The qualified PCB board has great requirements for the quality of PCBA processing. So, what are the parameters commonly used in the quality assessment of PCB multilayer circuit boards?
Glass transition temperature (Tg), thermal expansion coefficient (CTE), PCB decomposition temperature (Td), heat resistance, electrical properties and water absorption of PCB are commonly used to evaluate the quality of PCB board. Next, let us explain the glass transition temperature and thermal expansion coefficient for you in detail:
Glass transition temperature (Tg)
Copper clad laminate at a certain temperature, the substrate is hard and brittle, called glassy state: above this temperature, the substrate becomes softer and the mechanical strength becomes significantly lower. The critical temperature that determines the properties of the material is called the glass transition temperature.
If the temperature of Tg is too low, PCB will be deformed and components will be damaged at high temperature.
II. Coefficient of thermal expansion (CTE)
CTE quantitatively describes the degree of expansion of the material after heating. CTE refers to the elongation of a material per unit length for every 1 ℃ increase in ambient temperature. Lead-free welding requires a lower coefficient of thermal expansion of PCB board because of its high welding temperature. Especially for the multilayer PCB circuit board, the CTE in the Z direction has a great influence on the layer durability of the metallized hole. Especially in many times of welding or repair, after many times of expansion and shrinkage, the metallized hole layer will be broken.
