As the capabilities of electronic devices continue to grow with new features and functionality, the sizes of these devices are steadily shrinking. For the printed circuit board designer this usually means that you will have to pack more circuitry into smaller board sizes. Although advanced fabrication processes and next generation components offer some help with the increased circuit density, there are still the signal integrity requirements of higher speed designs that must be accommodated.
What this all boils down to is that if you aren’t already working with multilayer PCB designs, you probably will be soon. There’s much more to a successful multilayer design then just adding layers to your CAD system though. You must have a plan before you start of how the layer stackup of the board will be configured in order for the best multilayer circuit performance. Here are some ideas on how to prepare for your multilayer design, and how your PCB design CAD tools can help.
What Are the Key Considerations When Planning for Multilayer Printed Circuit Boards?
The main consideration when evaluating the need for a multilayer printed circuit board is the function and performance of the design. As the component and circuitry density increases on the PCB, you will need more room to route all the traces. In addition, these designs will require specific board layer stackup configurations in order to promote better signal and power integrity. Here are some of the considerations that you should keep in mind as you decide whether or not you need to design a multilayer board:
Room for routing: It may sound simplistic, but the more circuitry that you have in your design, the more board space you will need in order to route the traces. In addition, increasing the amount of components on the board will also consume the space that used to be available for routing.
Controlled impedance routing: These traces have to be routed at specific trace widths and spacing, and often are required to be on inner board layers sandwiched between power and ground plane layers.
Microstrip or stripline configurations: To help with signal integrity, sensitive trace routing is usually routed on a board layer with a ground plane beneath it, or on a layer sandwiched between two ground planes.
Clear return paths: To help reduce noise in your circuitry, you should have a clear return path on a plane layer. As the amount of circuitry in your design increases, the more plane layers you may need.
Noise isolation: For better board performance you will need to isolate noisy circuitry from other sensitive circuitry. This could require additional power and ground layers in order to separate analog and digital circuitry, or to shield power supplies.
On the other hand though, a multilayer design will increase the cost of manufacturing which also needs to be considered. Higher speed designs may require more exotic board materials, and the fabrication costs will go up depending on the board layer configuration that is used. Design time may also increase as the circuitry density and board layers get more complex adding yet another cost.
Here is where the services of your PCB contract manufacturer can be helpful. They have plenty of experience building all kinds of different printed circuit board layer configurations, and they can help you make the decision as to what will be best for your design. They will be able to give you different options on board materials, stackup configurations, and even suggest layout strategies before you start. They may even be able to send you board stackup data for your PCB design CAD tools to work with.
The 3D layout of the top etch and components of a PCB
How Your PCB Design CAD Tools Can Help Perfect Multilayer Circuit Performance
PCB design CAD tools today usually offer a lot more than simple schematic capture or place and route capabilities. The best tools will give you the ability to evaluate the needs of your design before and during its layout. Here are some of the features and functions that your tools should have in order to help you:
Simulation: With circuit simulators you can see how your design will function as you capture the schematic. This will help you with part selection and determining the layers required for the layout. While you are in layout you can use signal and power integrity tools to help you create a clean place and route.
Calculators: Your layout tools will have calculators for determining the width of the layers needed in your design based on your controlled impedance routing requirements. This information will also be used by the layer stackup generators in your tools when building the board layer configuration.
IPC-2581: More and more PCB design CAD tools are now incorporating IPC-2581 capabilities into their tools. In addition to the ability to transfer fabrication and assembly files to your contract manufacturer, it also gives the tools the ability to pull in information from the CM such as recommended layer stackup configurations.
Together with the input from your contract manufacturer, your design tools will be able to help you with your multilayer PCB stackup planning. You will be able to balance the layer cost, what it will take for multilayer PCB fabrication, and plan for a 6-layer board stackup, a standard 8 layer PCB stackup, or even working with a 4 layer PCB blind via configuration.
The same 3D PCB layout showing the inner layers under the components
Multilayer Printed Circuit Board Tips
To help you with your multilayer PCB planning, we’ve compiled some information here that you should find useful.
The Importance of Careful Multilayer PCB Stackup Planning
A multilayer circuit board will cost more to produce than a single or double sided board, and that cost will increase with each layer pair that you add. But there is more to the cost of fabricating the board then just the layer count, and in some cases you may end up saving expenses by adding layers. Trying to force more circuitry into fewer layers can increase the number of drilled holes and circuit density, ultimately increasing the manufacturing costs. Before you decide which board layer stackup configuration is best, you need to count all of the costs.
If you’d like to learn more about this subject, read about it here.
The Layer PCB Cost Considerations of Multilayer Designs
To fully count the costs of manufacturing your printed circuit board, you need to understand the determining factors of its fabrication and assembly. These include the board materials, the board size, and the number of layers in the design. You also need to consider the type of board finish, the size and amount of drilled holes in the board, and the aspect ratio. There are many other factors that you should consider as well in order to make a fully informed decision.
If you’d like more information about these considerations, you can find it here.
Understanding Multilayer PCB Fabrication Steps
Before you go much further, you may find it helpful to understand just how the multilayer PCB fabrication process works. There are basic steps when fabricating a multilayer board such as exposing photoresist material to create the correct copper images on each layer pair, and then laminating the layers together. Understanding how these processes all work together can help you to create a layout that is best suited for multilayer fabrication.
To learn more about the multilayer PCB fabrication process, take a look here.
The Cross-section Editor in OrCAD PCB Designer
Layout Considerations for the 6-Layer Board Stackup
Once you’ve settled on designing a multilayer board, you may choose to build a board with six layers. The 6-layer board has been the workhorse of the PCB design industry for many years, but how you configure the layers can have a dramatic effect on how the board will perform. With the wrong layer configuration your circuitry could be susceptible to EMI and other signal performance problems. Depending on what your circuitry needs are will determine what the best configuration is for your 6-layer board.
For an example of a 6-layer board stackup, try this:
For more information on the different layer configurations of a six layer PCB, click here.
Layout Considerations for the Standard 8 Layer PCB Stackup
In some cases it may be better to work with a PCB that has eight layers. Although there are many different ways that eight layers can be configured, there is one standard approach that is more commonly used. How you work with those layers and set them up for routing directions, return paths, and other design considerations is the key to success in using a standard 8 layer stackup.
To learn more about the standard 8 layer PCB stackup, read about it here.
A New Approach for the 4-Layer PCB Blind Via
In some cases, such as designing high density interconnect (HDI) circuitry, it may be better to work with advanced fabrication techniques than more traditional methods. Blind and buried vias are one such option that allow for smaller holes to create room for more routing. Another option is to use 3D printing to remove the traditional via manufacturing steps.
If you’d like to learn more about theise blind vias, you can read about it here.
Microvias may be helpful in an HDI multilayer design
Put Your PCB Design CAD Tools to Work
The key to success in your printed circuit board design is to understand and plan for what configuration of board layers will best fit your circuitry needs. To accomplish this you need both reliable fabrication input from your contract manufacturer, and the best PCB design tools to work with. As we discussed earlier, having the ability to easily configure the board layers in your design and exchange board layer configuration data with your CM will go a long way towards helping you to be successful.
The good news is that the PCB design system with the features and functionality to build your layer stackup configuration the way you need it is already available. OrCAD PCB Designer is the tool set you need to expertly take your design all the way from the initial concept, to the final manufacturing files.
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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