Glass Transition Temperature (Tg) is an important concept in PCB manufacturing, which refers to the temperature at which a material transitions from a glassy to a rubbery state. This transition involves a change in the form of molecular motion within the material and has a significant effect on the performance of the PCB. Below is a detailed description of the properties and effects of glass transition temperature:

Properties:

Temperature Stability:
Below Tg, the material exhibits rigid and hard brittle, glass-like properties. Above Tg, the material exhibits a soft and bendable rubbery state.
Molecular Motion:
Below Tg, the motion of the molecular links in polymers is mostly frozen, whereas above Tg, the molecular links have more degrees of freedom to move.
Coefficient of Thermal Expansion (CTE):
Tg has a significant effect on the coefficient of thermal expansion of a material. Typically above Tg, the CTE will be much higher than below Tg, meaning that the material has different dimensional stability at different temperatures.
Mechanical Properties:
Below Tg, the material has a high modulus of elasticity and exhibits good mechanical properties. Above Tg, the modulus of elasticity of the material decreases sharply.
Electrical Insulation Properties:
Tg also affects the electrical insulation properties of the material. Above Tg, the dielectric constant and dielectric loss of the material may be affected due to increased molecular motion.
Chemical Stability:
Below Tg, the chemical stability of the material is good. Above Tg, the material may be more susceptible to chemical changes due to increased molecular motion.

Role:

Determines the Operating Ambient Temperature of the PCB:
The Tg value determines at what temperature the PCB substrate becomes mechanically unstable. Therefore, the Tg value should be higher than the PCB’s expected operating temperature to ensure long-term reliability.
Affects the Mechanical Strength and Stability of the PCB:
PCBs with high Tg values have better stability and mechanical strength in high temperature environments, which is especially important for high temperature application environments such as automotive and industrial equipment.
Ensures Circuit Integrity:
In high load or high frequency applications, electronic components may generate heat. PCBs with high Tg values maintain circuit integrity by preventing deformation due to elevated temperatures.
Improve Long-term Product Stability:
Electronic devices are often required to operate for long periods of time, so PCB stability is critical. A high Tg value helps to minimize mechanical and thermal stresses on the board during long-term use.
Affects PCB Manufacturing Process:
During the PCB manufacturing process, such as lamination, drilling, copper plating and other steps, the Tg value needs to be considered to ensure the processability of the material.
Affects the Thermal Design of the PCB:
In thermal design, Tg value is an important parameter for selecting the right PCB material, which affects the heat transfer and distribution in the PCB.
It is Related to the Reliability and Lifetime of the PCB:
PCB with high Tg values usually have better reliability and longer lifetime, especially in applications with high temperature, high humidity or high mechanical stress.

In summary, the glass transition temperature is a key parameter that must be considered in PCB design and manufacturing, and it directly affects the performance and application range of PCBs. Choosing the right Tg value for the substrate is critical to ensure the reliability and long-term stability of electronic products.