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What is fr4 board material

FR4 is the basic material for most rigid circuit boards. Many electrical engineers and people involved in printed circuit boards are familiar with FR4 board materials. However, many people don’t know what FR4 is, let alone why it is the most popular PCB base.

fr4 board material
fr4 board material

FR4, also known as FR-4, is both a name and a grade. The name is applicable to glass fiber reinforced epoxy laminates used in printed circuit board manufacturing. However, the name is also used as a rating grade for epoxy laminates. The name essentially represents the basic quality of the laminate, indicating that various plates and designs are below FR4 grade. “Fr” in the name stands for flame retardant, while 4 is different from similar materials.

The material widely known as FR4 is a composite structure. The most basic layer of this material is a thin cloth like sheet woven with glass fibers. Glass fiber gives FR4 the necessary structural stability. Then, the innermost glass fiber layer is surrounded and bonded by a flame retardant epoxy resin. In addition to other physical properties, the resin gives the material rigidity.

How to select FR4 thickness

When ordering laminates for PCB projects, the designer or electrical engineer must specify the thickness of the FR4 PCB. Measure in inches, such as one thousandth of an inch or a few millimeters that is most suitable for setting. The thickness of a sheet of FR4 varies greatly depending on the needs of the project, but is often between ten and three inches.

Although the thickness of the board does not seem to be an important factor in PCB design, in fact, it is an essential function. The thickness of a circuit board affects many aspects of its function, which is why several factors should be considered in determining the thickness of a circuit board. These include the following.

Space: thinner may be better

If space concerns the designer, a thinner FR4 plate is preferred. This is the main factor in making smaller devices such as USB connectors and many Bluetooth accessories. Even for larger projects, smaller FR4 PCBs tend to save equipment space.

Flexibility: application dependent

Thinner plates may bend some capacity. While flexibility is unusual, it can be a positive or negative feature, depending on the application.

More flexible substrates are often preferred in some applications where the product is periodically stressed or bent. For example, those who use boards for medical and automotive applications often prefer flexible boards because these PCBs are often stressed and flexed.

However, flexibility may damage the PCB manufacturing process and cause serious problems in future assembly. When operated by a machine, if a welding machine is used, the more flexible plates may bend, resulting in the components being welded at an angle. In addition, such bending may damage the newly placed components and connections on the plate.

When to use FR4

Many electronic applications choose to use epoxy based FR4 boards. Their strength, reliability and relatively low cost make them a reasonable default choice for electronic engineers. However, FR4 is not ideal in all cases, especially in high frequency design. For these designs, high frequency laminates are often the more common choice.

  • Cost: FR4 will be cheaper
  • Signal loss: lower DF leads to lower signal loss
  • Signal loss: lower DF leads to lower signal loss
  • Temperature management
  • Dielectric constant: choose correct DK value
  • Operating environment
  • FR4 and high frequency laminate



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