There’s nothing better in the world than a six layer chocolate cake. I do enjoy a single layer cake, but there’s no comparison to its larger and more decadent six layer cousin. A six layer cake however will take more time to prepare, more skill to bake, and it will be more expensive than a simpler cake. It will also take a little more time at the gym to compensate for the extra calories, but in the end it is really worth it.
If you’ve only laid out simple one or two layer circuit boards, you are going to find that laying out a multilayer PCB will have some similarities to a six layer cake. Like the cake, it will take more time, more skill, and more cost, but it will also be a very enjoyable design challenge without the burden of having to go to the gym the next day. There will be some new design methodologies to learn though, and we’ll give you an introduction to those here in this multilayer PCB design tutorial.
The First Step of a Multilayer PCB Design Tutorial; Setup and Prep
In some cases, older legacy PCB footprints may not be adequate for a multilayer design, and you need to find out if there are any additional requirements necessary. Depending on the CAD system being used, you may have to add layers or attributes to a footprint for multilayer use. Here is where having access to a more advanced PCB design system with links to online library services can be a real benefit. It makes it much easier to have the latest and most accurate PCB footprint source data to work with.
The main difference between a double layer and multilayer board setup, will be in planning your layer stackup. The following are some of the points that you will need to consider while planning your board layer stackup:
Performance: How fast the circuitry will operate at, and the operating environment of the final board, may make a difference in the materials that the board will be fabricated with. There are more advanced materials than FR-4 that may be better suited for your application depending on what the needs are, but those materials may affect parameters such as your impedance calculations. Here is where the help of your PCB manufacturer will be an invaluable source of information.
Cost: The fabrication materials as well as the layer count and configuration, will have a direct bearing on the overall cost of building the board. Here, again, you need to work with your manufacturer to consider all of the options.
Density: The routing density of your board is another factor when determining the configuration of your board layer stackup. It is very painful when you have to go back and add layers to a board design after you’ve already started your layout. Not only do you have to reconfigure your CAD database, but you may have to make a lot of changes to your layout. On the other hand, if you start with too many layers you will be paying more for the boards then you should.
Circuitry: You also need to understand the needs of your circuitry in order to create the most optimum layer configuration. For instance, sensitive signals may require a stripline layer configuration for their best performance, which will mean adding additional ground planes. Areas of analog and digital circuitry will need to be separated with their own ground planes, and onboard power supplies will need isolation. All of this could have an effect on the layer configuration, and this should be planned for before layout starts.
Once you’ve gathered your data and created your board layer stackup in the layout database, it will be time to start placing and routing the board.
With tools like this board outline generator, creating board layer stackups is a much easier
A Different Perspective on Place and Route
When working on a multilayer layout, one of the first things that will be different is how much you need to start thinking in terms of “3D” design. A two layer board only requires you to consider it in terms of ‘top,’ and ‘bottom.’ Now you are in a world of multiple layers, and there are different things happening internally that could affect the top and the bottom. For instance, you may not want to place a noisy part in a certain location, because of the sensitive routing on an inner layer underneath it.
As far as the tools go, placing components will be the same as with a double sided board, but the landscape that you are working with will be different. For instance, you don’t have to worry about leaving as much space for routing channels between your parts since they will be mostly routed on the inner layers. There will still be a need for short direct routes on the surface layers for sensitive circuitry, but for the most part, you now have more room to work with. This is a good thing too because with a multilayer board, there are probably a lot more components that need to be placed.
Internal trace routing and power planes will be a joy to work with, but at the same time there are some important considerations here as well:
Multilayer boards will typically have more components and therefore more routing than a double sided board, so plan ahead for it. Depending on the technology of the board, some of this routing may have specific routing widths and spaces or other requirements, such as differential pairs or impedance controlled traces.
Some routing will require a stripline layer structure and must be routed on layers adjacent to ground planes. Additionally, sensitive routing must be crossed perpendicularly on adjacent internal signal layers to help reduce any possible broadside coupling or crosstalk.
Ground planes will have a lot of vias in them for connectivity, but those vias could affect signal return paths. This requires carefully planning your routing to avoid blocking up the planes.
Split planes need to be laid out so that sensitive signals don’t cross the splits and thereby ruin their return path. A situation like this can create a lot of noise on the board.
Once the placement and routing is done and checked, the rest of the design work will be similar to a double sided board. Now you are ready to have the boards built.
Multilayer PCB layout requires a 3D perspective to circuit board design
Finalizing the Design with Documentation and Output Files
To get your multilayer design out for manufacturing, you will need to create the same kind of documentation that you’ve always created, but with a few exceptions. First of all, there will be more details needed on your manufacturing drawings. Your fabrication drawing will need a multilayer board stackup detail, and notes detailing the specifics of how the board will be built. Second, if you are using Gerber files for your manufacturing outputs, you will obviously need to generate additional files for the multiple board layers. Here is where using an advanced set of CAD tools can be very helpful in creating and managing your manufacturing output files.
Fortunately, there are PCB design systems available that already have the tools you need for successful multilayer circuit board design. OrCAD PCB Designer is the type of advanced system that will give you access to online CAD library services, board outline creation wizards, and manufacturing and documentation generation utilities.
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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