Lesson · Problem 11
Move to Four Layers
A stackup gives current a home
You should be able to
- Read a stackup and say which layer is the reference under a signal.
- Connect a ground pin to that plane with a via, not with a schematic symbol alone.
PCB Fundamentals · Grounding / Return Paths
What it is
A four-layer stack in this course has named copper layers. A ground plane on an inner layer is the return for the signals above it. Coverage is how much of that layer is actually ground copper. A via from a device ground pin into that plane is the connection; the symbol ground is not enough.
The usual teaching stack here is signal, ground, power, signal. A trace on L1 then faces ground on L2. Power on L3 is a supply, not a return for those signals. If you move the trace to L4, it faces power unless you give it a ground it can actually use. A via is how a pin reaches the plane. Drawing the ground symbol does not pour the layer.
Why this matters
Two layers run out of room once power, ground, and fast signals all want a continuous reference.
What you are building
A four-layer board whose signals on top face ground on the next copper layer.
Prerequisites
This lesson uses via, return path, ground zone, microcontroller, SWD, decoupling capacitor. It introduces stackup, reference plane, layer order, plane coverage.
Component guide
What the parts are. The requirements panel is still the list that is graded.
Stackup
- What it does
- The ordered copper layers. This course uses signal, ground, power, signal.
- Why it is here
- A trace on the top then faces ground on the next copper layer.
- Symbol
- Four named layers, not a thicker two-layer board.
- Constraints
- L2 is ground. L3 is power. A ground symbol does not pour a layer.
- Beginner mistake
- Pouring ground on the bottom while the signals look at a power shape.
- What an engineer checks
- The layer each net is allowed to use, in the requirements.
Interview lens
State the stack out loud: signal, ground, power, signal. Power is not the return for the top traces.
What PCBGrade measures
Layer use, ground coverage, and the via limit. A ground symbol is not a pour.
Where this shows up
Moving a trace to the bottom without a ground under it is how a “four-layer” board still misbehaves.
Terms
- Stackup
- The ordered list of copper and dielectric layers in the board.
- Reference layer
- The copper the signal uses as its return, usually the nearest ground. Engineers also call that sheet a reference plane.
What is happening electrically
A signal and its return are a pair of conductors. On two layers they fight power and ground for the same copper. A four-layer stack gives the return a sheet of its own. In the stack this course uses, L1 faces L2 ground, so a trace on top has a nearby return. L3 is power. It can be a reference only for a trace that actually faces it and can use it. L4 faces L3, not L2.
Why the geometry matters
Pour ground on L2 and keep that layer for ground. Via every ground pin into that pour. A ground symbol on the schematic does not fill the layer. A signal that jumps to L4 leaves the L2 sheet unless you give it a return it can use. Extra vias on a short net add length and a chance to change reference.
How an engineer reasons
Name the copper directly under the trace before you celebrate a short route. If that copper is ground and continuous, the return is local. If it is a power shape, or a gap, the return has to find ground somewhere else. Coverage is how much of L2 is actually that ground.
Worked example
Put the ground pour on L2 and keep that layer for ground. The next inner layer is for 3.3 V, not for signals. Via the MCU ground pins into L2. SWD should stay over that reference. The requirements name the coverage fraction and the via limit.
Good and bad
Top signals over an L2 ground pour, with MCU ground pins viaed into it, match the stack. Ground poured only on the bottom, while top signals look at a power shape, is a pour that does not serve those traces. The board can still be “four layers” and still miss the reference.
Common mistake
Pouring ground on the bottom while the fast traces jump to a layer that does not face it.
Where this rule stops
Signal, ground, power, signal is the teaching stack here, not the only legal stack in industry. Some boards put ground on both inner layers. The rule that survives is: know which copper the trace faces, and connect the pins to that copper with vias.
Before the challenge
The challenge names the stack and the coverage it wants. Pour ground on the reference layer and via the ground pins into it. A symbol is not a plane.
Ready for the challenge
You can put ground on L2 and via a ground pin into that layer.
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