Definition

Glass Transition Temperature (Tg) is the temperature at which an amorphous polymer transitions from a hard, glassy state to a soft, rubbery state. For PCB laminates, standard FR-4 has Tg of 130-140°C, mid-Tg is 150-160°C, and high-Tg is 170-180°C. Above Tg, the Z-axis CTE increases dramatically (from ~50 ppm/°C to ~250 ppm/°C), stressing plated through-holes. Lead-free assembly typically requires Tg ≥150°C to withstand higher reflow temperatures.