Relative Permittivity
Material property value defines the relative permittivity of a laminate measured perpendicular to the plane of the circuit board. The z-axis dielectric constant determines the capacitance between parallel copper layers and the characteristic impedance of stripline circuits. Designers rely on this value to calculate the trace widths and dielectric thicknesses required for specific electrical targets.
Most laminates exhibit an anisotropic nature, meaning this value differs from the permittivity measured in the horizontal directions.
Permittivity Stability
Dielectric properties of the resin and glass system change as the operating frequency of the circuit increases. A stable z-axis dielectric constant is necessary for wideband applications to avoid shifts in signal timing and impedance. Resin systems are formulated to maintain a flat response across the gigahertz range.
Testing at multiple frequency points allows engineers to select the best material for their specific data rate.
Temperature Sensitivity
Thermal fluctuations cause the physical dimensions of the board to change, which indirectly alters the electrical characteristics of the substrate. As the board expands in thickness, the z-axis dielectric constant typically decreases due to the change in resin density. This variation can detune sensitive RF filters or cause impedance mismatches in high speed links.
Advanced materials with high glass transition temperatures are used to minimize these thermal effects in harsh environments.
