The flame retardant properties of PCBs (printed circuit boards) are mainly achieved by flame retardants in the substrate materials to ensure that flame spread is suppressed at high temperatures or short circuits.
★ Flame retardant standards and levels
PCB flame retardancy generally follows UL94 standards, and common levels include:
📍UL94 V-0：The highest flame retardant level, the flame will be extinguished within 10 seconds without melted droplets.
📍UL94V-1: The flame is extinguished within 30 seconds without droplets.
📍UL94V-2: The flame goes out within 30 seconds, but a small amount of melted droplets are allowed.
📍UL94HB: Only slow horizontal combustion speed is required.
★ Types and principles of flame retardants
halogen flame retardant (containing bromine/chlorine)
Common substances
Polybrominated biphenyls (PBB), polybrominated diphenyl ethers (PBDE), tetrabromobisphenol A (TBBPA).
Principle of flame retardant
Hydrogen halide radicals (HBr/HCl) are released at high temperatures, trapping·OH and·H radicals in the combustion chain reaction, reducing flame energy, promoting the formation of a carbon layer on the substrate surface, and isolating oxygen. High flame retardant efficiency and low cost. However, toxic smoke (dioxins) is produced during combustion, which pollutes the environment, and its use has been restricted by EU RoHS and other regulations.
📍Phosphorous flame retardant
Common substances
Phosphate esters (such as TPP), ammonium polyphosphate (APP), red phosphorus.
Principle of flame retardant
High temperature decomposition produces polyphosphoric acid, which promotes the dehydration of the substrate into charcoal and forms an insulation layer. Release PO·free radicals and inhibit the flame chain reaction. Low smoke, low toxicity, and good environmental protection, but may reduce the thermal stability of the material and have strong moisture absorption.
📍Nitrogen flame retardant
Principle of flame retardant
Decomposition absorbs heat at high temperatures, reducing the surface temperature of the material. Produce gases such as NH and N ˇ to dilute the oxygen concentration. It is non-toxic and non-corrosive, and is often used in conjunction with phosphorus, but the efficiency of using it alone is low.
📍Inorganic flame retardant
Common substances
Aluminum hydroxide (ATH), magnesium hydroxide (MDH), zinc borate.
Principle of flame retardant
ATH (decomposition temperature ≈200℃) and MDH (decomposition temperature ≈300℃) decompose and absorb heat, reducing the substrate temperature. Dilute the oxygen and form a protective layer. It is non-toxic, smoke-suppressing, and low-cost, but the addition amount needs to reach 40%-60%, which may affect the mechanical properties and dielectric properties of the PCB.
★ Flame retardant application of PCB substrate
📍FR-4 epoxy resin substrate
Traditionally brominated epoxy resin (containing TBBPA) is used.
Halogen free alternatives: Use phosphorus/nitrogen flame retardants, or add aluminum hydroxide.
📍High-frequency materials (such as PTFE, ceramic-filled substrates)
It usually relies on the high heat resistance of the material itself, or adds a small amount of phosphorus/nitrogen flame retardant.
Flexible PCB (polyimide substrate)
Commonly used phosphorus flame retardants or coating treatments.
★ Halogen free flame retardant
EU RoHS, REACH and other regulations restrict the use of halogens (requiring chlorine/bromine content of <900ppm).
Phosphorus-nitrogen synergistic system: Improve flame retardant efficiency.
Nanocomposite technology: such as layered silicate (montmorillonite) improves the strength of the carbon layer.
Bio-based flame retardants: Research on environmentally friendly materials such as phytic acid and chitosan.
Reprinted from Weixin Official Accounts-Home of PCB Engineers
