Intra-pair Trace Length Matching - Intra-pair Trace Length Matching - XAPP1392

• The differential signal and common mode signal in a microstrip or via have different propagation velocities due to the different effective dielectric constants of the surrounding materials seen by the signals.
• The propagation velocities of the differential signal and common mode signal differ in an inhomogeneous stripline because the core and pre-preg around the signal conductors have different dielectric constants and loss tangents. There are also different spatial distributions of resin and glass weave around the signal/GND conductors.
• Due to the different propagation velocity for the two modes, the skew on a microstrip should be compensated just after where the skew occurs, similar to an inhomogeneous stripine. Eliminating length compensation by routing bend/turning with arcs is preferred rather than compensating the skew with a single big loop or several small serpentines.
• Similarly, due to the different propagation velocity in a via, the skew should be compensated before a trace enters the via, even if both ends of the via are stripline with a similar dielectric constant. That is, the skew compensation should be made on the same layer as where the skew occurs.
• The skew on a homogeneous stripline can be compensated with reversed (mirrored) bends, a single big loop, or several small serpentines. When compensating skew with serpentines, maintain sufficient spacing between adjacent legs to avoid self-coupling between the segments. The goal is to maintain the impedance and validate the compensation structure with simulation.

One significant contributor to mode conversion is glass weave skew. Resin and woven glass fibers have different electrical properties in terms of dielectric constants and loss tangents. When one leg of a differential pair is routed on resin and the other is routed on glass fiber, skew is generated due to different propagation velocities. There has been significant industry research in this area that characterizes the impact of glass weave and identifies methods to mitigate its effect [10, 11, and 12]. Some commonly used methods to mitigate the weave skew effect are listed below. Appropriate methods should be picked based on the balance between cost and implementation effort to minimize the glass weave skew effect.

• Angled or zig-zag routing
• Jagged routing per differential pair pitch
• Rotating the panel
• Matching differential pair pitch to weave pitch
• Multi-ply with different weave pitches
• Advanced weave, for example, glass fiber with low dielectric constant (closer to the resin dielectric constant), tighter weave

Figure 1. Intra-pair Skew Examples