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What is the dielectric constant of FR4?

FR4 dielectric constant is not one fixed number that applies to every glass-epoxy board. The value used in design depends on the material system, reinforcement and resin construction, frequency, test method and relevant conditions. A defensible value is a documented input for a particular model. Start with the selected laminate supplier鈥檚 data and confirm that the number matches the intended electrical calculation.

In common PCB usage, Dk refers to relative permittivity. It influences electric-field behavior, propagation and transmission-line geometry. Bulk material Dk is different from the effective permittivity of a structure whose fields occupy more than one medium. A surface microstrip, for example, has fields in dielectric and above the board, so its effective value cannot simply be replaced with an arbitrary FR4 catalogue number.

This page owns the material-data and sensitivity question. The PCB impedance calculator and design guide owns trace-geometry planning, while the FR4 PCB page connects material choices to fabrication. Keeping those tasks separate makes it easier to find the appropriate next step.

Read the conditions before copying the number

As a concrete example of conditional data, the lists a typical Dk of 4.2 at 1 GHz using IPC-TM-650 2.5.5.9. That is an identified product, method and frequency example. It is not a universal FR4 constant or an acceptance limit for every board ordered from a fabricator.

The official provide a useful reminder that construction-specific data belong with the proposed stackup. A table may distinguish glass and resin combinations or frequency conditions. Preserve the applicable row and notes rather than copying one familiar value into all dielectric regions.

Material Dk, design Dk and effective permittivity

A material test value describes the result of a particular measurement. A design value may be supplied or extracted for use in a circuit model under defined assumptions. Effective permittivity describes the propagation behavior of a particular field structure. These ideas are related, but treating them as interchangeable can create a systematic error before the geometry solver even runs.

Rogers discusses measurement and circuit-design interpretation in . The practical lesson for FR4 work is to match the input to the model and document the source. It is not a reason to substitute a Rogers laminate value into an unrelated FR4 construction.

For a homogeneous, nonmagnetic ideal TEM medium, propagation speed is c divided by the square root of relative permittivity. Real PCB structures add geometry, conductor and frequency effects. A microstrip with solder mask is not the same field structure as an ideal embedded line. Differential coupling introduces another geometry task. Use the appropriate solver with the correct layer data when dimensions are being released for manufacturing.

Explore sensitivity without pretending to solve a trace

This worksheet compares two entered Dk assumptions using an explicitly limited homogeneous TEM model. It shows how propagation delay, an equal-delay length and a fixed-geometry impedance ratio change. Frequency is recorded as context; the worksheet does not calculate material dispersion or derive Dk from frequency.

For a 100 mm ideal path, Dk 4.0 gives approximately 0.667 ns delay. Dk 4.4 gives approximately 0.700 ns. The difference is about 32.6 ps under this model. The equal-delay length at the larger Dk is about 95.35 mm. These figures illustrate sensitivity; they are not recommended PCB routing lengths.

The fixed-geometry impedance ratio is the square root of Dk A divided by Dk B for the idealized homogeneous structure. The result deliberately has no absolute ohm value. It cannot tell you whether a particular trace is 50 惟, select a microstrip width, or verify a differential pair. Those tasks require the actual trace and dielectric geometry.

Changing any input invalidates the previous brief. The source field is required so a result can be traced to an assumption. Use illustrative mode to learn the relationships, then start a blank project with your own documented values. Downloaded text stays under your control; Contact does not receive the inputs automatically.

Why reinforcement and resin belong in the electrical review

FR4 combines glass reinforcement and resin. The proportion and arrangement of those constituents affect the electrical structure. A trace may not sample the composite in exactly the same way as a bulk test specimen. This is one reason to review the actual construction when timing, phase or controlled impedance is sensitive to material assumptions.

For a multilayer, record Dk assumptions by dielectric region rather than assigning one number blindly to the whole board. Different cores and prepreg constructions may require different documented inputs. The PCB prepreg page explains how glass style, resin information and pressed thickness travel together in a useful stackup proposal.

A discussion about glass weave should remain tied to the circuit鈥檚 sensitivity and the available manufacturing options. Do not prescribe a universal routing angle or guaranteed skew improvement from a generic drawing. If local material nonuniformity matters to the channel, include it in the engineering review and agree an appropriate validation approach.

Dk is not the entire signal-integrity model

Dielectric loss, conductor loss, roughness, geometry, launches, vias and return-path discontinuities can all matter. A laminate with a particular Dk is not automatically suitable for a channel-loss budget. Likewise, a high-Tg designation addresses a different material characteristic and should not be used as shorthand for low electrical loss.

Choose the depth of analysis from the product requirement. A short low-speed control connection may not need the same characterization as a long high-frequency channel. The useful question is whether the uncertainty in the material and geometry could change the design decision. If it can, resolve the uncertainty with the supplier, model and measurement plan before relying on a single computed dimension.

For RF manufacturing choices, the RF PCB page connects material options and fabrication concerns. For a broader comparison across substrate families, use PCB materials. These pages provide context; neither should be treated as a universal substitution list based only on matching Dk.

A practical Dk workflow for a design release

First identify the electrical task: impedance, timing, phase, resonance, capacitance or channel loss. Next select a model appropriate to the physical structure. Obtain the material and construction inputs that model requires, with frequency and document references. Then compare sensitivity to the uncertainties that are large enough to affect the result.

If the result is sensitive to pressed spacing, communicate that requirement in the stackup review. If it is sensitive to Dk, agree the material identity and the interpretation of its data. If conductor roughness or transitions dominate the result, changing only Dk may not address the actual problem. Keep the model scope visible to avoid solving the wrong variable precisely.

Before manufacturing, freeze the approved stackup with the board revision. Include any controlled-impedance targets and the agreed verification request. A coupon measurement establishes information about the supplied construction under its test conditions; it should not be described as a universal measurement of every circuit鈥檚 frequency response.

The PCB manufacturing service can review the fabrication package and material restrictions. A useful enquiry includes the proposed stackup, relevant nets, target behavior and the source of the Dk assumption. You can attach the worksheet brief manually to show why a material or spacing change deserves attention.

Common data mistakes worth catching early

Do not mix dielectric constant with dissipation factor because both appear in the same table. Do not omit units from thickness fields while preserving many decimal places in Dk. Do not use a nominal board thickness where a solver requires the distance from a trace to its reference plane. Each of these mistakes can produce a polished result from the wrong physical inputs.

Avoid overinterpreting calculator precision. A result displayed to several decimal places reflects arithmetic formatting, not the certainty of the laminate data or manufactured geometry. Report the assumptions and relevant tolerance basis with the result. Where input uncertainty is substantial, a sensitivity comparison is often more informative than one apparently exact value.

Finally, retain the model and material revisions when sharing a design. A teammate should be able to reproduce the calculation and understand why the selected value was used. That record is useful during engineering changes, supplier discussions and the next board revision, even when no advanced simulation is required.

FR4 dielectric constant questions