Board Materials

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Board Material Values

The material the printed wiring board is fabricated with determines its dielectric constant.
The dielectric constant in turn determines the time in which signals propagate over the board [Propagation velocity].

Permittivity [dielectric constant] is a measure of the ability to support an electrostatic field related to capacitance. The units [E_r] are Farads/meter.
The numbers for Printed Wiring Boards [PWB] varies all over the place, and seems to be hard to control.
How ever for any particular board material, E_r will be lower for top traces [Microstrip] and higher for traces embedded [Stripline] within the board material.
Signal Velocity [V_p] = C / [E_r]^1/2. 'C' is a constant at 30cm/ns.

Signal Rise Time vs Trace Length

The graphic above shows how the Board Material's Permittivity and the Circuit's Rise Time effect the maximum allowable trace length.
The blue vertical line assumes a rise time of 1.1nS, while the horizontal lines assume a particular board material [Orange for FR4, and Purple for Polyimide].
The graph indicates a trace length which exceeds Length > t_r / [ 6 x t _pr ]. The worst case trace length which must be terminated exceeds Length > t_r / [ 2 x t _pr ].
The difference between the two equations will relate to the Q of the circuit. The Q of the circuit is defined by the following calculation:
     Q = (L/C)^1/2/ R_s
     Frequency_Ring = 1 / (2 * 3.1415 *(LC)^1/2)
     Voltage Overshoot = V*e^{-3.1415/(4Q2-1)1/2}

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