What is FR-4 PCB material?

We’ve all heard the abbreviation FR4 being thrown about at some point when shopping for a PCB fab house, but what does it actually mean? And what implications does it have when choosing a PCB material? This post will help clear the fog and introduce some popular materials available for rigid PCBs.

Most likely, you will have seen FR4 as the standard option for small batch or prototype PCBs like in Seeed Fusion. FR-4 refers to a grade of material rather than a material itself and has many sub gradings and types such as TG130, High TG, FR4-Rogers.

The FR4 option on the PCB order page is the grade designation for the epoxy-fiberglass that often form the PCB core and prepreg layers. It is the properties of this base that gives the PCB the electrical isolation and mechanical strength required to endure increasingly demanding applications.

Single Sided PCBs with Exposed FR4 Substrate
Single-Sided PCBs with Exposed FR4 Substrate

In a typical PCB, the core provides the rigidity and the foundation on which the PCB traces can be ‘printed’ onto. In addition, the FR4 core and laminates form the electrical isolation separating copper layers.

For double layer boards, an FR4 core separates the top and bottom copper layers whereas, for multilayer PCBs, additional layers of FR4 prepreg are sandwiched between the inner core and the outer copper layers. The desired final thickness of the PCB can be controlled by mixing different laminate thicknesses. This arrangement is referred to as the stack-up.

For example, a typical 4 layer board of 1.6 mm thickness may have a core of thickness 1.2 mm. Two sheets of 0.12 mm prepreg topped with copper, solder mask and silkscreen roughly make up 1.6 mm. To build a 1.2 mm board, the core can be replaced with a 0.8 mm thick core.

Typical Multi-layered PCB Stack-up (4 layers)
Typical Multi-layered PCB Stack-up (4 layers)

The core is actually made up of a substrate layer covered in copper, thus it is also known as a copper-clad laminate. The stated thickness of the core usually excludes the copper.

To accommodate different final copper thicknesses e.g. 1 oz, 2 oz, etc. cores can be purchased with varying plating thicknesses e.g. 1.5 oz copper weight. After plating, they will build up to 2 oz boards.

Sheets of FR4 Grade Fiberglass Laminates and Copper Clad Core
Sheets of FR4 Grade Fiberglass Laminates and Copper-Clad Core

The name FR4 comes from the NEMU grading system where the ‘FR’ stands for ‘fire retardant’, compliant with the UL94V-0 standard. You may have noticed that on the Seeed Fusion order page the FR4 option is followed by TG130. The TG refers to the transition glass temperature – the temperature at which the glass-reinforced material will start to deform and soften. For Fusion’s standard boards this value is 130°C, which is more than enough for most applications. Special High TG materials can withstand temperatures of 170 – 180°C and can be ordered online using the Advanced PCB service.

Most FR4 laminates owe their flame resistance to its bromine content, a non-reactive halogen commonly used in industries for its flame retarding properties. This gives FR4 materials obvious advantages as a stock PCB material in terms of fire safety while in the field. It is also a little reassuring if your soldering skills are not up to standard.

However, bromine is a halogen which are highly toxic chemicals that are released in the environment when the material is incinerated. Even small amounts are sufficient to cause serious harm to humans or even death. To reduce the use of such hazardous substances in our everyday products, halogen-free FR4 laminates are readily available.

Other similar grades used for PCBs include FR2, which is a type of fire retarding fiberglass resin bonded paper and G10 which is not flame resistant at all. FR2 is cheaper and so has its uses in mass production of low-end electrical equipment. G10, a predecessor to FR4, has all but been taken over by the safer FR4 standard. Its only use in PCBs now is in designs that desire this flammable property.

There are also specialized FR4 laminates designed for high-speed applications, like Rogers and Taconic materials. These materials have stable dielectric constants (Dk) and low dielectric loss tangents (Df) that are essential for good signal integrity at high frequencies. Production of these laminates is difficult, however, so they constitute a significant portion of the total cost of the boards.

In choosing the right laminate for your application, electrical, mechanical, chemical and thermal properties all should be taken into account. But most will find low TG FR4 perfectly acceptable, which is why it remains the most popular laminate today.

This article has covered more information on rigid PCB materials than the average hobbyist will ever need, but should you ever find the use for High TG, halogen-free or more specialized materials, you can check out the Seeed Fusion Advanced PCB service for more options including high-frequency boards and rigid-flex PCBs.

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