The Last Mile of PCB Design: How to Clean Up Your Layout
A PCB layout is only complete once it fully reflects the information needed for successful assembly. Even after a board is fully routed, it needs to be checked and reviewed to make sure the assembly will be correct. There are aspects of a PCB layout that can’t be defined in design rules, which requires a designer to check things manually.
In this article, we’ll look at what a designer should check to ensure a PCB layout is ready for assembly and that the design intent is accurately reflected in the PCBA. As we will see, these points are not always caught by a DRC engine because they require insight into design intent. Make sure to follow these PCB layout cleanup steps when preparing your design for prototyping or volume production.
What to Clean Up in Your PCB Layout
The list below is by no means exhaustive, but it does list some of the common areas in a PCB layout that require attention before manufacturing. Make sure to check each of these areas.
Top and Bottom Silkscreen Layers
Silkscreen on the top and bottom layers will contain important information for assembly, testing, debugging, traceability, and much more. Silkscreen should be checked for any of the following errors:
- Check for overlapping or too-close reference designators. If a design rule for this is not enabled, then it won’t trigger in the DRC, so make sure to check for silkscreen visually.
- Make sure reference indicators are not falling onto a pad or un-tented via. This is another issue that clearance rules might not catch depending on how they are defined.
- Make sure part numbers are located in an area where they can be easily seen and read. Perform the same check for company logos, CE/FCC markings, ESD markings, etc.
- Set the appropriate font size for reference designators so that they are easily seen without optical aids.
- Check components have polarity indicators applied in the footprints, pin 1 indicators, and outlines that do not fall beneath component bodies.
- Place polarity indicators for power inputs, including voltage/current levels.
This list of checks can be performed visually and with the DRC engine in your PCB design software.
These reference designators are overlapping, which could be clearly seen by only turning on the surface layers.
Hole Sizes and Tolerances
When routing, it is tempting to place whatever via makes sense in the moment. This leads to a large number of inconsistent hole sizes throughout the PCB layout. Sometimes, these hole sizes will be non-standard, so your manufacturer will have to adjust the hole size to match their available tooling. Make sure you run through your vias in your layout and set them to a consistent hole size. Use as few different drill sizes as possible, and make sure these drills are standard sizes.
The other side of hole sizes is tolerances. You’re free to specify whatever tolerance you want, as long as you specify something. The problem is that manufacturers might not be able to hit your tolerance target. A safe value for a drilled hole tolerance (either drilled size or finished hole size) is +/- 3 mils (or +/- 0.075 mm).
This drill chart contains only standard drill sizes with tolerances included.
Tolerances come up in other areas as well, most notably in your fabrication notes. These specifications are used to define tolerances in your layer stacks, trace widths, impedance targets, board dimensions, and anything else that needs to be fabricated to precision. If you don’t account for these points, your fabrication house will set values for you, and they might not hit your application specifications, so make sure you communicate your requirements clearly.
If you took time to develop mechanical requirements at the beginning of the design process, then you’ll already have a list of mechanical items to check before sending the board out for fabrication and assembly. Mechanical elements in the board like connectors and critical components should be checked for positioning in the PCB layout, even after the layout has been completed.
Is this hole in the right location along the board edge? Make sure you measure hole positions.
If you want to bypass some of these mechanical checks, make sure to lock the mechanically important elements in place in your PCB layout. Things like mounting holes, connectors, fiducials, and some of the major components (like large processors) should be locked in position early and should not be moved unless absolutely necessary.
Any Remaining DRCs
Even after running all these design checks, there could still be some outstanding DRCs that need to be cleared up. Sometimes, running the checks shown above and implementing any fixes can create new DRCs. Therefore, as a final check before creating outputs, run another DRC check to ensure there are no outstanding rules violations. Always clear these up before you finalize the design and prepare outputs for manufacturing.
The status window in OrCAD PCB Editor will show you if you have any remaining DRC errors in your PCB layout.
When you’re ready to send your board off for manufacturing, make sure to follow these PCB layout cleanup tips to ensure successful production. Any time you need to create a new product, make sure you use OrCAD, the industry’s best PCB design and analysis software from Cadence. OrCAD users can access a complete set of schematic capture features, mixed-signal simulations in PSpice, and powerful CAD features, and much more.
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