Board Materials

[Series Termination] [Parallel Termination] [Thevenin Termination]
[AC Termination] [Differential Termination] [Termination Examples]
[PWB Materials] [Signal Reflection Calculation] [PWB Terms]
[PWB Info] [Logic Design Page]

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].

Printed Wiring Board Characteristics
Material Permittivity Propagation velocity
FR4 Outer trace2.8 - 4.5141 - 179mm/nS
FR4 Inner trace4.5141mm/nS
Rogers 4003 3.38---
PTFE 2.6---
GETEK3.8 - 4.2---
Nelco 4000-80003.5 - 4.4---

Permittivity [dielectric constant] is a measure of the ability to support an electrostatic field related to capacitance. The units [Er] 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, Er will be lower for top traces [Microstrip] and higher for traces embedded [Stripline] within the board material.
Signal Velocity [Vp] = C / [Er]1/2. 'C' is a constant at 30cm/ns.

The Board Materials Permittivity vs. Signal Rise Time determines the maximum line length
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 > tr / [ 6 x t pr ]. The worst case trace length which must be terminated exceeds Length > tr / [ 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/ Rs
     FrequencyRing = 1 / (2 * 3.1415 *(LC)1/2)
     Voltage Overshoot = V*e-3.1415/(4Q2-1)1/2

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Modified 6/13/15
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