Definition

Decomposition Temperature (Td) is the temperature at which a PCB laminate begins to chemically decompose, measured as the temperature at which 5% weight loss occurs (by TGA test). Td is always higher than Tg and represents a hard limit; exceeding Td causes permanent damage. Standard FR-4 has Td around 310-330°C, while high-performance materials may exceed 350°C. Td matters for lead-free assembly where peak reflow temperatures approach 260°C, leaving less margin than leaded assembly. Multiple reflow cycles cumulate thermal stress.