What goes around comes around
Do you believe in karma? There are some that say whatever you believe is effectively true as it will determine your actions. That may be true; however, there are some realities that are independent of whether anyone believes in them or not. Actions having reactions is one of these. Or if you like, what goes around comes around.
An inescapable reality of PCB design is that your signals must have a return path for your circuits to function. This is true for all signals, analog, digital and power. For simple, less complex circuits; such as single and double-layer boards, traces are sufficient to provide the necessary return paths. However, for smaller boards with more complex routing, multilayer PCBs with vias is required.
For multilayer boards, it is essential that good stackup layer tips are employed, which includes determining the best usage for signal, ground and power planes. Dedicating a layer(s) to power is less common than signal and ground power planes and poses some unique challenges. Let’s explore these by determining the best techniques for a 4 layer power plane design.
4 Layer PCB Stackup Design
A multilayer PCB is defined as having “3 or more” conductive layers, where each internal layer may be for signals, ground or power. Although, it is possible to design a 3 layer board with a power plane, practically, a 4 layer PCB, shown in the figure below, is most likely the smallest stackup that will contain a power layer.
4 layer PCB structure
As shown in the figure above, a 4 layer stackup includes two internal layers. This means that your board will include blind vias. Most often, one of these will be a signal layer and the other a ground plane. However, both internal layers may be ground planes, as there are EMI advantages to having parallel ground layers.
Another stackup alternative is to have a power plane and a ground plane; however, it is highly inadvisable to have two internal signal layers adjacent to each other. There are also situations where multiple power planes may be needed, but there are significant challenges to be considered for these designs, as discussed below.
4 Layer PCB Power Plane Design Challenges
When designing multilayer boards of any number of layers there are some stackup tips that should be followed to ensure the operation and help facilitate your board’s manufacturing. These include; configuring your stackup symmetrically and minimizing the distance between ground and power planes. Adhering to these and other multilayer stackup design guidelines pose challenges for 4 layer power plane designs, as listed below.
Multiple power planes
For high functionality PCBs, it is not uncommon for components to have different voltage level requirements for power. This can present significant power integrity (PI) challenges when only a single layer is available for power. The best way to accommodate this is to partition the power plane into different regions; however, current requirements, via number and placement must be considered to ensure that all of your components are adequately supplied.
Multiple signal types
The miniaturization of today’s denser, more complex boards usually means there are multiple signal types. For example, it is common to have power, digital and RF signals propagating on and through a single PCB. A typical solution is to use internal signal layers; however, for 4 layer boards with a power plane, this may not be the best option as the other internal layer may need to be a ground plane. If a ground plane is used, then partitioning components into like signal types and good ground design becomes critical to minimize EMI and maximum signal integrity.
For 4 layer boards, the requirement for symmetry can only be satisfied by using two internal power planes. This can be advantageous when multiple powers are needed, especially for two-sided PCBs, where components are mounted on the top and bottom layers. However, it does mean that traces will have to be relied upon solely to provide returns.
One of the reasons to use a 4 layer PCB is to provide additional heat dissipation for 2 layer designs. For 4 layer boards with thermal issues, it may be possible to use plane partitioning and multiple vias to aid in thermal dissipation, while still satisfying power requirements.
As shown above, there can be significant challenges to designing 4 layer PCBs with a power plane. Addressing these challenges requires that you weigh and employ good design techniques, which is most easily achieved with the best PCB design tools at your disposal.
How to Create the Best 4 Layer PCB Power Plane Design
Designing multilayer boards are typically a much more complicated process than layout single or double-sided PCBs. Choosing the best materials, setting layer dimensions, arranging the stackup, selecting the number, types, sizes and best locations for vias are all considerations that must optimally determined. And doing so is not a straightforward proposition. Instead, you must balance the advantages and disadvantages of different design techniques to arrive at the best design.
Making the best decisions for multilayer boards such as 4 layer PCB power plane designs improves with time. However, implementing them most effectively and efficiently depends on the capabilities and functionality of your PCB design tools that you use.
3D graphical analysis of 4 Layer PCB
One of the most powerful tools for designing multilayer boards, especially when multiple options have to be contrasted is real-time design integration as included in Cadence’s PCB Design and Analysis tools. Additionally, the ability to perform Analog/Mixed-Signal Simulation within OrCAD PCB Designer ensures that your design will meet its operational objectives as well as ensure board manufacturability before you send your design off to be fabricated and assembled.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.
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