Lesson · Problem 7
Give Current a Return Path
Return current under the trace
You should be able to
- Explain why return current follows the ground under a fast trace.
- Route around a void without treating a long detour as free.
Grounding / Return Paths · Routing
What it is
A ground zone is a region of copper tied to ground, not a visual fill. The return path is the route that return current actually takes in that copper. A void is a hole in the pour. Routing a fast net across the void makes the loop larger even when the signal trace itself looks short.
Why this matters
Current that leaves on a signal trace comes back on the lowest-inductance copper, which for a fast edge is the ground directly under the trace. A gap in that pour forces the return to detour.
What you are building
A trace over a ground zone, and a version of that zone with a void.
Prerequisites
This lesson uses trace, net, routed length, copper cross-section. It introduces return path, ground zone, void, loop area.
Component guide
What the parts are. The requirements panel is still the list that is graded.
Return path
- What it does
- The copper the current uses to come back, usually ground under the signal.
- Why it is here
- A forward trace with no nearby return makes a large loop.
- Symbol
- Ground copper on the layer the rule names.
- Beginner mistake
- A ground symbol on the schematic with a slot in the pour under the trace.
- What an engineer checks
- Which nets must stay over the ground zone, and what a void does to the score.
Interview lens
“Where does the return go?” is the question. The answer is the copper under the trace, not the ground symbol.
What PCBGrade measures
Whether the trace stays over the ground zone and how the void changes that. It is geometry, not a field solver.
Where this shows up
A slot under a trace, cut to “separate” something, is a classic source of noise.
Terms
- Return path
- The copper the current uses to get back to the source.
- Ground plane or zone
- A sheet of ground copper. A zone is the pour you draw; it is not automatic.
- Void
- A kept-clear hole in that copper.
What is happening electrically
Current that goes out on a trace comes back. At DC it may spread through the ground copper. A fast edge returns on the lowest-inductance path, which is the ground directly under the trace. A hole in that copper forces the return to travel around the hole. The loop area grows even when the forward trace looks short.
Why the geometry matters
Draw the ground zone so the fast nets sit over continuous copper. The void is a kept-clear opening. Routing across it is the mistake the figure shows. Routing around it is expected, and the new path can still be too long.
How an engineer reasons
Look at the forward trace and ask where the return is allowed to flow. If the pour is on the back, the front trace still needs solid ground underneath, not a gap. Keep the detour modest so the length limit still holds.
Worked example
Three fast nets stay over the back-side ground zone and out of the void. The zone has a minimum area. Connectivity and design-rule checks still apply.
Good and bad
A trace that stays on the pour has a short forward path and a short return. A trace that cuts the corner across the void has a short forward path and a long return. A trace that walks the long way around the board avoids the void and can fail the length limit anyway.
Common mistake
Crossing the void, or taking a detour so long that the routed length fails the limit. A modest detour around the void is expected.
Where this rule stops
“Under the trace” is the right picture for a trace over a nearby ground plane. It is not a claim about every frequency, and a slow DC net is less sensitive to the same gap. This challenge grades the fast nets, the void, and the length. It does not compute inductance.
Before the challenge
The challenge has a ground zone and a void. Keep the fast nets over the pour. A detour around the void is expected, and it can still be too long.
Ready for the challenge
You can point at the return and say what a gap in that copper does.
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