Learning Tips: PCB Circuit Design*

Learning the art of crafting your own PCBs? Yay! There are manywebsites that go in to great detail on how to use the various kinds of software, the nitty gritty of how to design a circuit, and all that jazz.

I’ve always found that there are tips, tricks, and “oh, hey, that thing I just scanned over was really important after all” moments. Maybe you’ll find a list of those things to be helpful? Yes? You’re in luck! This post is a collection of those tips, tricks, and moments.

Have one or a few to add? Please submit them in a comment below!

Physical Practicality:

  • Print out a 1:1 version of your PCB on paper, tape it to cardboard, and test fit all mechanical before releasing your board.

PCB Layout

  • Create your board frame on a 0.05″ grid. Make the lower left corner start at 0,0.
  • Stick parts on a 0.05″ grid. You should not break this rule unless you have a very good reason.
  • Separate the power grounds, signal grounds, analog grounds, digital grounds, control grounds.
  • Keep power and ground traces close to each other.
  • Keep inputs and outputs separated and isolate to prevent oscillations.
  • Place Capacitors that bypass supply voltages or decouple very close to the IC chip pins.
  • Give good spacing and clearance between tracks and pads
  • Put Reference designation on the schematic and on the PCB silkscreen to help troubleshooting
  • Put several test points that can isolate each section of the board from the power supply. This can help with troubleshooting or separate the power supply if it gets blown.
  • Put connections on the edges of the PCB
  • If you have 7 input molex connectors and 7 output molex connector and their is a SHORT from VCC to ground it will damage every single input and output IC chips.
  • Any LED should be labeled with its purpose (power, status, D4, Lock, etc).
  • Idem for connectors, pins, switches, and switch states: e.g Vin, Port1, Batt, 5V, TX, Power, +, -, Charge, On, Off, USB etc.
  • When applicable, it is better to avoid having vias go through the silkscreen when adding labels.
  • Group components together. For example the resistors surrounding a transistor in your schematic will also be grouped together on the PCB.
  • To minimize the chances of interference, rout data lines using short and symmetrical traces. Keep power carrying traces at about 7 mil from data traces to reduce capacitance/coupling.
  • When possible try to keep the traces size to 10mil. and use thicker traces for power lines. 12mil=100mA max, 16mil=500mA max etc.
  • Avoid 90 degree corners. Straight lines with 45 degree corners are preferable.
  • Where applicable use a ground pour on top/bottom layers.
  • To prevent pours from shorting to traces make sure you use a 10mil isolation setting on any of the ground pour.
  • Lay the PCB out at least twice. When finished, put it away for a day. Then come back and walk through the whole design again.

Schematic Layout

  • Use a GND symbol for all the GND connections.
  • Use appropriate power symbols for All VCC, 5V, 3.3V etc.
  • Add color notes to separate a complex design into various smaller bits (for example,charge circuit, accelerometer, etc).
  • Use DRC all the time!

Footprints

  •  All footprints need a reference designator {{refdes}}. If you come across a part on a board that doesn’t have this, you should change it and save the library. Verify ALL symbols and footprints you use and add known-good ones to your own library. For parts requiring it a pin one marker should be defined.
  • Order parts from Adafruit or Sparkfun? They’ve got their own libraries. Download them, but be sure to double check the footprints just in case.
  • All footprints need silkscreen indicators showing mechanical sizes, dimensions, or anything wired about the part.
  • To prevent it from flaking off easily silkscreen within a footprint or board should not go over pads or metal that will be exposed.
  • Top component layer should be marked by a red center cross.
  • Package outline layers should outline the actual package size.
  • The Top Courtyard layer should include all of the pins.
  • When adding a footprint make sure you add a solder mask.
  • Know your soldering skill level. Don’t think your up for soldering to the small pad that’s included for the part in your library? Create your own library with longer pin traces, to increase ease of soldering.
  • Every new footprint should have a human readable description.

Bookmark:

Credits, hat tips, and thanks to:

*This post was originally written for the OSH Park blog, and published on November 5, 2013.

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