A PCB stackup is the fundamental architecture of your printed circuit board, defining how conductive and insulating layers work together to create a functional electronic system. This critical aspect of PCB design influences everything from signal integrity to manufacturing costs, making it essential for both novice and experienced designers to master.
One of the fundamental tasks of electrical engineering is converting an electrical drawing, called a schematic, into a functional physical object. This task is usually accomplished by
- designing a printed circuit board (PCB) that has the same electrical connections as the schematic and can accommodate the same components,
- generating manufacturing files for this printed circuit board,
- sending these files to a facility that specializes in fabricating PCBs,
- and then, when boards have been successfully fabricated, installing the components.
This process involves a number of noteworthy details, but one of them is particularly crucial and yet sometimes overlooked—namely, the board “stackup.” A PCB’s stackup is not an obvious feature of the design when you’re looking at an image of the layout or inspecting a physical board. And yet, stackup influences how the board’s traces will be laid out, what its final size will be, where its components will be located, how much it will cost to manufacture, and—perhaps most importantly—how successfully the board will operate. Stackup affects crosstalk, signal integrity, power integrity, ground quality, and resistance to electromagnetic interference (EMI). Thus, choosing the right stackup for your PCB is an essential step in achieving electromagnetic compliance (EMC) and optimizing your product’s electrical performance.
What Is Board Stackup?
The stackup of a printed circuit board is the number and arrangement of its conductive layers. A conductive layer is where almost all of the PCB’s electrical action occurs (the exceptions are electrical coupling between layers and conductive structures, called vias, that connect one layer to another). The conductive material on these layers is copper, and through a process called etching, a uniform surface of copper is converted into a network of traces, polygons, and other shapes that carry electric current from one component to another.
The various types of electrical functionality required in a circuit board are not distributed randomly among the board’s conductive layers. Rather, designers “assign” a specific category of electrical functionality to each layer:
- A signal layer carries ordinary digital and analog signals. These are usually routed as narrow traces and include such things as data lines for serial communication, microcontroller inputs and outputs, and sinusoidal audio waveforms.
- A power layer has one or more large areas connected to power-supply voltages. For example, one power layer might consist of three copper polygons: one for a 12 V input supply, one for a 5 V analog rail, and one for a 3.3 V digital rail.
- A ground layer is one solid rectangle of copper connected to “ground,” i.e., the theoretically zero-volt node associated with electrical current that is returning to the source.
In many cases, one layer will be assigned to both power and signal, or a layer that is mostly power may have a few signal traces that could not be efficiently routed on the signal layer. Also, a board with only one ground layer will not be one solid copper rectangle if the design requires multiple ground sections (however, it is usually better to avoid split analog/digital ground planes).
A basic circuit board has only two conducting layers: top and bottom. A two-layer board has copper on the top, copper on the bottom, and insulating material in between. This insulating material also plays a structural role, giving the board strength and rigidity. Two-layer boards are generally discouraged for professional engineering products, because they do not allow the designer to take advantage of features that greatly improve a board’s physical and electrical characteristics.
A much more common and effective stackup includes four conductive layers: top, bottom, and two internal layers. This stackup provides adequate component density and electrical performance for countless projects. A “four-layer board”—this phrase is heard frequently in engineering offices—is a standard starting point for professional designers. The following diagram of a four-layer PCB configuration, representing what you would see if you looked at the board’s edge with a microscope, depicts the structures involved in board stackup.
As you can see, conductive copper layers are separated by insulating core or prepreg layers; the name “prepreg” refers to the fact that the material is pre-impregnated with resin, and the most common core material is a fiberglass-and-resin composite called FR4. Other stackups—six layers, eight layers, etc.—simply extend this pattern: conductive layers are separated by insulating core and prepreg layers.
Aspects of Stackup Design
PCB designers face two important decisions when thinking about stackup:
1. How many conductive layers should the board have?
2. Which layers should be assigned to signal, power, and ground?
In some applications, a third question arises:
3. What is the optimal separation distance between power and ground layers?
Number of layers: A four-layer stackup provides a good balance of performance, routing complexity, and cost. Additional layers offer more flexibility with layer assignments and make routing more feasible and efficient in high-density boards.
Layer assignments: Layers should be arranged so as to reduce crosstalk between signal layers, maximize power integrity, ensure high-quality return paths, create shielding to mitigate the effects of received EMI, and reduce the amount of radiated EMI.
Power/ground separation: Designers can control the thickness of the insulating layer between adjacent power and ground layers. In many cases this variable is not a critical factor, but it does come into play in RF applications that require controlled impedance. It is also worth considering when you need more interplane capacitance to achieve adequate power integrity: In high-frequency digital circuits, discrete capacitors may not offer enough low-inductance decoupling capacitance. These circuits benefit from reduced insulation thickness between power and ground layers, because with less separation, these layers will have more interplane capacitance.
Recommended Four-Layer Stackups
There are various ways to arrange the power, signal, and ground layers in a four-layer PCB. No stackup is ideal for every application, but the following layer setup is a widely applicable, high-performance option:
Having two internal ground planes allows for good return paths, creates electromagnetic shielding, and reduces EMI emissions. These ground planes also help to prevent crosstalk between the two signal layers, which are further protected by a separation distance that is the largest available in a four-layer stackup.
The following configuration is also recommended:
This stackup is effective from an EMC perspective. However, routing may be inconvenient, especially in a board with many components or high-pin-count devices, because component signals will need to be connected to a via and moved to one of the internal signal layers. Also, the proximity of the signal layers, combined with the absence of an intervening ground plane, will contribute to crosstalk. If this stackup is chosen, the designer should keep crosstalk in mind when routing traces on the signal layers.
Recommended Six-Layer Stackups
A six-layer board offers a great deal of flexibility in layer arrangements, and finding the optimal stackup is dependent upon identifying and responding to the application’s key constraints and objectives. A general recommendation is to include two external signal layers (top and bottom), two internal ground layers, one internal signal layer, and one internal power layer. Let’s look at one specific six-layer arrangement that is an excellent stackup for high-performance boards:
- top signal/component layer
- first internal ground plane
- internal power plane
- internal signal layer
- second internal ground plane
- bottom signal/component layer
Advantages of this stackup are as follows: The ground planes provide shielding on both sides of the board, the paired power/ground planes (layers 2 and 3) provide interplane capacitance, and crosstalk is reduced because there are no adjacent signal layers. We can even choose the thickness of the insulating layers such that the internal signal layer is closer to the ground plane and farther from the power plane; this reduces radiated EMI associated with traces on one layer being routed across split voltage rails on an adjacent power plane.
Optimizing Your Stackup
If your PCB stackup is contributing to EMI issues or EMC testing failures, DENPAFLUX can help. Our AI-powered PCB-analysis software identifies EMI concerns quickly and efficiently, and we also provide a full range of EMC consulting services. Our experts can give you the personalized advice you need for improving your stackup and achieving EMC compliance. Our goal is to make your EMC problems disappear—give us a call today, and let’s talk about how we can make that happen.
Conclusion
Proper PCB stackup design is crucial for creating reliable, high-performance circuit boards. By following these guidelines and best practices, designers can achieve optimal signal integrity, minimize EMI, and ensure successful manufacturing outcomes.
