...
Changing layers is typically unavoidable when routing on a 4-layer PCB. As depicted below, transitioning from Layer 1 to Layer 4 (or vice versa) occurs in the midst of source and sink devices. In such cases, without a bypass capacitor to facilitate high-frequency signal flows, the return current continues to flow through the ground plane. Consequently, the combined current loop area (A2 + A3) significantly increases compared to the area A shown in the preceding figure. This expansion results in heightened electromagnetic field strength and an increased potential for electromagnetic interference.
...
One solution is to incorporate The simplest way to mitigate this issue is to relocate the via closer to either the source or the sink. As depicted in the figure below, the via is repositioned near the sink. This adjustment significantly reduces the combined current loop area (A4 + A5), thereby ensuring that the electromagnetic field remains at a lower level.
...
If relocating the via is not feasible, mitigating this issue can be achieved by incorporating a bypass capacitor near the transition point, where one end connects to the ground plane and the other end connects to the power plane. As depicted below, the return current flows through the bypass capacitor back to the ground plane. This arrangement ensures that the current loop area (A4 A6 + A5A7) remains nearly identical to area A in the preceding figure, thereby maintaining a consistent electromagnetic field level.
...
Another solution involves adding a ground trace adjacent to the original net on Layer 4, providing a path for the return current. It's essential to terminate the ground trace with ground vias at both ends to ensure proper grounding and minimize impedance.
...