When the temperature rises to a certain area, the substrate will change from "glass state" to "rubber state", and the temperature at this time is called the glass transition temperature (TG) of the plate. That is to say, Tg is the highest temperature at which the base material remains rigid. Common PCB substrate materials not only produce softening, deformation, melting and other phenomena at high temperature, but also have a sharp decline in mechanical and electrical characteristics.
High Tg means high heat resistance. Generally, the board of Tg is more than 130 degrees, the high Tg is more than 170 degrees, and the medium Tg is more than 150 degrees. Generally, PCB with Tg (> 170 degrees) is called high Tg PCB. With the increase of Tg of the substrate, the heat resistance, moisture resistance, chemical resistance and stability of PCB will be improved. The higher the TG value, the better the temperature resistance of the sheet. Especially in the lead-free process, the application of high Tg is more.
With the rapid development of electronic industry, especially the electronic products represented by computers, they are developing towards high functionalization and multi-layer. It requires higher heat resistance of PCB substrate material as an important guarantee. The emergence and development of high density installation technology represented by SMT and CMT make PCB more and more inseparable from the support of high heat resistance of the substrate in the aspects of small aperture, fine wiring and thinning.