Prepreg

What Is Prepreg?

Prepreg (short for pre-impregnated) is a reinforcing fabric, usually woven fiberglass, saturated with partially cured (B-stage) resin. In a multilayer printed circuit board (PCB), prepreg sits between cores and copper foil and bonds them together during lamination, when heat and pressure make the resin flow and fully cure. The cured prepreg becomes the dielectric between layers, so its glass style, resin content, and pressed thickness set the spacing and dielectric constant (Dk) your impedance calculations depend on. Pressed thickness is also the value you know least precisely until your fabricator confirms the stackup.

How Prepreg Is Made and Used

A material supplier weaves glass fibers into cloth, runs the cloth through a resin bath, and partially cures it until it is tack-free but still reactive. Sierra Circuits describes this B-stage state as the property that lets prepreg bond layers: during final lamination the resin softens, flows around etched copper, and finishes curing.

A core starts from the same glass cloth and resin. The laminate maker presses plies of B-stage material between copper foils and cures them into a rigid sheet. Prepreg finishes curing in your fab's lamination press, so in the finished board both are fully cured. Core thickness is fixed when the laminate is made, and prepreg thickness depends on how your fab presses it. A typical multilayer PCB stackup alternates cores and prepreg. Your fabricator stacks one or more prepreg sheets, called plies, to reach each target dielectric thickness, and the finished value comes from their approved stackup.

Prepreg vs Core

Property

Prepreg

Core

Cure State

Partially cured (B-stage) until lamination

Fully cured (C-stage)

Copper

Supplied without copper; bonds cores and foil

Copper clad on one or both sides

Thickness

Set by pressing; depends on resin content, glass style, and copper density

Supplied at a specified thickness and tolerance

Role

Bonding and insulation between layers

Structural base for etched signal and plane layers

Layout Input

Use your fabricator's pressed thickness and Dk

Use the specified core thickness and Dk

Common Prepreg Glass Styles and Thicknesses

A glass style number identifies the cloth construction. Finished thickness also depends on resin content, and suppliers sell the same glass style at more than one resin content. Isola publishes these values for its IS400 laminate and prepreg:

Glass Style

Resin Content

Listed Thickness

Notes

106

74%

2.3 mil (0.058 mm)

Thin, resin-rich layers

1080

65%

3.1 mil (0.079 mm)

Thin to mid-thickness layers

1080

69%

3.5 mil (0.089 mm)

Same cloth with more resin

2116

53%

4.7 mil (0.119 mm)

General-purpose bonding layer

7628

46%

7.9 mil (0.201 mm)

Thick, glass-rich, coarse weave

Other material systems list different values, and the thickness your fab presses depends on the copper around each layer. Confirm it with your fabricator before you calculate impedance.

Why Prepreg Matters for Layout

  • Impedance: Dielectric height and Dk set the trace width and gap you need for a target impedance, so a different prepreg construction means different routed geometry. See Controlled Impedance PCB and Dielectric Constant.
  • Resin flow: Resin fills the space around copper during lamination, so pressed thickness depends on the copper density of the neighboring layers. Treat a catalog value as a starting point for stackup planning.
  • Fiber weave skew: Glass has a higher Dk than the resin around it, so the two traces of an equal-length differential pair can carry signals at different speeds depending on each trace's position over the weave. See Fiber Weave Effect.

Frequently Asked Questions

What Is the Difference Between Prepreg and Core?

Prepreg is partially cured glass cloth and resin that bonds layers during lamination. A core is fully cured laminate with copper on one or both sides. Cores come in fixed thicknesses, and prepreg thickness is set by pressing.

Why Is It Called Prepreg?

The name is short for "pre-impregnated." The glass cloth is impregnated with resin before it reaches the PCB fabricator.

Does Prepreg Affect Impedance?

Yes. The thickness and Dk of the prepreg between a trace and its reference plane set the trace width you need for a target impedance, so a stackup change changes the routed geometry.

Why Is My Pressed Prepreg Thinner Than the Datasheet?

Resin flows into the gaps between copper features during lamination. How much it thins depends on copper distribution, resin content, glass style, and press conditions, so use your fabricator's approved stackup instead of a fixed reduction factor.

Related Terms

PCB Stackup · FR-4 · Dielectric Constant · Controlled Impedance PCB · Fiber Weave Effect

Relevant Resources

Sources

Isola Group. "IS400 Laminate and Prepreg." Accessed September 30, 2026. https://www.isola-group.com/pcb-laminates-prepreg/is400-laminate-and-prepreg-isola-group/.

Isola Group. "Laminate & Prepreg Manufacturing." Presentation. Accessed September 30, 2026. https://www.isola-group.com/wp-content/uploads/Understanding-Laminate-Prepreg-Manufacturing.pdf.

Sierra Circuits. "What Is a Prepreg in PCB Manufacturing?" Accessed September 29, 2026. https://www.protoexpress.com/blog/prepreg-the-slice-of-cheese-in-your-pcb/.

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