Impedance Calculation Parameters: FR4 Substrate, Copper Thickness, and Prepreg

Impedance control is one of the foundations of reliable high-speed and high-frequency PCB design. Whether the target is a single-ended 50 Ω trace or a differential pair inside a dense high-speed channel, every stackup calculation depends on a small set of physical parameters: the thickness of the substrate, the weight of the copper, the width of the trace, and the presence of solder mask. Get these values right, and the calculated impedance matches the fabricated board. Get them wrong, and the best layout will still return a board that misses its target.

This article examines the material-side parameters that feed an impedance calculation. It explains how FR4 substrate thickness is specified, how the copper foil notation on a core is read, and how the common FR4 prepreg styles differ in thickness and dielectric constant. The goal is to give designers and engineers a practical reference they can use when building or checking a stackup.

Highlights:

• The copper notation on a core (H/H, 1/1, 2/2, H/1) describes the copper weight on each face, and substrate thickness can be quoted either with or without that copper included.
• Prepreg style (106, 1080, 3313, 2116, 7628) sets both the dielectric thickness and the dielectric constant, so the two must always be selected together.
• Dielectric constant and thickness are material-specific. Values for Tg ≤ 170 FR4, N4000-13, and IT180A/S1000-2B differ and cannot be treated as interchangeable.

FR4 Substrate Thickness in Impedance Calculations

In an impedance calculation, the substrate thickness of an FR4 core can be defined in one of two ways. The first is a thickness that does not include the copper foil. The second is a thickness that does include the copper foil. Both conventions appear in real stackup documentation, and confusing the two shifts the calculated dielectric height, and therefore the impedance.

When a core is specified using copper weight notation, four common combinations appear: H/H, 1/1, 2/2, and H/1. In this notation, H/H, 1/1, and 2/2 indicate that the two faces of the core carry copper foils of 0.5 oz, 1 oz, and 2 oz respectively; the letter H stands for the half-ounce foil. The mixed designation H/1 indicates that the two faces carry 0.5 oz and 1 oz copper. Reading the notation correctly is essential, because the copper weight on each face affects the finished dielectric spacing after lamination.

The distinction between copper-inclusive and copper-exclusive thickness matters most when a stackup is translated from a material datasheet into an impedance model. A datasheet thickness that already accounts for copper produces a different dielectric height than a bare dielectric thickness. Designers should confirm which convention a given value follows before entering it into a field solver.

Impedance Calculation Parameters

An impedance calculation draws on the full set of parameters that describe the trace and its surroundings. The substrate and prepreg define the dielectric height and the dielectric constant. The copper foil defines the conductor thickness. The trace width and the solder mask coating complete the picture. Because each parameter is coupled to the others, a change in any one of them propagates through the result. This is why stackup data, material data, and layout geometry must be kept consistent throughout the design.

Common FR4 Prepreg Parameters

Prepreg is the partially cured glass fabric that bonds the core and the copper foil together during lamination. Its style determines both how thick the dielectric layer will be and what dielectric constant the impedance model should use. The table below lists the theoretical and actual thickness and the dielectric constant for the five common prepreg styles (106, 1080, 3313, 2116, and 7628) across three material groups.

Reading the table requires attention to the material group. The Tg ≤ 170 group represents a general-purpose FR4 system, with dielectric constants that rise from 3.6 for the thinnest 106 style to 4.2 for the thickest 7628 style. The N4000-13 group holds a constant dielectric constant of 3.7 across all five styles, which simplifies modeling. The IT180A and S1000-2B group sits at the higher end, ranging from 3.9 to 4.5. Thickness also grows with style, so selecting a thicker prepreg to build up dielectric height simultaneously changes the dielectric constant in the first and third groups.

These relationships explain why prepreg selection cannot be treated as a purely mechanical step. The layer that provides the required spacing also determines the electrical environment of the trace above or below it. A stackup should therefore be built with the specific prepreg styles and material group in mind, and the impedance model should use the thickness and dielectric constant taken from that same group. Matching the two keeps the calculation aligned with what the fabricator will actually laminate.

Dielectric Layer Thickness and Dielectric Constant (Shengyi and Equivalent Materials)

The dielectric layer in a PCB stackup is built from cores and prepregs. Its thickness and dielectric constant are not abstract material properties. They set the impedance of every trace, the propagation delay of every net, and the coupling between adjacent copper features. For impedance design, the relevant value is not the nominal catalog thickness of a prepreg sheet. It is the dielectric thickness that remains after lamination, when resin has flowed around the copper features and the copper foil has been pressed into the prepreg. The core thickness and dielectric constant data below provide the starting point for that calculation. The prepreg thickness tables provide the reference values used to estimate resin flow under common copper conditions.

Core Thickness and Dielectric Constant Values

The first table lists core thickness in both millimeters and mils, together with dielectric constant values for two material rows. One row represents a laminate with Tg ≤ 170. The other row represents IT180AS1000-2, identified here as the 180A (IT) material. The table is used when selecting a core for a stackup and when entering the core dielectric constant into an impedance field solver. The millimeter values define the mechanical thickness of the core. The mil values provide the same thickness in the unit commonly used in stackup drawings and impedance models. The dielectric constant values apply to the corresponding core thickness and material row.

The two material rows do not have the same dielectric constant at a given core thickness. The IT180AS1000-2 row is higher than the Tg ≤ 170 row at each listed thickness. This difference must be carried into the impedance model. Do not mix dielectric constant values from one row with thickness values from another material set unless the laminate supplier has confirmed that the combination is valid for the material actually used. The table also shows that dielectric constant is not constant across all core thicknesses within one material row. For the Tg ≤ 170 row, the values move from 3.6 at 0.051 mm to 4.2 at 0.18 mm, then return to 3.95 at 0.21 mm and 0.25 mm, and rise again at 0.36 mm and above. The IT180AS1000-2 row follows a similar pattern at a higher level. For accurate impedance design, use the value that corresponds to the selected core thickness, not a single average value for the entire board.

When multiple prepreg sheets are combined, the dielectric constant is taken as the arithmetic value. This rule applies to the combined dielectric layer, not to each individual sheet in isolation. The arithmetic value is a practical stackup input for the combined dielectric. It does not replace the material data for each prepreg type, but it provides a consistent way to represent a dielectric layer built from more than one prepreg sheet.

Board thickness accuracy also depends on the incoming material. Use the measured thickness of the incoming laminate when calculating stackup thickness. When calculating the interlayer dielectric thickness for impedance design, use the actual incoming thickness and calculate according to the copper distribution ratio. This means the stackup drawing should not rely only on nominal core thickness. The actual laminate thickness, the copper distribution on each layer, and the prepreg flow behavior all contribute to the final dielectric spacing. The core table provides the material constant and the nominal thickness reference. The prepreg tables and the thickness formulas below provide the adjustment for copper distribution and resin flow.

High-Tg FR4 Prepreg Thickness Under Different Copper Conditions

The following table gives thickness values in mil for common high-Tg FR4 prepregs under different copper conditions. The prepreg types listed are 106, 1080, 3313, 2116, and 7628. Their nominal prepreg thicknesses are 2.34 mil, 2.88 mil, 4.1 mil, 4.82 mil, and 8.04 mil, respectively. The table separates the 0.5 oz copper case from the 1 oz copper case. Within each case, five interface conditions are listed: Copper/Gnd, Gnd/Gnd, Copper/Signal, GND/Signal, and Signal/Signal. The routing ratio on power and ground layers is taken as 75%, and the routing ratio on signal layers is taken as 25%. These values are for reference only. In actual design, calculate according to the line distribution ratio.

0.5 oz Copper

The row labeled resin-flow fill thickness gives the corresponding fill thickness used in the calculation. For 0.5 oz copper, the fill thickness rises from 0.16 for Copper/Gnd to 0.96 for Signal/Signal. The effective dielectric thickness for each prepreg type falls as the interface moves toward more signal copper. For the 106 prepreg, the nominal thickness is 2.34 mil, while the table value is 2.18 mil for Copper/Gnd and 1.38 mil for Signal/Signal. For the 7628 prepreg, the nominal thickness is 8.04 mil, while the table value is 7.88 mil for Copper/Gnd and 7.08 mil for Signal/Signal. The same prepreg style therefore does not have a single dielectric thickness. Its effective thickness depends on the copper interface and on the amount of resin that flows into the copper-free areas.

The Copper/Gnd and Gnd/Gnd columns represent interfaces with a large ground area. The Copper/Signal, GND/Signal, and Signal/Signal columns represent interfaces with more signal routing. Because the assumed routing ratio is 75% on power and ground layers and 25% on signal layers, the table is a reference set rather than a universal answer. A layer with dense signal routing has a different copper distribution from a layer with a large ground pour. That difference changes the resin flow and therefore changes the dielectric thickness. When the actual line distribution ratio is available, use it instead of the 75% and 25% assumptions.

1 oz Copper

For 1 oz copper, the resin-flow fill thickness row begins at 0.31 for Copper/Gnd and reaches 1.86 for Signal/Signal. The effective dielectric thickness is lower than in the 0.5 oz case for the same prepreg type and the same interface. For the 106 prepreg, the value is 2.03 mil for Copper/Gnd and 0.48 mil for Signal/Signal. For the 7628 prepreg, the value is 7.73 mil for Copper/Gnd and 6.18 mil for Signal/Signal. The trend is consistent across the table: thicker copper and more signal copper reduce the remaining dielectric thickness after lamination. The prepreg nominal thickness remains the same, but the dielectric spacing between copper surfaces does not.

This is why a stackup cannot be built by adding nominal prepreg thicknesses alone. The 106 prepreg is listed as 2.34 mil, but its effective thickness in a 1 oz Signal/Signal interface is 0.48 mil. If the nominal value is used in an impedance model, the model will overestimate the dielectric thickness and produce an incorrect impedance result. The same issue applies to the larger prepreg types, although the absolute difference is greater for thicker prepregs. The table provides a controlled reference for the common case. The formulas in the next section provide the general calculation method when the copper thickness and residual copper ratio are known.

Calculating Effective Dielectric Thickness After Resin Flow

Impedance calculation involves parameters that describe the actual geometry after lamination. The nominal prepreg thickness is only the starting point. During lamination, resin flows into the spaces around the copper features. The copper foil or copper pattern occupies part of the space that would otherwise be filled by dielectric. The remaining dielectric thickness depends on the copper thickness, the residual copper ratio, and whether the prepreg is in contact with copper on one side or on both sides. Two interface types are used in the calculation: type 1, between a core and copper foil with single-sided fill, and type 2, between inner cores with double-sided fill.

Type 1 applies to the interface between a core and copper foil, where the prepreg is filled from one side. The calculation is: measured thickness = theoretical thickness ? copper thickness × (1 ? residual copper ratio). The theoretical thickness is the nominal prepreg thickness before the copper effect is applied. The measured thickness is the effective dielectric thickness after lamination. For a surface layer, the residual copper ratio is 100%. For a bare board, the residual copper ratio is 0. When the residual copper ratio is 100%, the subtraction term becomes zero. When the residual copper ratio is 0, the full copper thickness is subtracted. The formula therefore ties the dielectric thickness directly to the copper distribution on the interface.

Type 2 applies to the interface between inner cores, where the prepreg is filled from both sides. The calculation is: measured thickness = theoretical thickness ? copper thickness 1 × (1 ? residual copper ratio 1) ? copper thickness 2 × (1 ? residual copper ratio 2). Each side of the prepreg has its own copper thickness and its own residual copper ratio. The two subtraction terms are applied separately. This matters when the two inner layers have different copper weights or different copper coverage. A single average copper thickness or a single average residual copper ratio is not sufficient for type 2. The two interfaces must be described separately because each one removes a different amount of resin volume from the prepreg.

The formulas are applied to the dielectric layer that is being modeled. For a single prepreg sheet between two copper features, the result is the effective thickness of that sheet. For a dielectric layer built from more than one prepreg sheet, the same logic is applied to each sheet, and the stackup is assembled from the resulting effective thicknesses. The dielectric constant is then taken from the material data, with the arithmetic value used when multiple prepreg sheets are combined. This keeps the thickness calculation and the dielectric constant calculation consistent with the physical construction of the layer.

The table values for high-Tg FR4 prepregs are reference values. They assume 75% routing on power and ground layers and 25% on signal layers. When the actual line distribution ratio is available, use it to calculate the residual copper ratio for each interface. For impedance-critical layers, extract the copper distribution from the layout instead of relying on the table assumption. A ground layer with a large continuous pour and a signal layer with dense traces do not have the same residual copper ratio even when they use the same copper weight. The difference changes the resin flow and the final dielectric thickness.

The calculation also depends on the copper thickness. The prepreg tables separate 0.5 oz copper from 1 oz copper because the copper thickness changes the amount of resin displaced. When the copper thickness is known, use that value in the formula. Keep the units consistent between the theoretical thickness, the copper thickness, and the measured thickness. The prepreg table is given in mil, so the calculation should use mil for all thickness terms. If the core thickness is taken from the millimeter table, convert it to the same unit before combining it with prepreg values in a stackup model.

For stackup design, the sequence is straightforward. First, select the core and read its thickness and dielectric constant from the core table. Second, select the prepreg types and identify the copper interface for each prepreg sheet. Third, determine the copper thickness and residual copper ratio for each interface. Fourth, apply the type 1 formula for a core-to-foil interface or the type 2 formula for an inner-core-to-inner-core interface. Fifth, use the resulting effective dielectric thickness in the impedance model. The nominal prepreg thickness is not used directly in the model because it does not represent the dielectric spacing after resin flow.

The same method applies to the core thickness itself. The board thickness accuracy note requires the use of the actual incoming thickness when calculating stackup thickness and interlayer dielectric thickness. The core table provides the nominal thickness and dielectric constant, but the incoming laminate measurement is the thickness that should be used for the final stackup. This is especially important when the stackup contains several cores and prepreg layers. A small thickness error in one layer can shift the impedance of a trace that references that layer. Using measured incoming thickness and calculated resin flow keeps the stackup model aligned with the manufactured board.

The dielectric constant values in the core table and the effective thickness values from the prepreg calculation serve different roles. The dielectric constant describes the material property of the core. The effective thickness describes the geometry of the dielectric space. Both are required for impedance calculation. A correct dielectric constant with an incorrect thickness produces an incorrect impedance. A correct thickness with a dielectric constant taken from the wrong material row also produces an incorrect impedance. The two data sets should be used together, with the material row, core thickness, prepreg type, copper weight, and residual copper ratio all traceable in the stackup documentation.

For documentation, record the source of each value used in the stackup. Record the core material row and thickness, the prepreg type and nominal thickness, the copper weight on each interface, the residual copper ratio, and the calculated effective thickness. This record allows the impedance model to be checked after the board is manufactured. If the measured impedance differs from the model, the record shows whether the difference comes from the core dielectric constant, the prepreg flow calculation, the copper distribution assumption, or the incoming laminate thickness. The tables and formulas in this section provide the calculation framework. The actual design values should come from the materials and copper distribution used in the specific board.

Impedance Calculation: Inner and Outer Layer Copper Thickness and Line Width

Impedance calculation depends on the copper thickness and trace geometry entered into the field solver. For inner and outer layers, the same nominal base copper foil does not produce the same calculated copper thickness. The first table lists the nominal base copper specification and the corresponding calculated copper thickness T used for inner-layer and outer-layer impedance calculations.

Nominal Base Copper and Calculated Copper Thickness

The inner-layer calculated copper thickness is 0.65 mil for 18 μm base copper, 1.25 mil for 35 μm base copper, and 2.56 mil for 70 μm base copper. The outer-layer calculated copper thickness is 2.2 mil, 2.9 mil, and 4.2 mil for the same nominal base copper values. These calculated values, rather than the nominal foil thickness, are the T1 values used in the impedance calculation. Using the nominal foil thickness directly would omit the difference between the inner-layer and outer-layer copper thickness used by the calculation model.

The table is therefore a lookup for the copper thickness term in the impedance model. The layer type must be selected first, and the nominal base copper must then be matched to the corresponding calculated thickness. The inner-layer and outer-layer values are not interchangeable, even when the nominal base copper is the same.

Line Width and Spacing Inputs

The second table defines the upper trace width W2, lower trace width W1, and spacing S1 used in the calculation. W0 and S0 are the customer-designed line width and spacing. The offsets convert the design dimensions into the calculation inputs for each layer type and base copper thickness.

• The parameters in the table are the copper thickness T1 used during impedance calculation and the values of the upper and lower line width.
• W0 and S0 represent the customer-designed line width and spacing, respectively.

For inner layers, the upper line width W2 is smaller than W0, while the lower line width W1 remains W0 and the spacing S1 remains S0. The reduction in W2 is 0.1 mil for 18 μm base copper, 0.4 mil for the first 35 μm entry, and 1.2 mil for the second 35 μm entry. For outer layers, the adjustments are larger and act in both directions. W2 is reduced by 0.6 mil for 18 μm base copper and by 0.9 mil for 35 μm base copper. W1 is increased by 0.7 mil for 18 μm base copper and by 0.9 mil for 35 μm base copper. S1 is reduced by 0.7 mil for 18 μm base copper and by 0.9 mil for 35 μm base copper.

Solder-Mask Correction for Outer-Layer Impedance

Impedance calculations are commonly performed with a model that does not include solder-mask coverage. In practice, solder mask affects outer-layer impedance, and impedance-controlled traces are generally covered by solder mask. Production tests have therefore established a correction for outer-layer impedance. The correction applies to outer-layer traces covered by solder mask. Outer-layer traces without solder-mask coverage and inner-layer traces do not require this correction.

The corrected value is calculated as:

Z0 (solder-mask covered) = Z1 (no solder-mask coverage) × 0.9 + 3.2

Here, Z0 is the impedance value for the solder-mask-covered trace, and Z1 is the value calculated without solder-mask coverage. The coefficient 0.9 and the constant 3.2 are the correction terms derived from production tests. The equation converts the uncorrected calculation result into the expected impedance after solder-mask coverage.

Worked Example: 50 Ω Target

If the required impedance is Z0 = 50 Ω, the calculation without solder-mask coverage is obtained by rearranging the equation:

Z1 = (50 - 3.2) / 0.9 = 52.0 Ω

Set Z1 as the calculation value in SI8000. The field solver is then run with Z1, the calculated copper thickness T1 from the first table, and the line width and spacing values from the second table. After the solver returns the uncorrected result, the solder-mask correction is applied to compare the result with the required outer-layer impedance.

When using these tables, keep the layer type, nominal base copper, and solder-mask condition consistent. The calculated copper thickness differs between inner and outer layers, and the line-width offsets differ by layer and copper thickness. Applying an inner-layer offset to an outer-layer trace, or omitting the solder-mask correction for a covered outer-layer trace, changes the input geometry and the resulting impedance target.

Related Reading: 

• Definition and Design Purpose of Characteristic Impedance in PCBs
• PCB Impedance Control: Manufacturing Factors and Parameter Relationships