Having a good understanding of different PCB material properties can certainly ease the task of choosing a material for a given set of circuit performance requirements. What better place to learn more about key PCB material parameters than starting with a PCB product data sheet?
Figure 1 is an example of a data sheet for a high-frequency PCB material. It lists numerous material properties, typical values for those properties, conditions under which those values were determined and even applicable measurement methods. The first two parameters pertain to the relative dielectric constant, ε r. Two different values are given through the thickness or z direction of the material, process and design values. These values differ and are determined in different ways. Process ε r is measured for raw substrate material clamped in a fixture described in test method IPC-TM-650 22.214.171.124, a test procedure established by IPC.® While this is an excellent gauge for PCB material process control and quality assurance, it has several attributes, such as entrapped air, which are not representative of the material’s use in a PCB application. The design ε r value is better suited as a reference for circuit design and modeling. It is determined by fabricating a microstrip transmission line of precisely controlled dimensions on a material under test.
Next on the data sheet is dissipation factor or tangent delta (tan δ), a parameter mainly responsible for dielectric losses in high-frequency circuits. It is determined by means of the same test method as used for process ε r, although it is often listed with multiple values for different test frequencies. In general, a PCB material’s dissipation factor will increase with increasing frequency; knowing values at different frequencies provides insight into the broadband loss performance of that material.
Since high-frequency circuits must often perform across a range of operating temperatures, a