FR4 Printed Circuit Board

FR4: The Backbone of PCBs for Electronic Engineers

According to Rush PCB UK, FR4 is the most common material that PCB manufacturers use to make printed circuit boards. Most individuals and engineers in the electronic industry are familiar with FR4 as the base material for building rigid circuit boards.

What is FR4?

The name FR4 or FR-4 is a combination of an acronym and a grade. The FR stands for Flame Retardant and the 4 is the mark of a grade of the material. FR4 is an epoxy laminated fiberglass reinforced sheet that printed circuit board manufacturers use. Grade 4 indicates the base quality of the laminate sheet.

Under the FR4 name, there exists a variety of sheet materials and designs, with the number differentiating them from others in the same class.

FR4 has a composite structure. Fiberglass, woven into thin cloth-like sheets, is bound with a flame-resistant epoxy resin, to form a base layer for a printed circuit board. While the fiberglass gives the material its structural stability, the epoxy gives FR4 its material rigidity. FR4 also has several other physical properties that make it so popular among engineers and designers as a base PCB material.

How PCBs Use FR4

The FR4 in a PCB forms its primary insulating backbone. Board manufacturers laminate the FR4 sheet with copper foil on both sides using adhesive and bond them with heat and pressure to form a copper clad. PCB manufacturers use the copper clad to build printed circuit boards. Depending on the design, they etch the copper foils to form circuits on which they solder electronic components.

Complex PCBs can have more than just two layers. Manufacturers etch Inner layers of copper foils to form specific circuits and bond all the layers to form a single multilayered PCB. Vias form the interconnections among the inner and outer copper layers. The outermost copper layers need a covering of solder masks to keep them from being tarnished and oxidized from chemicals in the atmosphere. A silkscreen layer helps in the assembly process.

Depending on the application, designers choose the FR4 and copper foil thicknesses. Unless the board has to withstand severe vibrations, designers use standard thickness FR4 material. The copper foil thickness depends on the current the board must handle.

When to Use FR4

Designers prefer to use FR4-based printed circuit boards for their mechanical strength, reliability, good electrical insulation, and relatively low cost. However, FR4 material is not suitable for boards handling high-frequency signals, which require special laminates for better signal integrity.

PCB Thickness

When ordering a printed circuit board for a project, the designer must specify the number of layers and their thickness. Depending on the needs of the project, the thickness of the FR4 board may vary significantly. This is an essential feature and affects many aspects of the functionality of the board.

Advantages of Using FR4

FR4 materials are popular among electronic engineers in the industry as FR4 offers several advantages:

  • Easy to design and fabricate
  • Low-cost materials
  • Makes compact boards
  • Provides good bonding with copper foils—high peel-off strength
  • High mechanical strength
  • Highly resistant to moisture
  • Good electrical insulating properties in humid environments
  • Can withstand high temperatures
  • Can be fabricated into multiple layers
Properties of PCB Materials

Properties of PCB Materials Ref:

Types of FR4 PCBs

Depending on the filler material, it is possible to make various types of FR4 boards such as:

  • Halogen-Free PCBs
  • Single-Layer PCBs
  • Double-Layer PCBs
  • Multi-Layered PCBs
  • Rigid-Flex PCBs


Being a widely relevant material, FR4 is popular mostly for its low cost and relative electrical and mechanical stability. Although FR4 material is popular for its extensive benefits, it is not the best material for high-frequency applications. For any requirements of PCBs made from high-quality FR4 material, please contact Rush PCB UK.