FR-4 is not one material — it is a family of woven-glass epoxy laminates spanning a range of glass-transition temperatures (glass transition temperature), resin systems and filler chemistries. The Tg number on the datasheet is the temperature where the resin shifts from rigid to rubbery. Below Tg the laminate is stiff and dimensionally stable; above it, the resin softens and the board's z-axis expansion climbs sharply.
What Tg actually changes
Two things. First, thermal margin during assembly: lead-free reflow peaks around 245–260 °C, and a board spends several minutes above 200 °C. Second, z-axis expansion: above Tg the resin expands far faster, which stretches the plated barrel of every via. More expansion means more stress on the copper barrel — and via cracking under thermal cycling is a classic failure of underspecified laminate, not of plating.
When standard FR-4 (Tg 130–140 °C) is fine
- General multilayer consumer and industrial boards, 1.6 mm and thinner.
- Boards assembled with leaded processes or moderate reflow profiles.
- Products without wide thermal-cycling requirements.
Standard FR-4 has the lowest cost and the widest availability, with Dk around 4.2–4.5. If the board is thin, runs cool and is not cycled hard, paying for high-Tg laminate buys nothing.
When the upgrade to Tg 170–180 °C earns its price
- Lead-free reflow on thicker boards — a thick board's inner layers lag the surface temperature, so the resin spends longer near Tg.
- Thermal-cycling environments (automotive, outdoor, power electronics).
- Boards with high aspect-ratio vias, where z-axis expansion loads the barrel.
- HDI and multiple lamination cycles — every additional press is another thermal excursion.
The number people forget: decomposition temperature
Tg is reversible — the resin softens and stiffens again. What destroys a board is Td, the decomposition temperature, where the resin chemically breaks down. Reputable laminates hold Td above 300 °C, well past any reflow profile. The practical failure mode from over-temperature assembly is not melted laminate; it is delamination from moisture, so bake requirements matter for stored boards.
Our stock position
We stock standard FR-4, high-Tg FR-4 (Tg 170–180 °C) and halogen-free variants compliant with IEC 61249-2-21, alongside the RF and flex grades listed in the laminate guide. Specifying a stocked grade is the single easiest lead-time saving — an exotic laminate that must be ordered turns a quick-turn into a procurement wait.