Hi,
Firstly, thank-you to the Toradex team for this fantastic layout guide, it has been very useful.
This doc has great details on the recommended stack-up for a 6 layer board, and track width and spacing for impedance requirements.
However, for these to work correctly, a certain dielectric constant value is required (based on the material between layers 1-2 / 5-6). Unfortunately I cannot find any reference to this in the documentation, and the value of 4.8 (pulled from the layer stackup in the Ixora reference design) does not appear to be correct when calculating these values.
Reverse calculating the required dielectric constant between layers 1-2 puts it somewhere in the range of 3.7 - 4. Does Toradex have recommended value you could share (or the value the Ixora Carrier board is manufactured with)?
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Hello, and thank you for your kind words about our layout guide! We’re glad to hear it has been helpful for your PCB design process.
You’re absolutely correct that achieving the target impedance (for example, 50Ω single-ended or 90Ω/100Ω differential) is heavily dependent on the correct dielectric constant (Dk or εr) between layers, as well as other stack-up parameters. However, the exact dielectric constant can vary significantly depending on the materials and processes used by different PCB manufacturers. The values (such as 4.8 or 4.2) provided in stack-up examples or reference designs (like Ixora) should be treated as guidelines rather than strict values, since even FR4 material can have a wide range of dielectric constant (typically ~4.0 to 4.8).
What is recommended:
- The impedance calculations in the layout guide are based on typical manufacturer-provided data, but for your production board, you should always consult your specific PCB manufacturer. They can provide the real εr value for the exact material and thicknesses they will use.
- Manufacturers may use slightly different materials even for “FR4,” and they’ll have the most accurate value for their supplied pre-preg and core laminates in your chosen stack-up.
- The actual stack-up and the corresponding dielectric constants are usually determined in collaboration with your manufacturer (source).
Practical steps:
- Ask your chosen PCB supplier for the detailed stack-up and εr values for each dielectric layer in their process. Supply them with your required impedance values (e.g., 50Ω SE, 90Ω diff) and trace geometries for them to confirm or tune your design.
- PCB manufacturers have specialized tools (like Polar Instruments) for calculating and adjusting impedance based on their exact materials and process tolerances.
- As noted in the Toradex documentation, basic impedance calculators in layout tools are often not accurate enough; relying on the manufacturer’s expertise is the best approach.
Summary:
Toradex does not specify a single “official” dielectric constant value for the Ixora board or similar designs because this is dependent on manufacturing specifics. The critical step is for you to obtain the stack-up and supported εr directly from your PCB fab for reliable impedance control. Take the stack-up and recommended values as a starting point, but always validate with your supplier.
For further references and detailed recommendations, please visit the following documentation page:
https://developer.toradex.com/hardware/apalis-som-family/carrier-boards/ixora-carrier-board
Best regards,
Toradex AI Assistant.
