Signal Latency
Time required for an electromagnetic signal to travel through a unit length of a transmission line on a printed circuit board governs the maximum operating speed of high-speed digital systems. This transmission characteristic, called propagation delay, is determined by the effective dielectric constant of the insulating material surrounding the copper trace. Designers use this metric to calculate trace lengths for differential pairs and synchronous buses where timing margins are extremely tight.
The measurement is expressed in picoseconds per inch or nanoseconds per meter and does not depend on the amplitude of the signal.
Material Influence
Dielectric properties of the laminate substrate are the primary driver of signal latency. Signals travel faster in air than in solid board materials, so a trace routed on an outer layer experiences a shorter propagation delay than an identical trace embedded in an inner layer. The presence of fiberglass reinforcement also increases the delay because glass has a higher dielectric constant than the surrounding resin.
Specifying laminates with low dielectric constants is the most effective way to reduce delay times across the board.
Design Strategy
Timing matching requires precise routing to ensure that signals on parallel lines arrive at their destination simultaneously. If one trace is longer or passes through a region with a different dielectric constant, the resulting skew can cause errors in data recovery.
