FR4

What it is

FR-4 (also written FR4) is the substrate material used for most rigid printed circuit boards. It's a composite of woven fibreglass cloth impregnated with an epoxy resin, cured into a flat, hard sheet. Copper foil is laminated to one or both sides; copper-clad FR-4 laminate is the starting material for many conventional rigid PCBs.

The name comes from the NEMA FR-4 designation: “FR” means flame retardant, while “4” identifies the glass-reinforced epoxy laminate category. FR-4 is a material family rather than a single precisely-defined formulation; modern PCB laminates are commonly specified using IPC-4101 in practice. There are older and cheaper grades (FR-1, FR-2 — paper phenolic) that you'll occasionally see on very low-cost consumer boards, but FR-4 is the workhorse. Aluminium-backed, ceramic, PTFE and CEM laminates also exist for specific applications.

How a PCB is built up around it

A basic single-layer PCB is one sheet of FR-4 with a thin copper foil (typically 1 oz/ft² ≈ 35 μm) laminated to one face. Etching the copper leaves behind the tracks and pads that carry current. Double-sided boards have copper on both faces, with plated through-holes or vias providing connections between the layers where required.

Multilayer boards combine copper-clad FR-4 cores with sheets of uncured resin-and-glass “prepreg”. During lamination, heat and pressure cure the prepreg and bond the layers together. A typical four-layer board has two copper-clad cores separated by prepreg, with additional prepreg and copper foil forming the outer layers. Six-, eight- and higher-layer boards use the same basic principle but can have many different stackups.

Why it's the default

• Rigidity + dimensional stability.It doesn't warp much with temperature and it keeps its shape over time.
• Adequate electrical properties. Its dielectric properties are suitable for many digital and lower-frequency applications, though dielectric constant and loss vary substantially with formulation, glass content, frequency, and measurement method — so any single “the Dk of FR-4 is X” number you see quoted should be treated as an approximate typical value, not a specification.
• Cheap and universally available.Every PCB fab in the world stocks it, and the economies of scale make anything else noticeably more expensive.
• Flame retardant. FR-4 formulations are designed to provide flame-retardant properties. The exact flammability performance depends on the particular laminate and its applicable certification, such as a UL 94 rating.
• Easy to work with. It cuts, drills, and mills predictably, and accepts standard soldering processes without surprises.

Where it falls short

• Higher-frequency behaviour. At higher frequencies FR-4's relatively high dielectric loss and variation in dielectric properties can become significant. Glass-weave effects can also introduce local variations in effective dielectric constant and impedance. FR-4 is used in plenty of designs into the GHz range, but for demanding RF and high-speed work specialised low-loss laminates (Rogers, Isola, etc.) may be preferred.
• Temperature. FR-4 is available in a range of Tg (glass transition temperature) grades. Traditional FR-4 may have a Tg around 130 °C, while high-Tg formulations can exceed 170 °C. Tg marks a significant change in the epoxy's mechanical behaviour rather than a simple melting point. For demanding thermal cycling or high-temperature work, the laminate's Tg, decomposition temperature (Td), thermal expansion and other properties all need to be considered together.
• Flexibility. It is relatively rigid and isn't suitable for repeated flexing. Flexible circuits generally use polyimide or another flexible substrate.

Fibreglass dust

A practical note if you ever hand-drill or cut FR-4: the dust is fine glass fibre and epoxy — an irritant to lungs, skin and eyes. Avoid generating dust where possible, use local extraction, and wear appropriate respiratory, eye and skin protection (a proper particulate respirator rather than a nuisance dust mask if you're producing much of it). Wet cutting also reduces airborne dust. Fab-house drilling relies on strong dust extraction, and sometimes wet or other dust-control techniques, for exactly this reason.