Meaning
The distance separating conductive copper sheets in a multi-layer circuit board determines the capacitive and inductive coupling between traces. Optimizing the layer dielectric thickness is a primary method for controlling the characteristic impedance of high-speed transmission lines and minimizing crosstalk. This parameter is specified in the board fabrication stackup document.
Impedance Control
Impedance calculations for microstrip and stripline traces rely heavily on this spacing. If the dielectric is too thin, the capacitance to the reference plane increases, which lowers the trace impedance. To maintain the target impedance, the trace width must be adjusted accordingly.
This relationship means that any change in the board stackup requires a redesign of the trace geometry.
Signal Attenuation
Thin dielectric layers also increase the risks of signal attenuation and dielectric breakdown under high-voltage conditions. In RF boards, the dielectric material and its thickness affect the propagation velocity and the overall losses of the signal. Choosing a thicker dielectric or a material with a lower loss tangent can improve signal strength.
This choice is important for high-frequency or long-range wireless applications. It directly impacts the efficiency of the antenna feedlines and the sensitivity of the receiver.
Manufacturing Tolerance
Fabrication tolerances must be carefully managed because variations in the thickness of the prepreg layers can cause the trace impedance to deviate from the design value. Manufacturers provide stackup sheets that specify the expected tolerances for each material layer. These sheets must be reviewed during the design phase.
