A high temperature circuit board is generally defined as an instance where the glass transition temperature (or Tg) is greater than 170°C. The glass transition temperature is one of the most important properties of any epoxy, representing the temperature where the polymer transition from a hard material to a soft one. High temperature PCBs should aim for a continuous thermal load around 25°C below the Tg. 

When PCB manufacturers design high temperature circuit boards, they must be aware of two things: heat distribution, and materials that are able to withstand extreme heat. Because materials play such a critical role in this type of PCB design, the process is also known as High-Tg PCB manufacturing, and requires expertise in thermal design.