What is the Difference Between Prepreg and Laminate?

March 13, 2020 , in Blog

You may have heard the saying that those who can do and those who can’t teach. Recalling these words always lightens my heart and provides me with a good chuckle. Having spent a reasonable amount of time as both “one of those who can do” and “one of those who teach,” I often wonder about the types of people who would find solace in this phrase. I have known no one whom I could accurately say can’t do yet can teach. However, I have known many people who can do and teach. That is perhaps because adequately dispensing knowledge first requires that you thoroughly understand the material and are able to use it yourself.

PCBs prior to assembly

Fabricated circuit boards

Thoroughness is not only applicable to academia. In fact, the best PCB development occurs when a white box approach to manufacturing, wherein the design and manufacturing stages are transparent and integrated, is implemented. This means that the contract manufacturer (CM) understands the design intent and the designer understands the benefits of DFM that match the CM’s capabilities. One design decision that routinely is overlooked is board material, which may be due to a lack of thorough understanding of the variety of options and their compositions. Let’s shine some light on how the PCB stackup is constructed, specifically the difference between prepreg and laminate, which should aid you in selecting the material best suited for your design.

What is the PCB Stackup?

The PCB stackup is one of the most important considerations when designing circuit boards. In fact, optimal board design requires that you adopt an overall design perspective that incorporates verticality. This includes selecting the number of layers and ground planes, locations and types of vias, as well as the materials for the stackup. Choosing the right material begins with understanding the various levels of the PCB stackup and the terminology used to identify them, as listed below.

Common PCB Stackup Terms

  • Layer

A layer for a PCB refers to a level of the stackup composed fully or partially of copper. A layer, which carries electrical signals, may be an external or surface layer or an internal layer.

  • Plane

A plane is a solid copper layer of the PCB stackup that is typically used for grounding or power.

  • Core

The stackup core is a material level that has copper layers on both sides. For single and double-sided PCBs, this arrangement may comprise the entire board stackup.

  • Base

Base refers to the inner dielectric material of the PCB core.

  • Substrate

The PCB substrate is a dielectric material to which a copper layer is attached (either one or both sides). The substrate material defines the board’s rigidity or flexibility, as well as electrical properties; such as board impedance.

  • Prepreg

Prepreg is a type of base material, fiberglass or fabric, that has been pre-impregnated or reinforced with resin, typically epoxy, or polyimide that is partially cured.

  • Laminate

PCB laminate may contain an inner prepreg; for example, copper clad laminate (CCL), which is a common PCB material, and copper on one of both sides. The laminate may be the PCBs core.

As shown above, the common terminology used to describe the composition of the PCB stackup can be confusing. And one of the ambiguous usages is differentiating between prepreg and laminate.

The Difference Between Prepreg and Laminate

Let’s see if we can definitively distinguish between prepreg and laminate. Prior to taking this on, it is probably useful to understand that between every pair of copper layers, there is an insulation layer. This insulation layer may begin as prepreg or a laminate. Now, let’s compare prepreg and laminate.

Partially or uncured core material Definition Fully cured core material
To bond two copper layers together or to bind one copper layer and a core while maintaining separation between the two materials being bonded Usage Dielectric material with copper on one or both sides used for isolation between copper layers
Between two cores or a core and copper foil. Stackup location Between copper layers or planes. May form the core of the board stackup.

Hopefully, this table provides some clarity and differentiation between prepreg and laminate and how they are used in your board’s fabrication. It may also be helpful to note that core, substrate, and laminate are sometimes used interchangeably.

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Making the best material selection for your design begins with understanding the different parts of the PCB stackup and their uses. Next, you need to partner with a contract manufacturer (CM) that can accommodate the needs of your board build. Tempo Automation is the fastest manufacturer of high-quality PCBAs in the industry for prototyping and low-volume production. One reason we can do this is the wide range of capabilities we offer for your PCB stackup.

And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.

If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on the difference between prepreg and laminate or other PCB material issues, contact us.

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