I had an old friend who once decided to build a custom carrier that would fit on top of the luggage rack of his car. He carefully drew up the plans, bought the materials, and began construction of it in his basement. As he worked, he carefully calculated the load points so it would sit precisely on the luggage rack, and he added other enhancements to increase its versatility.
The carrier was magnificent once he completed it, but there was one big problem that he hadn’t accounted for: The size of the completed carrier was so large that he couldn’t fit it through the door to take it out of the basement. Fortunately, with my and our other friends’ help, he eventually managed to squeeze it through, but he never heard the end of our ribbing him about it.
Since I don’t have a basement I’m not likely to make the same mistake, but that story does serve as a good reminder. We should always make sure we have considered all of the details of a project before we design ourselves into a corner. This is really important, especially when you are considering the layer stackup of a printed circuit board and its impact on assembly and manufacturing. You need to make sure that you have all of the needs of the board met without unnecessarily increasing the complexity and cost of manufacturing. Here are some PCB stackup guidelines that can help.
PCB Stackup Guidelines for High-Speed Circuits
Many years ago, you could design many printed circuit boards without much thought to signal integrity because the signal speeds of the boards weren’t fast enough to warrant any concern. That is no longer the case, however, as even an average PCB design now needs signal integrity consideration.
Designers must allow for the space that will be needed for controlled impedance routing and to guard against congested layouts that can create crosstalk. High-speed transmission signals need enough room to be routed in such a way as to avoid using vias, and measured trace lengths and trace length matching all need additional room, as well. All of these extra routing requirements may result in your circuit board needing additional layers with their stackup configuration precisely planned.
Designing for the optimum performance of high-speed transmission lines requires the use of microstrip and stripline board layer stackup configurations. Knowing what type of signals along with their electrical requirements are being routed will determine which configuration or combination of configurations should be used in your layer stackup.
Microstrip is easier to design and tends to have more predictable manufacturing results, but your board may not have the room to route all of these signals on the exterior layers. Microstrip and stripline configurations also require precise placement of power and ground planes in the stackup.
By carefully planning out the best layer configuration for all of these factors before you commit to the design, you can ensure better signal integrity and have a positive impact on your design’s layer count requirements.
Analog Design Requirements for Circuit Board Layer Stackups
High-speed design requirements aren’t the only factors that can affect your PCB layer stackup, however. Your analog circuitry may have an effect, as well. In some cases, the use of analog devices may dictate the configuration and content of your circuit board layers.
Op amps and other devices typically require the absence of any ground planes underneath them, as those can provide unwanted capacitive coupling that may impact the performance of these parts. This can require specific layer stackup configurations, or the use of split planes to pull the planes back from underneath analog parts.
For PCBs with both analog and digital circuitry, the different circuitry must be kept isolated from each other. This may require unique layer stackup configurations to support all the necessary power and ground planes.
Designing the power delivery network on your board can be one of the most difficult parts of your PCB design. It must deliver clean power for noise-sensitive devices, as well as providing the ground and shielding requirements, and it has to avoid sensitive areas of analog circuitry.
At the same time, power and ground plane splits must be carefully placed so that sensitive signals do not cross the splits, which could potentially ruin their integrity by choking off their signal return paths. The more your power and ground requirements increase, the more diligence you must take in carefully planning your circuit board layer stackup.
Circuit Board Material Considerations
Another part of planning your circuit board layer stackup is deciding what materials will be used. While FR-4 or one of its variants has been the traditional choice for PCB fabrication throughout the years, it may not be the best choice for newer high-speed designs. Standard FR-4 can exhibit some of the following problems, depending on the operational and environmental conditions of its intended use:
- Signal loss: As transmission line frequencies increase, the higher dissipation factor in FR-4 may contribute to signal loss.
- Impedance control: With a 10% tolerance in dielectric constants, FR-4 has a harder time holding precise impedance values than other materials that are rated at 2% or less.
- Mechanical instability: FR-4 has limitations to excessive voltage and heat, which could lead to the breaking down of its insulating qualities allowing electricity to be conducted.
- Absorbing moisture: Where some of the newer materials have a moisture absorption rating of 2%, FR-4’s rating can be significantly higher, up to almost 50%.
There are newer materials available for printed circuit board fabrication that can take the place of standard FR-4. These materials include enhanced epoxies, polyimides, and polytetrafluoroethylene (PTFE), which is more commonly known as teflon. These materials perform better than FR-4 in one or more of the problem areas listed above, but they can be more expensive and more difficult for the fabricator to work with.
How Your PCBA CM Can Help with PCB Layer Stackup Configurations
The key to successfully planning and configuring your PCB design’s layer stack is to get knowledgeable advice upfront before you start placing and routing components. One of the best sources that you can go to for this kind of advice is from your PCB manufacturer. As industry expert Rick Hartley says in the high-speed design classes he teaches, follow the manufacturer’s specifications and recommendations. “Don’t insist on what you want, find out their process and design to that. This what they do day in day out, and they are good at it.”
