IPC Certified Interconnect Designer (CID) Practice Exam

In a four-layer board, the most important factor for placing critical nets is giving them a short, low-impedance return path and a stable reference, which is achieved when you route them on an inner layer that sits next to a continuous plane. Layer 2 is commonly used as a reference plane (usually ground) or, if routing resources demand, as an inner signal layer. Layer 3 then becomes the other interior option: it can host a power plane (to support decoupling and clean power delivery) or serve as a second inner signal layer if you need more routing density while still maintaining impedance control. This arrangement helps maintain controlled impedance for traces adjacent to the planes, reduces loop area for high-speed signals, and improves EMI/shielding and crosstalk performance. If you put critical nets on Layer 3, Layer 2 should still be a solid plane to ensure a strong, nearby return path. The takeaway is that inner layers next to planes are favored for critical nets to achieve reliable impedance control and return-path integrity, while outer layers are typically reserved for less sensitive routing.