PCB DEPARTMENT MANAGER
FR4 (Flame Retardant 4), created by NEMA (National Electrical Manufacturers Association) in 1968, stands for flame retardant. Structurally, it is a composite material consisting of woven fiberglass fabric with epoxy resin binder.
Example of FR4 weaving
Having a good strength-to-weight ratio, FR4 is highly resistant to harsh environmental conditions. With little moisture absorption, it is widely preferred as an electrical insulator with heat and mechanical strength. It is compatible with all types as the final coating (Finished Type) in PCB production.
“The main body of multilayer PCBs is called the core (Fr4 core), and the outer insulation structures are called PP (Fr4 Prepreg).”
Today, it is a standard for in-hole coating. It is not that suitable for molded (cutting, drilling holes) production. It is possible to use single-layer, double-layer and multi-layer PCB types. Today, we can call it the “standard” and the first material type that comes to mind for PCB. “The main body of multilayer PCBs is called the core (Fr4 core), and the outer insulation structures are called PP (Fr4 Prepreg).” Here are some main parameters that may be important when selecting the FR4 material.
Dielectric Constant – Permittivity
It is an important parameter for signal integrity and impedance in high frequency electrical circuits. For most FR4 materials, this value is in the range of Er, 2.2 to 4.7. The dielectric constant varies with frequency and usually decreases as the frequency increases.
Loss Factor
It is a measure of power that is lost due to the material. The lower this value, the less power is lost. At the same time, this loss also changes with frequency. The loss tangent is important in very high frequency circuits (usually above 1GHz).
CTI
It measures the ability of the PCB material to resist electrical deterioration in the Z-direction (inter-surface) of the PCB. It is expressed in Volts.
Tg
It is the temperature at which the material transforms from a reasonably hard, glass-like substance into a more flexible and bendable, plastic-like material. This value is important both for board-placement and the working environment performance. A normal Tg is considered to be 130°C, medium Tg is considered to be 150°C, and high Tg is considered to be 170°C and above.
Td
This is a measure of deterioration of the material. The analysis method measures the time when 5% of the material is lost with weight. This creates a breach of reliability and delamination. High reliability requires “PCB Td ≥340°C”.
Coefficient of Thermal Expansion (CTE)
It is the expansion rate during heating of the material. The CTE of a material is usually much higher than that of copper, which can cause coupling issues when the PCB is heated.
Halogen Material (HalogenFree)
It refers to the materials that do not contain fluorine, chlorine or bromine. Halogen-containing materials produce toxic gases when burning. Therefore, halogen-free material may have to be preferred according to the application.
CAF (Anti-CAF Material)
CAFs are metal filaments that can grow from copper through perforated coating along glass fibers embedded in PCB materials. Eventually, they can create a short / open circuit that negatively affects the reliability of products in the field. Anti-CAF material should be preferred to prevent this problem.
