Issue link: https://iconnect007.uberflip.com/i/1533085
34 DESIGN007 MAGAZINE I MARCH 2025 • Buses and associated control signals should all be routed on the same layer or a symmetric layer of the stackup. However, if they are routed on different layers then they must be routed to matched delay. • e propagation delay of a serpentine trace is less than the delay through an equivalent-length straight trace. • e clock (or strobe) signal in source syn- chronous interfaces should have the lon- gest delay within the bus group. • Placing a copper pour on one or both sides of a trace effectively creates a coplanar waveguide, which can reduce the imped- ance at the coupling point on the trace. • Ground pours may be effective on high- impedance analog two-layer boards but do not significantly reduce crosstalk on low- impedance digital multilayer boards since the traces are closely coupled to the plane. • e PCB designer needs to be able to visu- alize the connectivity of the return current flow to avoid large loop areas that increase series inductance. • Each signal layer should be adjacent to, and closely coupled to, a reference plane, which creates a clear, uninterrupted return path. • Discontinuities tend to divert the return current increasing the loop area, induc- tance and delay. • Crosstalk can occur either between traces on the same layer or through broadside coupling between traces on adjacent layers. • Broadside coupling is harder to detect beca- use we typically focus on trace clearances. • Traces routed in parallel and broadside experience more crosstalk than those routed side-by-side due to the larger cou- pling area and tighter spacing. • Route adjacent signal layers orthogonally to each other in the stackup to minimize the coupling region. • Space unrelated critical trace segments by three times the trace width. DESIGN007 Resources Beyond Design columns by Barry Olney: "Intercon- nect Impedance," "To Pour or Not to Pour," "Return Path Optimization" Barry Olney is managing director of In-Circuit Design Pty Ltd (iCD), Australia, a PCB design service bureau that specializes in board-level simulation. The company developed the iCD Design Integrity software incorporating the iCD Stackup, PDN, and CPW Planner. The software can be downloaded at www. icd.com.au. To read past columns, click here.