Common parameters used to evaluate PCB quality include glass transition temperature (Tg), coefficient of thermal expansion (CTE), PCB decomposition temperature (Td), heat resistance, electrical properties, and PCB moisture absorption. Below, we'll explain the glass transition temperature and coefficient of thermal expansion in detail:

1. Glass transition temperature (Tg)

Below a certain temperature, the copper-clad laminate substrate is hard and brittle, known as the glassy state. Above this temperature, the substrate softens and its mechanical strength significantly decreases. This critical temperature, which determines material properties, is called the glass transition temperature.

If the Tg is too low, the PCB will deform at high temperatures, potentially damaging components.

2. Coefficient of thermal expansion (CTE)

CTE quantitatively describes the degree of expansion of a material when heated. CTE is the length per unit length of material elongated for every 1°C increase in ambient temperature. Due to the high soldering temperatures required for lead-free soldering, PCB require a lower coefficient of thermal expansion. Especially for multi layer PCB, the Z-direction CTE significantly impacts the interlayer resistance of plated-through holes. Especially during multiple soldering or rework, the metallized hole layer will break after multiple expansion and contraction.