PCB Stackup and Impedance Control
A PCB's stackup is the invisible backbone of the board's electrical behavior. The same schematic can give very different signal-integrity and EMC results with a different stackup. Especially in high-speed designs, leaving the stackup to chance invites hard-to-solve problems. This article covers the fundamentals of stackup planning and controlled impedance.
Why does the stackup matter?
The layer order determines which reference plane the signals flow over, the return current's path, the inter-plane capacitance and the overall EMC behavior. A good stackup offers the return current a short, uninterrupted path, both reducing radiation and improving signal integrity. That's why the stackup should be planned before routing.
Reference (ground/power) planes
High-speed signals should be adjacent to a reference plane (preferably an uninterrupted ground); because the return current flows in this plane, immediately beneath the signal. Backing a signal layer against a solid ground plane shrinks the loop area. Adjacent power and ground planes create a natural high-frequency capacitance that helps power distribution — this topic is covered in detail in the PDN article.
What is controlled impedance?
In high-speed or RF signals, the transmission line's characteristic impedance (typically 50 Ω single-ended, 90–100 Ω differential) must be kept at a certain value; otherwise reflections and signal distortion occur. Microstrip structures are used on outer layers, and stripline on inner layers. The impedance is set by the trace width and the distance to the reference plane.
Factors that determine impedance
- Trace width: a wide trace, low impedance.
- Dielectric thickness: the distance between the trace and the reference plane; as it increases, the impedance rises.
- Dielectric constant (Er): the material's electrical property (for example FR-4 ~4.3).
- Copper thickness: the trace thickness affects the impedance somewhat.
When working with the manufacturer, specifying the target impedance lets them adjust the stackup and trace widths accordingly (impedance-controlled fabrication).
Common stackup examples
The layer count is chosen according to the design's speed and density. More layers mean a better reference plane and easier routing but raise the cost.
| Layers | Typical structure | Where it suits |
|---|---|---|
| 2 layer | Signal + signal/ground | Low speed, simple, cheap |
| 4 layer | Signal–GROUND–POWER–Signal | An ideal start for modern designs |
| 6 layer | More planes + inner signals | High speed, dense routing |
| 8+ layer | Multiple planes/signals | Very high speed, dense BGA |
Practical recommendation
For most designs critical in terms of speed and noise, 4 layers (Signal–Ground–Power–Signal) is a good start: the outer signal layers are backed against the solid planes immediately beneath them. Although 2 layers is attractive for cost, it causes problems in high-speed designs because providing an uninterrupted ground plane is hard.
Common mistakes
- Thinking about the stackup after routing: if the stackup isn't planned from the start, the reference planes and impedance are spoiled.
- Passing a fast signal over a split plane: the return current is cut, and radiation and distortion increase.
- Sending to production without an impedance target: your impedance may not hold with the manufacturer's default stackup.
- An asymmetric stackup: unbalanced copper distribution can cause the board to warp.
Frequently asked questions
Is 2 layers enough?
In low-speed, simple designs, yes. But if there are high-speed signals or EMC sensitivity, 4 layers offering an uninterrupted ground plane is much safer.
Who determines the controlled impedance?
The designer specifies the target impedance, and the manufacturer adjusts the stackup and trace widths accordingly. Without communication, the target impedance may not hold.
Is FR-4 always suitable?
In most applications, yes; but at very high frequencies FR-4's losses increase and low-loss special materials may be needed.
Conclusion
A good stackup is the combination of solid reference planes, short return paths and controlled impedance, and it must be planned before routing. This topic is closely related to the EMC/EMI and PDN articles. When planning the components on the board, you can price your bill of materials in bulk with the BOM tool.