Regardless of the laminated structure of the multilayer PCB, the final product is a laminated structure of copper foil and dielectric. The materials that affect circuit performance and process performance are mainly dielectric materials. Therefore, the choice of PCB board is mainly to choose dielectric materials, including prepregs and core boards. So what should be paid attention to when choosing?
1. Glass transition temperature (Tg)
Tg is a unique property of polymers, a critical temperature that determines material properties, and a key parameter for selecting substrate materials. The temperature of the PCB exceeds Tg, and the thermal expansion coefficient becomes larger.
According to the Tg temperature, PCB boards are generally divided into low Tg, medium Tg and high Tg boards. In the industry, boards with a Tg around 135°C are usually classified as low-Tg boards; boards with a Tg around 150°C are classified as medium-Tg boards; and boards with a Tg around 170°C are classified as high-Tg boards.
If there are many pressing times during PCB processing (more than 1 time), or there are many PCB layers (more than 14 layers), or the soldering temperature is high (>230℃), or the working temperature is high (more than 100℃), or the soldering thermal stress is large (Such as wave soldering), high Tg plates should be selected.
2. Coefficient of Thermal Expansion (CTE)
The coefficient of thermal expansion is related to the reliability of welding and use. The selection principle is to be as consistent with the expansion coefficient of Cu as possible to reduce thermal deformation (dynamic deformation) during welding).
3. Heat resistance
Heat resistance mainly considers the ability to withstand the soldering temperature and the number of soldering times. Usually, the actual welding test is carried out with slightly stricter process conditions than normal welding. It can also be selected according to performance indicators such as Td (temperature at 5% weight loss during heating), T260, and T288 (thermal cracking time).