Electronics Manufacturing

What is FR4 Material: Properties, Constituents and Fabricate Steps

What is FR4 Material?

The most widely and extensively used material for manufacturing printed circuit boards is FR-4 material, with excellent electrical and mechanical properties. It is regarded as a general  printed circuit board material FR4. The abbreviation FR stands for flame-retardant or flame-resistant material. It is self-extinguishable. The number 4 in FR-4 is the code for the flame-retardant material. FR-4 belongs to the class of epoxy FR4 laminated fiberglass. FR-4 is composed of fiberglass cloth bonded by epoxy as an adhesive. FR4 PCB material is available in many colors: blue, green, white, black, etc. Normally, the thickness used is 1.6 mm and 1.2 mm. There are five grades for FR material. They are FR-1, FR-2, FR-3, FR-4, and FR-5.

FR4 Material Properties

High Mechanical Strength: It displays excellent mechanical strength. It can withstand mechanical stress, which occurs during manufacturing processes, assembly operations, and other handling operations. It exhibits great dimensional stability when exposed to high temperatures and other environmental factors.

Excellent Electrical Insulation: While FR4 displays excellent electrical insulation properties, it does not allow an electric current to flow between conductors on a PCB. This prevents short circuits, other electrical damage, and fire.

Excellent Thermal Stability: Another FR4 material property is excellent thermal management properties.

Low Moisture Absorption Rate: The moisture absorption rate of FR4 material is low. If immersed in water for twenty-four hours, the moisture absorption is 0.01%

Flame Retardant: FR4 is a self-extinguishable flame-retardant material. Fire will not spread in the event of accidents.

Lower Cost: FR-4 costs less than its counterparts, which makes it a very good choice for all manufacturers.

The Constituents of FR-4 Laminates

FR4 Laminates

Reinforcement: Woven glass Fiber cloth is normally the reinforcement laminate for  FR-4, which provides mechanical support and electrical properties. The typical constituents of E-grade glass used in FR-4 are shown in the below table.

Constituent Composition %
Silicon dioxide 52-56
Calcium oxide 16-25
Aluminium oxide 12-16
Boron oxide 5-10
Sodium oxide and potassium oxide 0-2
Magnesium oxide 0-5
Iron Oxide 0.05-0.4
Titanium oxide 0-0.8
Fluoride 0-1

Resin: The FR4 laminated fiberglass resin structure normally consists of bi-, tetra-, and multi-functional epoxy groups. Curing agents, flame retardants, fillers, etc. are added to the resin system to accelerate the performance of the laminate.

Curing Agents: Curing agents are used to harden the PCB. Curing agents known as dicyandiamide, commonly known as DICY, and phenol novolac are widely used in the PCB industry.

Flame Retardants: They are added to reduce the flammability of the laminate. They suppress combustion by forming gas barriers against oxygen. Nowadays halogen-free flame retardants are used

Fillers: Fillers are added to resins to achieve a lower coefficient of thermal expansion and to avoid cracks in the plated through holes. Fillers also help to retard the flammability of the material and reduce the cost. Fillers used for FR-4 are silica and aluminum silicate.

Accelerators: They are added to the resin system to speed up the curing rate. Imidazole is one of the widely used accelerators used for the FR-4 PCB.

The Fabricate Steps of FR4 Material

A PCB’s most basic building blocks are resin and reinforcement composites. FR4 PCB material is a composite of woven fiberglass reinforcement. The steps to fabricate FR-4 are listed below.

  1. Raw materials like glass are melted in a furnace to form fiberglass filaments,  which are combined to form fiber yarn.
  2. Yarns are weaved to form fiberglass cloth.
  3. A coupling agent like organosilane is coated on the fabric to facilitate bonding between organic resin and inorganic glass. Resin is obtained from petrochemicals in raw form.
  4. Curing agents, flame retardants, and fillers are added to the resin.
  5. Prepreg is made from glass fabric along with epoxy resin. Multiple prepregs are thermally combined to make a laminate.
  6. Copper foil is deposited over the laminate to get a copper-clad laminate.
  7. Many prepregs and laminates are stacked to make a multilayer PCB.
  8. The drilling of through holes and vias is done and plated with copper.
  9. A solder mask is applied to the board, exposing the areas to be soldered.
  10. Components are mounted on the PCB.
  11. The wave soldering process solders the board to obtain the final PCB circuit.

FR4 PCB Board

Disadvantages of FR4 material

Not Suitable for High-frequency Applications: The circuit board material FR4 dielectric constant changes with frequency and is non-uniform in high-speed, high-frequency applications. Besides, circuit board material FR4 has a higher dissipation factor, resulting in higher signal loss. So FR4 materials are not preferred for high-frequency applications.

Lower Operation Temperature than PTFE Material: The dimensional stability of circuit board material FR4 degrades with high power, high voltage, and high temperature. With high temperatures, the dielectric constant changes, which leads to a change in impedance. But PTFE materials can withstand very high temperatures and also keep a steady dielectric constant.


Due to the versatile great FR4 material properties, it is a suitable material for both single-layer and multi-layer PCBs. Its lower cost, widespread availability, and large-scale usage make it the best choice for manufacturing and design in most PCBs.

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