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What is FR4 printed circuit board

FR4 is a printed circuit board based on glass-reinforced epoxy laminate material. It is the most used material because it is cheap in comparison with other materials and it can be used in many applications.  

 

FR4 Types

You can find different FR4 boards:

  • Single layer boards, with only copper in the top of the substrate, or double layer, with two copper layers, one in each side.
  • With different substrate thickness. It can be found thickness from 0.1 mm up to 3.2 mm. The required thickness will depend on the application it will be used. Typical thinner thicknesses are used in multilayer boards in which the FR4 board is used to distribute signals to other boards, in order to not increase the thickness of the PCB. Higher thicknesses are used when a high robustness of the PCB is required because a large PCB is designed.
  • With different copper layer thickness. Typical copper thicknesses are 17, 35 or 70 um.
  • Different panel sizes can be bought, the use of one or another will depend on the equipment the manufacturer has, and depending on the final size of the requested PCBs.  Typical panel sizes are 12x18 inches, 18x24 inches and 21x29 inches. When you are designing a panel, you have to consider that not all the panel can be used to draw your PCBs. Around 2 inches length and 2 inches width will be used for other purposes, like fiducial holes and other manufacturer tasks and also to avoid the panel can bend if work is performed close to the borders.

 

FR4 Properties

  • It has a dielectric loss tangent of around 0.017 to 0.025 while, for example, Rogers laminates have typical values of 0.002 and alumina PCBs have values of around 0.0002. The dielectric loss tangent indicates the insertion losses of the material with the frequency. As can be seen FR4 has higher values and this is the reason it can be used only for low frequency applications. Because at high frequencies, the insertion losses of the lines are higher, in comparison with other, more expensive, options.
  • Another important parameter is the dielectric constant. The typical value is around 4.6. This value is slightly dependent on the manufacturer and also the thickness of the substrate, but it is a value that typically provides good results when calculations are performed from it. From this parameter, the relation impedance/thickness of the line can be obtained. This is very useful when designing RF and digital circuits.
  • It is flame resistant.
  • It has barely zero water absorption
  • It has a very high dielectric breakdown, higher than 60KV. This implies a very high voltage is required to change the properties of the material
  • It maintains its electrical and mechanical properties in different environmental conditions as dry or wet.
  • Typical temperatures in which FR4 can work is from -55 to +125ºC.
  • There are improved FR4 versions that provides other characteristics such us lower dielectric values or higher temperature ranges. These boards are not standards and not all the vendors will offer them.

 

FR4 Uses

  • FR4 is used worldwide for different applications, commercial, civil, automotive or even military applications.
  • It is used for DC, digital and low frequency RF applications.
  • Practically the total of DC boards are done with FR4 since it is the cheapest material, with very good performance and because it is the material all the manufacturers have the possibility to use.
  • FR4 is a very good option for digital applications, in which, FPGA, microcontrollers, ADCs, DCAs, buses, etc are used.
  • FR4 boards can be used for low frequency applications. Up to 1-2 GHz the properties of this material can be compared with other more expensive options like Rogers laminates or alumina. From MHz up to 1 GHz the results will be slightly worse but the different in price can help to decide for the FR4 option. From 2 GHz the differences are higher and typically other materials are selected. With FR4 and the corresponding components, a wide catalogue of RF circuits can be correctly designed and fabricated as amplifiers, switches, attenuators, etc.

 

And that is all. In this post we explain when to use metal core PCBs.

 Roberto
 2018-12-06 09:08
 Help    Engineering   
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