An Introductory Multilayer PCB Design Tutorial
There’s nothing more amazing on earth than a six-layer chocolate cake. I like a single layer cake but it’s not comparable to its more substantial and extravagant six-layer cousin. A six-layer cake will require longer to bake and requires more expertise to bake and is more costly than a simple cake. Additionally, it will require some time in the gym to offset the additional calories, but ultimately, it’s definitely worth it. If you’ve just created simple two or one layers of circuit boards, you will notice that the process of laying out multilayer PCB will bear some resemblances to a 6 layers cake. Similar to the cake, it will require longer, more skill and cost but it will be an extremely enjoyable design challenge, without the stress of going to the gym on the following day. There will be innovative design methods to master, however, and we’ll provide a brief introduction to them in this tutorial for designing multilayer PCB.
In certain instances, footprints of PCB from the past might not be sufficient for multilayer designs so you must discover whether there are any other requirements that are required. Based on the CAD software employed, you may need to add additional layers, or even attributes in the footprint to allow multilayer usage. This is why having access to a higher-quality PCB design tool with online library services could be beneficial. It is easier to access the most up-to-date and accurate PCB footprint data source to use.
The major difference between a double and multilayer setup is in the planning of the stacking of layers. Below are some of the things you should be aware of when designing your board’s layer stack up
Performance: The speed that the circuitry can operate and the operating conditions of the final circuit can affect the materials the board is constructed using. There are higher-tech materials that are more advanced than FR-4, which could be better suited to your particular application based on the requirements and how they influence parameters like your impedance calculations. This is where the assistance by your PCB manufacturer can be an important source of details.
- Cost: The materials used in the fabrication process, and the configuration and layer count will directly impact the total cost of creating the board. Again, here you must work with your manufacturer to look at the various options. Not like altium designer price which is higher in a market.
- Density: The density of routing that your design has is an additional element to consider when you are deciding the layout of your layer stack up. It can be very stressful when you must return and add layers to a layout after having already begun the layout. Not only will you need to modify your CAD database it is also possible to make many adjustments in your design. In contrast, it is possible that if you start using too many layers, you are likely to pay more for boards than you must.
- Circuitry: It is also important to know the requirements of your circuitry to develop the most optimal layer configuration. For instance, signals that are sensitive might require a stripline configuration to ensure their highest performance, which may need to be complemented by more ground planes. The areas of digital and analog circuitry must be separated by the ground planes of their respective circuits and onboard power sources require isolation. All of this will impact the layout of layers and must be considered prior to when design begins.
After you’ve collected your information and built your board and layer stack-ups in the layout database it’ll be time to begin putting as well as routing your boards.
A Different View of Place and Route
If you’re working on a multilayer layout one of the primary things that will be different is the extent to which you have to think about a “3D” layout. A two-layer board will require you to think as a combination of ‘top’ and ‘bottom. You are now within a realm of multiple layers with a myriad of internal events that could influence the top and bottom. As an example, it might not be able to put the loud component in a particular place due to the sensitive routing on the internal layer beneath it.
When it comes to the tools are concerned, putting components will be exactly the same as for the double-sided board but the terrain you’re working on will differ. In particular, you won’t need to think about leaving enough space to route channels between your components since they’ll be primarily directed to the inner layers. There’s still the need for short direct routes through the surface layer for sensitive circuits however for the majority part, you will have more space to use. This is good also because, with multilayer boards, it is likely to have many more elements that must be put in.
The internal power and traceroutes can be fun to use, but at the same time, there are some crucial considerations to be made also:
- Multilayer boards typically contain many more components, and consequently more routing options than a double sideboard, so you need to prepare for it. Based on the technology used by the board, certain of these routings may require particular widths and spaces for routing or other requirements, like differential pairs, or impedance controlled tracks.
- Certain routings will require the use of a stripline layer and is required to be run on layers that are adjacent to the ground planes. Furthermore, routing that is sensitive must be traversed perpendicularly to nearby internal signal layers to minimize the risk of crosstalk or broadside coupling.
- Ground planes are likely to have lots of vias for connectivity, but these vias may influence the signal return routes. It is important to plan your routes to prevent blocking planes.
- Split planes must be laid out in a way that the signals that are sensitive do not cross the splits and thus disrupt their path back. This could create lots and noise in the boards.
After the routing and placement is completed and verified then the rest of the design work will be identical to a double-sided board. Then you can build the boards.
Finalizing the Design by Output Files and Documentation
To make your multilayer PCB model be manufactured it is necessary to prepare the same type of documentation you’ve produced, but with some minor changes. The first is that there will be additional details that you need to include in your manufacturing drawings. The drawing for fabrication will require details for stacking up multilayer boards as well as notes on the particulars about how your board is constructed. In addition, if you’re making use of Gerber files to create making outputs for your factory, then you are likely to require additional files to accommodate the various layers of the board. This is where sophisticated CAD tools can prove useful in the creation and management of the files for your manufacturing outputs.
It is good to know that there are multilayer PCB design software with the tools required to succeed with the multilayer design of circuit boards. The OrCAD PCB Designer is an advanced system that can give the user access to internet-based library services for CAD including board outline creator wizards, the tools to create documentation and manufacturing.