Beyond the glass transition temperature the z-axis expansion increases dramatically, and the more likely that damage might occur. The lower the number, the better. The thicker the PCB, the more susceptible it is to z-axis expansion faults. The lower the number the better, loss tangent measures the loss of signal energy. The lower the number the better.
Lower values allow faster signal propagation and permit thinner impedance controlled PCBs.
The lower the number, the better. The higher the number, the more that temperature rise increases Er. Laminate PPO epoxy resin structure with better behavior than FR-4. Very stable with temperature variations. Etch width may be limited.