Multilayer PCB Stackup Planning Considerations

Key Takeaways

• To help you determine where the balance is between functionality and cost, below are details you should consider for your multilayer PCB stackup planning.

• You will need to consider many factors to determine the total cost of your circuit board, including size, shape, and thickness. 

• Designs that used to work well with simple single or double-layer boards may now require multiple layers in order to support the high-speed circuitry that they contain.

Multilayer PCB stackup planning requires balancing the board’s needs with its cost

When we plan out the layer configuration for our PCB designs, we are usually trying to get the biggest bang for the buck. Although it would seem that fewer layers means less expense, that may not be the whole story. To help you determine where that balance is between functionality and cost, let’s look at some of the details you should consider for your multilayer PCB stackup planning.

Multilayer PCB Stackup Planning: What Are the Costs?

The plain and simple fact is that the basic cost to fabricate a multilayer PCB will be more than a single- or double-sided board. Generally, a four-layer board will cost almost twice as much as a single- or double-layer board of the same size. From there, each additional set of layers will add an extra 20% or so. There is, however, a lot more than just the layer count that goes into determining the total cost of a circuit board.

To determine the total cost of your circuit board, you will need to factor in many other pieces of the puzzle:

• Board size, shape, and thickness

• Surface finish

• Copper weight

• Number of drilled holes and different drill sizes

• Circuit density

These details and more must be considered to accurately understand the actual cost. It could be that in trying to save money on board layers, you will increase the number of drilled holes and circuitry density. Trying to force more circuitry into fewer layers is not necessarily the best option, as it could add to the design and manufacturing expenses due to the complexity.

On the other hand, sticking with a set number of layers in a multilayer design instead of looking for ways to reduce the layer count could also cost you more. The key is to thoroughly investigate your options to make an informed decision. That also means taking into consideration the signal performance of the board in your cost analysis as well. 

Multilayer printed circuit board with copper gold ISA
Source: Wikipedia user Sieobserver (Christoph Kappel)

High-Speed Design PCB Stackup Considerations

PCB design continues to evolve with more high-speed circuitry on even the simplest designs. Designs that used to work well with simple single or double-layer boards may now require multiple layers in order to support the high-speed circuitry that they now contain.

In some cases, high-speed trace routing may require isolation from other traces in order to prevent crosstalk. Spreading the routing out to support this isolation may require additional routing layers in your design.

High-speed designs may also need non-standard board materials and specific layer thicknesses for impedance-controlled routing. To accommodate stripline routing requirements, multiple layers will be needed in order to sandwich signal layers with high speed transmission lines between two ground plane layers.

When working with dual stripline configurations, two adjacent signal layers must be sandwiched between a pair of ground planes. As you can see, not only does high-speed circuitry require additional signal layers to route traces on, but it also needs additional power and ground plane layers. 

Utilize a microstrip or stripline technique in your multilayer stackup
Source: Wikipedia user Binarysequence (Dave L. Jones)

Power and Ground Plane Stackup Planning

When high-speed transmission lines are routed on a surface layer of a PCB, they will require a solid ground plane beneath them. This is a microstrip routing configuration, and as we have already seen, stripline routing needs the signal routing layers to be sandwiched between two ground plane layers.

With these routing configurations plus the different voltages required by this circuitry, the number of power and ground plane layers in a design could dominate the board layer stackup. Here is where careful multilayer PCB stackup planning needs to come into play to manage the number of layers you will need carefully.

One thing that will help is to combine multiple voltages onto one plane layer. Splitting the power or ground plane can be very helpful in reducing the total number of board layers needed in the design, but it can also introduce new problems. The most important area of concern will be to avoid routing high-speed traces over the split ground planes.

If your routing requires a ground plane under it for a return path, splitting that plane into different voltages will ruin the return path and potentially cause signal integrity problems. This doesn’t mean that you shouldn’t use a split plane; just exercise caution with your routing when you do so.

Cadence Can Help With Your Multilayer PCB Stackup Planning

Another thing you can do to help yourself with multilayer PCB stackup planning is to use PCB design tools created to help you with this task. The different schematic capture, layout, and analysis tools from Cadence have many features and functions within them that you will find useful. In particular, the Allegro X Design Platform allow you to calculate your design’s requirements for creating a multilayer stackup and high-speed design features that will make your job easier.

Leading electronics providers rely on Cadence products to optimize power, space, and energy needs for a wide variety of market applications. To learn more about our innovative solutions, talk to our team of experts.