RT/Duroid 6002, Taconic TSM-DS3, RO3003 are typical choices). For MMIC packaging and 60+ GHz modules, non-reinforced PTFE laminates with Dk around 2.1 and Df ≤ 0.001 (TacLamplus, CuFlon) become necessary.

How do I measure or verify the Dk of a fabricated PCB?

The most accurate methods use resonator test coupons on the production panel — typically a microstrip ring resonator or stripline resonator measured on a vector network analyzer. The resonant frequency of the coupon directly yields the effective Dk. Many fabs can provide Dk measurement on production coupons per IPC-TM-650 2.5.5.5. For quick in-lab verification, TDR measurement of trace impedance followed by back-solving for Dk (given known trace geometry) provides an engineering-grade estimate. For the highest accuracy, split-post dielectric resonator (SPDR) measurement on material samples is the reference method.

Bottom Line

Dk and Df are the two parameters that determine whether your PCB can deliver the signal integrity your design needs. Dk controls velocity and impedance; Df controls loss. Both are frequency-dependent, both drift with temperature and moisture, and both are measured by specific methods you need to match when comparing across vendors. For low-speed designs under 1 GHz, FR-4 is usually fine. For RF above 3 GHz or multi-gigabit digital above 10 Gbps, specify materials with published Dk and Df at your actual operating frequency, measured by a repeatable IPC method, with tight tolerances. Work with your fab on stack-up definition, include copper roughness in your simulations, and use lot-specific measurements for tight impedance budgets. Get these fundamentals right and your boards will behave the way the simulation predicted.

Suggested Meta Description

Complete engineer’s reference on PCB dielectric constant (Dk) and dissipation factor (Df): definitions, formulas, frequency effects, material selection, and design tips.

Character count: 164 (slightly over the Yoast 156 sweet spot, but renders on most SERPs).

Tighter alternative under 156:

Engineer’s reference on PCB dielectric constant (Dk) and dissipation factor (Df): what they mean, how to use them, material comparison, and design tips.

Character count: 152.