An eight-layer board thickness is the sum of its dielectric gaps, copper layers and any coatings included in the mechanical definition. The label “1.6 mm PCB” does not tell you the distance between a signal trace and its reference plane. Keep those quantities separate when you specify a stackup or calculate impedance.
Start with a construction you can audit
Consider a symmetric foil construction with eight copper layers. Four gaps are prepreg and three gaps are dielectric cores. Number the copper L1 through L8, from top to bottom. For this arithmetic example, every copper layer is 0.035 mm thick and the solder mask allowance is 0.020 mm on each side. These are planning assumptions rather than a supplier commitment.
| Gap | Type | Calculation | Thickness |
|---|---|---|---|
| L1–L2 | Prepreg | 1 × 0.076 | 0.076 mm |
| L2–L3 | Core dielectric | 1 × 0.203 | 0.203 mm |
| L3–L4 | Prepreg | 2 × 0.130 | 0.260 mm |
| L4–L5 | Core dielectric | 1 × 0.203 | 0.203 mm |
| L5–L6 | Prepreg | 2 × 0.130 | 0.260 mm |
| L6–L7 | Core dielectric | 1 × 0.203 | 0.203 mm |
| L7–L8 | Prepreg | 1 × 0.076 | 0.076 mm |
Calculate the total without double-counting copper
Dielectric thickness = 2 × 0.076 + 2 × 0.260 + 3 × 0.203 = 1.281 mm. Copper thickness = 8 × 0.035 = 0.280 mm. Mask allowance = 2 × 0.020 = 0.040 mm. The calculated finished total is therefore 1.601 mm. If the drawing excludes solder mask, the same construction is 1.561 mm under that definition.
The core values in this table exclude copper. If your supplier gives a copper-clad core thickness that already includes foil, adding those foils again overstates the board thickness. Ask which convention the stackup sheet uses. Similarly, a starting outer foil value is not necessarily the finished outer copper after plating.
Understand what a glass style does—and does not—specify
A glass style identifies a reinforcement construction, not one universal pressed thickness. Resin content and laminate family also matter. The calculator’s selected material references include 1080 at 66% resin content and 2116 at 56% resin content. Consult the material supplier’s current construction table and the fabricator’s actual pressing data; a familiar glass-style number alone is not sufficient to release a design.
The two sheets in a prepreg gap also need enough resin to fill adjacent etched copper features. A nominal multiplication is useful for an early thickness budget, but the final pressed result depends on the copper pattern and processing. Treat the calculation as a discussion document until the fabricator approves the actual construction.
Why the impedance does not follow the total thickness
If L1 references the continuous L2 plane, its immediate dielectric height is 0.076 mm in this example. Increasing the center core from 0.203 mm to 0.403 mm adds 0.200 mm to the overall board but does not change that outer gap. The outer microstrip calculation should still use the approved L1–L2 height, together with actual copper thickness and material permittivity.
Conversely, changing the outer PP construction affects both total board thickness and outer-layer impedance geometry. You may need a different trace width to meet the same target. This is why selecting a nominal overall thickness in a calculator cannot uniquely determine a controlled-impedance trace.
Use tolerances as an agreement, not extra decimal places
An assumed ±10% band around 1.601 mm is 1.4409–1.7611 mm. That arithmetic does not establish a factory tolerance or a measurement method. A card guide, press-fit connector or enclosure may require a different nominal and tolerance. State the measurement area and whether mask, plated pads or other coatings are included.
- Check all seven dielectric gaps and all eight copper layers.
- Verify that paired gaps use the intended material and pressed thickness.
- Review copper distribution separately from dielectric symmetry.
- Record finished copper and the solder mask thickness convention.
- Obtain fabricator approval before routing to the proposed impedance geometry.
Try the calculation and retain the assumptions
Open the stackup designer, start with the 1.601 mm example and change one PP ply count. Observe the mirrored gap and calculated total. Export the construction CSV and attach it as design intent, with a clear revision identifier. Compare alternative layer allocations in the stackup library.
Material reference: Isola 370HR construction tables. Calculations on this page are original planning examples; no manufactured sample or test result is implied.