What FR-4 is
FR-4 is a woven glass fabric impregnated with epoxy resin, laminated under heat and pressure with copper foil on one or both sides. The glass style controls thickness and mechanical stability; the resin content controls flow, dielectric constant and how the laminate behaves when drilled.
Three properties that matter
- Tg (glass transition temperature) — where the resin softens. Standard grades sit around 130–140 °C, high-Tg grades above 170 °C.
- CTE (coefficient of thermal expansion) — the z-axis expansion during reflow. Thick boards with many layers accumulate more stress at the plated barrels.
- Dielectric constant and loss — adequate for most digital designs, but the reason RF work moves to specialised laminates.
What to put in the fabrication note
State the laminate grade or Tg class, the board thickness, the copper weight on each layer and the surface finish. If the design depends on impedance control, add the target impedance and reference layer. With those four items the fabricator can pick a build it can laminate repeatably instead of reverse-engineering the intent.
Where FR-4 stops
When the operating temperature exceeds what the resin tolerates, or when insertion loss at the target frequency dominates the loss budget, FR-4 is the wrong substrate. That is the point to evaluate metal-core, ceramic or high-frequency laminates.
