What Tg changes
Tg is the temperature where the resin changes from glassy to rubbery. Above it, expansion rises sharply and mechanical stiffness drops. In a lead-free reflow profile the board sees peaks well above 200 °C briefly, so the question is not whether the laminate softens, but whether it recovers without damage.
When the upgrade earns its cost
- Thick or high-layer boards — more dielectric and copper means more stored stress during reflow.
- Repeated assembly — double-sided builds and boards that go through selective solder afterwards see more thermal cycles.
- Large copper planes — heat spreads unevenly and local expansion differs from the rest of the board.
- Fine vias and small annular rings — barrel stress has less margin.
For a simple four-layer control board with one reflow pass, standard Tg is usually adequate. For a 12-layer backplane with heavy copper, the upgrade is cheap insurance.
What to specify
Give the fabricator the Tg class or laminate name, the stackup with copper weights, and the assembly profile if it is unusual. If the design is impedance controlled, the laminate change must be reflected in the trace geometry — a high-Tg material with a different Dk will not hold the same impedance at the same width.
