I’ve observed numerous 2-layer PCBs with ground pours on both the top and bottom layers, prompting me to inquire about the rationale behind this practice. Wouldn’t it be more advantageous to utilize the top layer for power and signals, reserving the bottom layer for the ground? This approach could streamline routing and leverage the capacitance between the planes. What are your thoughts on this strategy?
Oh what a great question! How you approach this would depend on your circuit. Let’s say you have some 12 bit ADC’s in your circuit. They might have some millivolt signals going into them and in that case having ground pours all around helps isolate and shield those traces. On the other hand, if you had some 50 ohm single ended traces, you would want ground underneath them to create a transmission line, and would want to avoid ground pours being too close to the signal traces causing capacitive loading. Many of the 2 layer designs I’ve done had power on the bottom and ground pour on the top, but either way the power and ground form a capacitor and help alleviate noise from the board. Bottom line, think of what the signals are doing and the best environment for them prior to making those decisions.
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I will address your question from a different perspective. Designing 2-layer boards is a bad design and creates many issues with today’s technology. Perhaps for very low-speed signals or DC-based maybe. What we know currently is the effect of Energy moving into the space of the Live Trace and its return. Power is not return, but the lower potential, call it ground. So what’s important is the Energy moving in the dielectric (space) in a board. So to answer your question, we need to think what is your circuit. Some folks have ground under and poor power on top and others have power under poor ground on top to balance the to layers, and if the board is small, there is almost no interlayer capacitance because the separation might be very far away. So the poring just creates problems rather than solves them. but the reality is Energy moves and travels from point A to point be through space. This concept is well understood by Microwavewave engineers because we are used to dealing with waves, e, and H fields. Please play this video and well explained and will enlight more hw_introisledungeon1e_h_all_120 (youtube. com)
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