Tips for Getting Started In Compact PCB Design
I mentor a graduate engineer who recently had their first encounter creating a high-density layout PCB for a design project. As a professional engineer, this was an interesting experience to witness how they approached this task. When it was finished, I asked my mentee what they had learned during the project. Their feedback is quite enlightening, so I thought I would share each of the points raised with a wider audience.
- During the schematic design, always think carefully about the layout and choose components with the right footprints. This is especially important in high-density layouts where there is a limited amount of space. When placing a component, consider whether you will be able to route underneath it or not.
- Do not automatically choose the component with the smallest footprint. Selecting a component with a larger footprint will not always take the most overall space. Sometimes placing a component with a larger overall footprint can open up the option for routing underneath it. Without this option, high-density designs can be much harder to route.
- In high-density designs that include a microcontroller, study your MCU so you know what features it has and what functions it can perform to do as much as possible with the MCU without requiring additional components to be added. For example, if you use an MCU that includes internal pull-down and pull-up components, you can eliminate any external pull- downs and pull-ups from your design.
- Remove any unnecessary components from your schematic design. For example, if you use an MCU’s in-built analog to digital converter function, you will probably add the additional external filtering recommended in the MCU’s datasheet. However, do you really need this filter if you are only using the ADC for a non-critical function such as battery charge monitoring? If removing the filter doesn’t adversely affect the functionality, this can save much-needed board space.
- When starting with your component placement, always add a via to the ground from the very start to ensure that there will be enough space should you need it.
- When starting off with your manual routing, always begin with the larger vias and wider traces. That way, the routing will be more straightforward, and you will know that if the via and trace do not fit, you always have the option to make them smaller or thinner.
- Group together components by their function, do not spread them around the whole of the board area. This will make the PCB easier to route as the traces will be much shorter between connections, saving PCB space, and making the device more immune to EMI (electromagnetic interference) and emitting less radiation.
- Route all the critical signals at the same time as when you place the components. This means that you will ensure that all the vital connections which require special attention will be routed the correct way. Otherwise, there is a good chance that you encounter a problem when routing these signals, which means you may need to start from the beginning using a different component placement.
- Always place any special components which require specific and strict placement, like connectors, first. Only after placing these components will you be able to see how much space you have left to place all the other components.
- Do not forget the basic rules of PCB design when designing your high-density layout. Sometimes when you are trying to fit eve+ry component in a small area, you can forget that there are electrical rules for the PCB design that you need to follow in addition to the Tetris-like challenge. These rules include placing decoupling capacitors as close to the IC supply pins as possible, implementing the best switching power supply design with minimal current loops, and placing TVS diodes in series with the other connections as near to the connector as possible.
- If this is your first high-density design, do not be afraid to just get started. You will probably need to redo the design again, but that’s okay; that’s how you learn. My graduate engineer had to redo their design four or five times, and they learned something new every time. If you think there is not enough room, then try and rethink the layout. Often there is plenty of room on the PCB; it just needs to be uncovered. My graduate thought there wasn’t enough room for everything. Still, then I threw a spanner in the works by suggesting he change all the 0201 SMD components to the larger 0402 SMD type and change the connector to a bigger one for easier hand assembly of a prototype. To complete this, he needed to open up more space for routing and managed to solve the problem.
- Use the PCB design software tools that you have available. For example, Altium Designer® has so many short keys and tools available to you during layout, saving you time and giving you an idea if your placement is right and if it will be routable. Create rules early on according to your manufacturer’s capabilities because if it turns out that the PCB cannot be manufactured, you could be in trouble.
- Do not be afraid to change your MCU pin functions during the PCB design process. Pins like GPIO’s can be easily changed most of the time, which could make your PCB layout simpler.
- Review your design several times because any errors found after you submit your work or order your PCB could be too late to fix. Mistakes can not only appear on your PCB layout but also in your schematics. For example, my graduate engineer found that a few of their components did not have any connections, and they did not notice this at first. If these errors had not been fixed, the mistakes would have been critical, and the device would not have worked. Once the device has been assembled, it is always much harder to discover why it’s not working because there are many more possible reasons beyond a simple design error.
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