Careful Multilayer PCB Stackup Planning: Your Best Bet to Save Time and Money

Throwing money at a problem can be an effective method to resolve the problem, as long as you have a lot of money to throw. The real-life scenario though is that we are usually trying to save our money instead of spending it as if it were growing on trees. But that doesn’t change the fact that to resolve some problems simply requires more resources. The key to this conundrum is to find out exactly where the balance is so that the money spent gives us the biggest bang for our buck.

This balance is exactly what we are trying to achieve when we plan out the layer configuration for our printed circuit board designs. Although it would seem that fewer layers means less expense while more layers adds to the cost, that may not actually be the whole story. To help you figure out where that balance is for your designs, let’s take a look at some of the details you should consider for your multilayer PCB stackup planning.

Count the Cost in Your Multilayer PCB Stackup Planning

The plain and simple fact is that the basic cost to fabricate a multilayer printed circuit board is going to be more than a single or double sided board. In general, a four layer board is going to cost almost twice as much than the same size single or double layer board. From there each additional set of layers will add on 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.

All of these details, and more will need to be taken into account to get an accurate idea of what the true cost will be. It could be that in trying to save money on board layers that 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 looking for ways to reduce the layer count could end up costing you more as well. The key is to thoroughly investigate your options so that you can make an informed decision. That also means taking into consideration the signal performance of the board into your cost analysis as well.

Multilayer PCB stackup planning requires balancing the boards needs with its cost

High Speed Design PCB Stackup Considerations

PCB design continues to evolve with more and 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 accomodate 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 would need to 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, it needs additional power and ground plane layers as well.

Utilize a microstrip or stripline technique in your multilayer stackup

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 requires the signal routing layers to be sandwiched between two ground plane layers.

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

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 amount of board layers needed in the design, but it can also introduce new problems as well. 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 up into different voltages will ruin the return path and potentially cause all kinds of 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.

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

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.