Glass transition temperature (Tg) is a critical thermal property of polymers and materials like PCB substrates. It represents the temperature at which a material transitions from a hard and relatively brittle “glassy” state into a softer, more pliable “rubbery” state.
Key Concepts of Glass Transition Temperature (Tg):

Behavioral Change:
Below Tg: The material is in a glassy, rigid state. It is hard and brittle, with limited molecular mobility.
Above Tg: The material becomes more flexible and rubbery as the molecular chains gain mobility.

Relevance to PCB Materials:
FR4 and Tg: Standard FR4 has a Tg typically in the range of 120°C to 130°C. High-Tg versions can have Tg values above 170°C, making them more suitable for applications that involve higher temperatures.
Thermal Management: Knowing the Tg is essential for ensuring that the PCB material will perform reliably under the operating conditions. Operating a PCB above its Tg can lead to mechanical failures, delamination, or even electrical failure.
Process Considerations: During the soldering process, the temperature can reach high levels. Using a material with an appropriate Tg ensures that the PCB substrate does not deform or lose its structural integrity.

Applications Based on Tg:
Consumer Electronics: Standard Tg materials (e.g., standard FR4) are sufficient for most consumer electronics, where operating temperatures do not typically exceed 100°C.
Automotive and Aerospace: High-Tg materials are preferred due to the higher operating temperatures encountered in these environments.
High-Power Electronics: Devices like power amplifiers, RF circuits, and LEDs may require substrates with higher Tg to manage heat dissipation effectively.

Glass Transition vs. Melting Temperature:
Tg vs. Tm: Tg should not be confused with the melting temperature (Tm). Tg is the temperature range where the polymer transitions from a glassy to a rubbery state, while Tm is the temperature at which a crystalline material melts.
Amorphous vs. Crystalline: Tg is a characteristic of amorphous (non-crystalline) regions of materials. Crystalline regions, if present, have a distinct melting point (Tm) rather than a Tg.

Why Tg Matters in Electronics:

Reliability: Operating above Tg can cause the material to soften, leading to potential mechanical failures, such as cracking or warping.
Dimensional Stability: Below Tg, materials maintain their shape and size. Above Tg, they may exhibit increased thermal expansion, leading to misalignment of components or circuit paths.
Soldering: During soldering, PCBs are exposed to high temperatures. Ensuring that the Tg is higher than the peak process temperature prevents damage to the PCB.

In summary, the glass transition temperature is a crucial factor in the selection of materials for PCBs and other applications where thermal performance is critical. Materials with higher Tg are used in environments with elevated temperatures to ensure reliability and longevity of the product.

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