PCBs (Part 1)

Part 1, 2

PCB design pitfalls… A quick glance on the net brought me to various interesting web sites:

“4 Circuit Board Design Mistakes to Avoid”
“PCB Design – The Top 5 Mistakes”
“6 PCB Design Mistakes to avoid designing printed circuit boards”
“7 Fatal Mistakes to Avoid on Your PCB Design”
“Top 9 mistakes that PCB engineers need avoid in PCB design”
“Top 10 PCB Routing Tips for Beginners”
“11 Myths About PCB Layout”

4, 5, 6, 7, 9, 10, 11 ? Is there really a limit to the number of PCB design pitfalls ? Probably not. From the very early times when I was using an ink pen to draw the tracks on plain copper before immersion in iron perchloride up to now, I had my glorious and my inglorious times which were more or less related to the success in making nice clean flawless PCBs.

So, based on my experiences, here is a list of reminders that I am sharing with you.

  • Footprints: Always get the components that you will solder on a PCB, check there dimensions, check your footprint libraries. Once routed, print the PCB copper layers and check the footprints. Personally, I am using my very own footprint libraries in order to avoid mastikes.
  • Not Plated Holes (NPTH): Unless your design is really screw-less, always provision holes for mounting your PCB on stands, posts or what so ever.
  • (Wire) Routing : If the copper layers look nice, the routing is probably fine. Spaghetti designs are almost always synonymous of trouble. Simplify and shorten routes. Check tracks width and maximize then when dealing with power supplies. Avoid routing tracks between pins.
  • Thermal relief: choose proper copper widths. Ban vias, plated holes, wide tracks located next to SMD pins. Avoid multiples tracks connect to a single pin.
  • Ground planes: Always ! However ground planes may have an impact on some components: use keep out areas if necessary. All tracks carrying weak signals must be buried.
  • Thermal vias: Although your prototype works great on the bench, the PCB may be used in harsh conditions and some components may insufficiently dissipate heat. Create thermal vias beneath power components and have sweet dreams.
  • Cuts: Did you check the overall dimensions of your PCB ? Check it again this way: print the PCB on plain paper, glue this page on cardboard, cut along the edges and try to adjust this mock PCB to its destination. Plain easy, plain beneficial.
  • Test points: Always ! Add test points to power supplies, critical signal points (inputs, outputs, driving pins, etc.). Whenever possible, group the test points in order to facilitate the work of the troubleshooter.
  • Silk prints: Always ! They are so usefull for identifying component references and orientations, test points, etc. Be strict while placing text, apply your own rules, e.g. on top or on the right. Always reference your PCB with a project name or code and above all with its revision name or code.
  • Wires: If you can, always tidy wires on one side. Don’t you ever connect wires here and there in the middle of nowhere. That’s rude.
  • Connectors: Less critical than wires however tidy connectors on the PCB edges, leave room around connectors so that clumsy fingers may be able to plug them.
  • Components: Use homogeneous component sizes. Try to orient polarized components in the same direction.
  • LEDs: So useful. LEDs are inexpensive and light indicators may save you a lot of time in the context of testing proof of concept prototypes. Feel free to get rid of the LEDs once all early tests pass.
  • Print geber files and analyze them individually. I am personally using a different software for designing PCB and for checking gerber files.
  • Check list: Build your own QC check list. Be strict, and whatever happens, even if your are in a hurry, use it: ultimately the time spent here will save you a great deal of time then ! If possible, ask someone else to counter check your works.
  • Backup: Once the gerber files is sent to the supplier, compress all related files and archive them (including libraries). Bringing corrections to the original files will drive you nuts. Create a “findings” file and a new version holding corrections.
  • Antennas: Think the PCB as a 3D environment ! Well, I think that if you are dealing with antennas you already know about the previous points of attention.

And now my very last advice: always leave one night in between your last final review and the upload of your gerber files to your favorite PCB maker. Most brains work hard during night, synthesizing thoughts and works so that, quite often in the morning, you may have an sparkling idea and get back in a hurry to one of those reminders that I tried to enumerate for you.

HTH

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