When engineers search for standard FR4 thicknesses, they usually expect a simple answer. Instead, they often encounter conflicting information.
One source lists a 0.10 mm core. Another shows 0.136 mm. A third refers to a 1080 prepreg with a thickness of 0.075 mm, while a supplier datasheet lists an entirely different value.
The confusion arises because several different concepts are being mixed together:
- Core thickness
- Finished core thickness
- Prepreg thickness
- Pressed dielectric thickness
- Glass style
- Resin content
- Lamination behaviour
All of these values are related, but they are not the same thing.
Understanding the difference is essential when designing stackups, comparing supplier capabilities, or specifying impedance-controlled PCBs.
Core Material — The Easy Part#
A core is a fully cured FR4 laminate with copper foil already bonded to both sides.
Unlike prepregs, cores are relatively straightforward because their dielectric thickness is fixed before lamination.
Typical core thicknesses available from most laminate manufacturers are:
| Core Thickness (without copper) | Common Name |
|---|---|
| 0.05 mm | Ultra-thin core |
| 0.075 mm | Thin core |
| 0.10 mm | Standard HDI core |
| 0.127 mm | 5 mil core |
| 0.15 mm | Thin multilayer core |
| 0.20 mm | Standard multilayer core |
| 0.25 mm | Standard multilayer core |
| 0.30 mm | Standard multilayer core |
| 0.36 mm | Heavy-duty core |
| 0.51 mm | 20 mil core |
| 0.80 mm | Thick core |
| 1.00 mm | Thick core |
These values refer only to the FR4 dielectric.
Copper thickness is normally specified separately.
For example:
| Construction | Finished Core Thickness |
|---|---|
| 18 / 100 / 18 µm | 0.136 mm |
| 35 / 100 / 35 µm | 0.170 mm |
This distinction is the first source of confusion. Some datasheets specify dielectric thickness, while others specify the finished core including copper.
Prepreg — A Different Material#
Prepreg is fundamentally different from a core.
A prepreg consists of woven glass fabric impregnated with partially cured resin.
During lamination, heat and pressure complete the curing process and permanently bond the multilayer together.
Because prepreg changes during lamination, it cannot be treated as a fixed-thickness material.
This is where many misunderstandings begin.
Why Prepreg Thickness Is Not a Fixed Number#
A common assumption is that prepregs behave like cores.
They do not.
A core may have a thickness of 0.10 mm before lamination and remain 0.10 mm afterwards.
A prepreg does not.
During pressing, the resin softens and flows. The glass fabric compresses. Excess resin migrates into available spaces within the multilayer construction.
The final dielectric thickness depends on:
- Resin content
- Glass style
- Copper distribution
- Copper height
- Lamination pressure
- Lamination cycle
- Number of prepreg plies
As a result, the same prepreg can produce different dielectric thicknesses in different stackups.
For this reason, PCB manufacturers usually work with pressed thickness targets rather than raw prepreg thickness values.
Resin Content — The Real Engineering Variable#
The most important characteristic of a prepreg is often not its glass style but its resin content.
The amount of resin determines how the material behaves during lamination.
| Classification | Typical Resin Content Range* | Typical Purpose |
|---|---|---|
| Low Flow | Approximately 40–50% | Controlled resin movement, cavities, rigid-flex applications |
| Standard Flow | Approximately 50–60% | General multilayer construction |
| High Flow | Approximately 60–70% | Filling uneven copper structures and large cavities |
| Very High Flow | Above 70% | Special applications requiring maximum filling capability |
*The resin content ranges shown here are representative industry values. Actual resin content depends on the laminate supplier, glass style, resin system and material series. Always consult the manufacturer’s datasheet when designing a production stackup.
Resin content is usually specified as a percentage of the prepreg weight.
For example, a prepreg with a resin content of 60% contains approximately 60% resin and 40% glass reinforcement by weight.
Higher resin content generally means:
- More resin available to flow during lamination
- Better filling of uneven structures
- Greater ability to compensate copper topography
- Less dimensional stability
Lower resin content generally means:
- Reduced resin flow
- Better thickness control
- Improved dimensional stability
- Less capability to fill large cavities
In many advanced multilayer designs, resin behaviour becomes more important than nominal prepreg thickness.
Glass Style — The Structural Part of the Prepreg#
Resin content defines how the prepreg flows during lamination.
Glass style defines the mechanical structure that carries the resin.
A prepreg is not just resin. It is woven glass fabric impregnated with partially cured resin. The glass fabric controls thickness stability, mechanical strength, dimensional behaviour, and how evenly resin is distributed across the layer.
Common glass styles include 106, 1080, 2113, 2116 and 7628.
Thin glass styles such as 106 and 1080 are often used where small dielectric spacing is required.
Heavier glass styles such as 2116 and 7628 provide more thickness and mechanical stability, but they also create larger dielectric spacing.
It is important to note that glass style and resin content are specified independently.
For example:
- 1080 RC50
- 1080 RC60
- 2116 RC52
- 2116 RC65
may all use the same glass fabric while exhibiting very different lamination behaviour.
This is one reason why simply specifying “1080 prepreg” is often insufficient for advanced multilayer constructions.
Prepregs Control Dielectric Spacing#
Many engineers assume that core thickness determines layer spacing.
In reality, prepregs often provide the fine adjustment required to achieve the target spacing.
By selecting different prepreg styles and resin contents, manufacturers can control:
- Dielectric thickness
- Impedance
- Overall board thickness
This allows a manufacturer to fine-tune the stackup while maintaining the desired electrical performance.
The electrical result may be identical even though the construction differs.
Low-Flow Prepregs and Controlled Cavities#
Prepreg selection becomes particularly important when cavities or embedded structures are involved.
A common misconception is that more resin flow is always beneficial.
In reality, some applications require exactly the opposite.
Consider a multilayer PCB containing an SMD component mounted on Layer 3 approximately 0.5 mm below the outer surface. A cavity is machined into the structure so the component can fit within the board thickness.
During lamination, the cavity must remain open and maintain its geometry.
To achieve this, manufacturers often insert a temporary PTFE (Teflon) filler into the cavity area. At the same time, low-flow prepregs are selected to minimise resin migration.
If excessive resin enters the cavity during pressing:
- Cavity dimensions may change
- Embedded component clearances may be reduced
- Additional machining may be required
- Reliability can be affected
In this case, the objective is not to fill empty space.
The objective is to prevent resin from flowing where it should not.
This illustrates an important principle:
Prepregs are not simply dielectric spacers. They are process materials whose flow characteristics directly influence the final PCB structure.
Common Prepreg Styles#
| Glass Style | Typical Pressed Thickness |
|---|---|
| 106 | 0.045 – 0.060 mm |
| 1080 | 0.060 – 0.080 mm |
| 2113 | 0.080 – 0.100 mm |
| 2116 | 0.090 – 0.120 mm |
| 7628 | 0.170 – 0.220 mm |
These values are approximate.
The final dielectric thickness depends on the stackup design and lamination process.
For this reason, stackup calculations should always be based on manufacturer data rather than generic internet tables.
Why Multiple Prepreg Sheets Are Often Used#
Another common misconception is that manufacturers always use a single prepreg sheet to create the required dielectric spacing.
In practice, multiple prepreg plies are often combined.
Examples include:
- 1080 + 1080
- 1080 + 2116
- 2113 + 2116
- 7628 + 1080
The reason is not simply thickness.
Using multiple prepreg sheets allows the manufacturer to control several important process variables simultaneously.
Better Resin Distribution#
During lamination, resin must flow around copper traces, pads and plane structures.
Multiple prepreg plies can provide a more uniform resin distribution than a single thick prepreg.
This reduces the risk of voids and dry areas.
Improved Thickness Control#
A target dielectric thickness may be achieved using different prepreg combinations.
For example, two thin prepregs may provide better process control than one thick prepreg.
Filling Uneven Copper Structures#
Multilayer PCBs rarely contain perfectly flat copper surfaces.
Heavy copper areas, dense routing and large copper planes create variations in surface topography.
Using multiple prepreg plies helps the resin adapt to these differences during lamination.
Material Availability#
Not every prepreg style and resin content is available from every laminate supplier.
Manufacturers often select combinations that are readily available and qualified within their process.
As a result, two PCB manufacturers may use different prepreg combinations while producing nearly identical finished dielectric thicknesses.
The final electrical and mechanical performance is usually more important than the exact prepreg combination used to achieve it.
Why Different Suppliers Show Different Values#
Many of the conflicting values found online are technically correct.
They simply refer to different stages of the manufacturing process.
One supplier may publish:
- Raw prepreg thickness
- Resin content
- Glass style
Another may publish:
- Pressed thickness
A third may publish:
- Finished dielectric thickness within a specific stackup
Without understanding the context, these values appear contradictory.
In reality, they describe different aspects of the same material system.
Summary#
Core thickness is usually straightforward because the dielectric thickness is fixed before lamination.
Prepreg thickness is fundamentally different. During pressing, resin flow, glass compression, copper distribution, and process conditions all influence the final result.
This is why experienced PCB manufacturers rarely specify prepregs by thickness alone. Resin content, flow characteristics, glass style, and the surrounding stackup are equally important.
A multilayer stackup is not assembled from fixed-thickness building blocks. It is engineered from cores, prepregs, resin systems and lamination processes to achieve a target electrical and mechanical result.
When evaluating a multilayer construction, it is often more useful to ask:
“What dielectric thickness must be achieved after lamination?”
rather than:
“What prepreg thickness should be used?”
The first question focuses on the final electrical and mechanical result. The second focuses only on one of the many variables used to achieve it.