I’ve been designing boards for a long time; however when I began my career, the boards I designs were pretty simple. Most of them were small low speed designs with very few restrictions on them. Test boards and control panels, I cannot tell you how many simple 2 and 4 layer boards I have designed in my career. That being said, as my skills increased, so did the complexity of the boards that I designed. A particular high point for me was when I began working on dense multi-layer high speed designs.
High speed PCB designs have tighter signal integrity requirements, and constructing your board for them becomes part of the overall signal integrity equation. Not only is the selection of board materials important for impedance, dissipation and other signal characteristics, but the order and number of layers of those materials play a vital role as well.
Plan out your designs and routing.
Stackup requirements for high speed routing
High speed signals are most often routed using either microstrip or stripline methodologies. These methods are designed so that a high speed signal running through a routed trace will have the correct physical structure for its return path. The physical structure requirements of these routing methods will have a great impact on your board stackup decisions.
Microstrip is the simpler methodology of the two. It has a transmission line, which is a routed trace on an external layer of the board, that is separated from its referenced ground plane by a layer of dielectric material. Alternatively, stripline has a referenced ground plane both above and below the transmission line. The transmission line is a routed trace on an internal signal layer which is embedded in dielectric material balanced between the two planes.
When planning your board stackup for a high speed design, you will need to take into account these routing methods. When routing microstrip, your board stackup will have to include a ground plane next to the external routing layers of the board. When routing stripline your board stackup will have to include ground planes on both sides of an internal signal layer where the high speed routing is being done. There are also other types of stripline which you may need to consider as well. For example, dual stripline requires that you surround two adjacent internal signal layers with ground planes.
Power and Ground on a PCB
Stackup requirements for ground and power planes
As we mentioned earlier, effective high speed routing using microstrip or stripline routing methodologies require a least one complete ground plane. I also recommended that you incorporate a complete power plane into the design.
Alternatively, when you are working with several regulated voltages, giving each voltage its own layer and ground plane can result in too many plane layers in the board stackup. This can be incredibly expensive or impractical to build.
One alternative is to split the ground and power planes to carry different voltages. This is a common practice in mixed signal designs where a single plane layer may have multiple splits. When doing this In a high speed design, special care must be executed. This is because a high speed transmission line routed over a plane split will divert the return path for that signal. When a return path is diverted, it can cause noise and crosstalk in the circuit. This because the return path of the signal has to loop outside of its expected path instead of flowing along next to the routed trace. Therefore, be sure to construct your board stackups so that high speed circuitry is not forced to cross plane splits or non-referenced ground planes.
There aren’t any hard fast rules as to how many layers constitute the best board stackup for high speed designs. That decision will have to be based on cost, materials and the needs of the design. There are however some important considerations for the configuration of the stackup that have been listed here that should be helpful to you.
Proper board layer stackup in high speed board design requires the best PCB software, and Altium Designer is a good one to get started with. Would you like to find out more about how Altium assists its users in the configuration of the board layer stackup for a high speed design? Talk to an expert at Altium.
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