I NTRODUCTION OF PCB MATERIALS
The materials used to make PCBs are very important. And because the price of different material can vary from 10% to 100%, so the price differences of PCBs made by different materials can be huge! But as we said before there is no "free lunch", usually low cost material come with low quality. In this section different PCB materials will be introduced.
1. Glass transition temperature Tg: A high Tg is very important for the PCBs to guard against barrel cracking and pad fracture during soldering operation
2. Coefficient of thermal expansion (Tce): SMD assembly process exposes the printed wiring assembly to more temperatur e shocks than typical through-hole process. At the same time, the increase in lead density cause the designer to use more and more 2 layers, making the board more susceptible to the problems concerned with the base materials ...Tce. This can be a particular problem with regard to Z-axis expansion of the material since this induces stress in the copper-plated holes, and becomes a reliability concern.
Dissipation factor: A measure index of the tendency of an insulating material to absorb some of the ac energy from electromagnetic field passing through it. Low values are important for RF application, but relatively unimportant for logical applications.
Electrical breakdown voltage DBV: The voltage per unite th ickness of an insulator at which an arc may develop through insulator.
Water absorption factor WA: The amount of water an insulating material may absorb when subjected to high relative humidity, expressed as a percent of total weight. Absorbed water increa ses relative dielectric constant as well as reduces Electrical breakdown voltage .
There are 6 major different materials--- FR1 FR2, FR3, FR4,CEM-1, CEM-3--- used to make PCBs .