FR4 is the most common material for printed circuit boards. This guide covers its key specifications, typical stack-ups, and when to consider alternative materials.
FR4 is a composite material made from woven fiberglass cloth impregnated with an epoxy resin binder. The "FR" stands for Flame Retardant, and it meets the UL94V-0 standard for flammability.
Important electrical and mechanical properties define FR4 performance.
The typical Dk for FR4 ranges from 4.2 to 4.8 at 1 MHz. This value varies with frequency, temperature, and resin content.
FR4 has a dissipation factor around 0.02 at 1 MHz, indicating signal loss. This loss increases significantly at higher frequencies.
FR4 has a Glass Transition Temperature (Tg) typically between 130°C and 180°C. Higher Tg grades offer better thermal stability. It provides good mechanical strength and rigidity.
Stack-up design is critical for signal integrity and manufacturability.
A simple stack-up with a core and prepreg, suitable for low-complexity designs.
A common configuration offering dedicated power and ground planes, improving EMI performance and signal routing.
Used for complex, high-speed designs, providing multiple signal layers sandwiched between reference planes for controlled impedance.
While versatile, FR4 is not ideal for every application.
For frequencies above 1-2 GHz, materials like Rogers (RO4000 series) with lower and more stable Dk/Df are preferred to minimize signal loss.
Applications like automotive under-hood or down-hole drilling may require polyimide or ceramic-based substrates for higher thermal endurance.
Polyimide films (e.g., Kapton) are used instead of rigid FR4 for flexible sections of a board.
Metal-core PCBs (MCPCBs) using aluminum or copper substrates are chosen for high-power LED lighting or power converters.
FR4 remains the workhorse material for most PCB applications due to its balance of cost, performance, and manufacturability. Understanding its specifications and limitations is key to selecting the right material. For high-frequency, high-temperature, or specialized mechanical requirements, alternative materials should be evaluated during the design phase.
