Signal integrity or SI is a measure of the quality of an electrical signal. When considering signals routed across a PCB, the goal is to deliver a signal in an “unimpaired” condition from output source to destination.
Some of the main issues that impair signal integrity are:
Crosstalk: Higher interconnect density has led to each net having signals that are closer together, thus leading to increased coupling capacitance between neighboring nets. As circuits shrink, problems from reduced noise immunity increase.
Ringing: happens when a signal causes the parasitic capacitances and inductances in the circuit to resonate at their characteristic frequency. Ringing can cause unwanted electromagnetic radiation to be emitted and can delay the arrival of the signal at it's destination.
Overshoot/Undershoot: Is the transient signal level beyond/in between the steady state level. A circuit is designed to minimize rise time while containing distortion of the signal within acceptable limits. Overshoot represents a distortion of the signal. In circuit design, the goals of minimizing overshoot and of decreasing circuit rise time can conflict. Overshoot often is associated with settling time, how long it takes for the output to reach steady state; see step response.
Signal Loss: from the skin resistance in conductors is a factor of high frequency current and the dielectric material. Current tends to crowd to the edges of the conductor, hence the term “Skin Effect”. The easiest ways to increase a trace’s cross sectional perimeter is to widen it. The other way is to use materials with a lower “dielectric loss.”