While laying out a PCB for a large company many years ago, I found myself in the middle of an argument between the lead electrical engineer and the lead manufacturing engineer. “This is how I want the parts to be placed,” yelled one of them while the other responded with, “No! It’s going to be this way instead!” Finally the project manager came up, grabbed them both by the scruff of the neck, and made them sit down and work out the problem together. That was good management!
The ability to see the problem, weigh the options, and make a decision is what good management is all about. And whether you are making decisions that involve people and projects, or simply how to come up with the right layer stackup for your printed circuit board design, it is still a process that needs to be managed. Here are some ideas on some layer stackup management techniques that might help you with your next PCB design.
PCB Layer Stackup Management Techniques Using Your Design Tools
There was a time where most of the circuit boards being designed used the same basic layer stackup. Two, four, and six-layer boards were the usual stackups, and occasionally you would see an eight-layer design come your way too. Normally the layers were all configured the same as well. A four layer board would have a power and ground plane in the middle, while a six layer board would add another signal layer on the top and bottom of the configuration.
Now we know that those older board layer stackups could cause a lot of signal integrity problems depending on their layer configuration. At the time our goal was to find ways to add more room for trace routing, but that was often done without considering the price that we were going to pay for signal performance. The standard six layer stackup that we used was actually very susceptible to EMI and other problems, but that doesn’t mean that the six layer board is off limits now. As long as the layers are configured correctly, a six layer stackup can do a lot to protect the board circuitry from interference.
The key to a successful board layer stackup is to use the capabilities of your PCB design tools to help you configure the layers in your design correctly. Design systems today often have many different features and tools built into them that can help with this, including the following:
Layer stackup generators: Most PCB design tools have some sort of layer generator that will use inputted values to create a board layer stackup. You usually have the ability to specify the number of board layers in the design along with their widths, materials, and characteristics. You will also be able to configure the layers for signal routing, power and ground planes, and split planes. This information is then used by the impedance calculator for determining trace widths for controlled impedance routing.
Impedance calculators: Using the data created by the layer stackup generator, the impedance calculator will give you the correct width for your controlled impedance lines. The relationship between these tools usually goes further than that though with this data also being made available to other portions of the system to configure design rules or drive circuit analysis tools.
Circuit analyzers: These tools will give you the ability to analyze your board for crosstalk, signal integrity, power distribution, return path integrity, and other performance metrics of your design. Although these tools are usually used more for perfecting the routing on your board, they can also be useful in helping you to make the decision whether or not to add or subtract layers from your board stackup.
Other capabilities, such as defining rigid and flex zones within your stackups, materials management, and types of dielectrics involved are important to ensure that your stackup tool has the capacity to manage properly.
With the layer stackup that you need completed, managing your stackups for future designs is also handled by your CAD tools. These stackups can be saved in order to use them in a new design at some point in the future. The stackup configuration information is also available for inclusion in manufacturing drawings and other documentation.
A six layer board being configured in a PCB design CAD tool layer stackup generator
Using Other Resources for PCB Layer Stackup Strategies
The features inside of your PCB design tools will give you the ability to create the stackup that you need, but before you can do that you need to know what will work best for your design. Fortunately, there is a lot of information available to you on this subject. Not only are there technical classes and seminars on PCB layer stackup configurations, but you can also find plenty of information online in different articles and forums. It also is very helpful to get insight from other designers that are working in the same technologies that you are.
One of the best sources of information though will be from working with the contract manufacturer that will end up building your printed circuit board. Your PCB CM has built their business on manufacturing various types of circuit boards for many different customers. Their expertise includes a thorough understanding of different PCB technologies, and the circuitry requirements of digital, analog, power, high speed, and RF designs. They also know circuit board fabrication materials, techniques, and will have a vast network of component vendors and suppliers that they work with. They will understand better than most the configuration that will work best for the type of board that you are designing, and you should get their input before you start your design. As industry expert Rick Hartley puts it; “this is what they do day in and day out, and they are good at it.”
With the proper configuration of layers, you will be able to design multi-layer boards like this.
Working Together With Your PCB Contract Manufacturer
As we have discussed, there are many features in your PCB design CAD tools that will help you create and manage your board layer stackups. At the same time, your PCB CM is also a great resource for board layer stackup information. You may be thinking at this point that if we could get them to work together, we would have the best of all worlds. Well you are right, and the tools for this exchange of information already exist.
The IPC-2581 format is a generic standard for printed circuit board and assembly manufacturing description data and transfer methodology. With it you can receive layer structure information complete with conductive and dielectric materials definitions, coatings, and material characteristics from a participating contract manufacturer. This information can then be imported directly into your PCB design CAD tools, eliminating the need for manually entering the board layer stackup data.
To manage your layer stackup information using IPC-2581, you need a PCB design system that is able to support this format. OrCAD PCB Designer from Cadence has the ability to both import and export IPC-2581 data. Not only does this give you the ability to pull in layer stackup information from your CM, but you can also use it to create accurate and reliable manufacturing data to get your boards built.
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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