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TECH ARTICLE don’t have lots of over lapping nets in a 2D drawing then it should be easy to do the same on the PCB. If I can keep it all to one side of the board I will; this allows for another circuit to sit on the other side. I have used this technique often with RPM monitoring circuits for fans and then have the PWM drive circuit on the other side of the board. They both have to feed to and from the microcontroller and device, in this case a fan, so it is good if they run in parallel.

run a 0-volt trace out and connect it back. I try and use “around the outside” rule. OK, not a great sounding rule but this means that I will have a track that runs around the whole of the outside of the board in a loop and then run feeds into these blocks. It’s also good to highlight the whole track and see where it’s running. Can you bridge a gap (Figure 3)? The better your 0-volt connections the more likely your board will work well!

For a digital layout, I will run the lines together as much as possible but I do not try to stay to one side of the board only. I will however use a rule: blue = horizontal, red = vertical. The blue and red refer to the bottom and top sides of the board as it is displayed in the CAD package. By running these lines only horizontal on one side of the board and vertical on the other, it will minimize the number of board layers required.

Once complete, I then tackle the flood plan. Now I know not everyone worries about this but I do a copper flood on top and bottom connected to 0 volts. By maximizing the 0-volt tracks the flood will reach all parts of my board and I will have good connections throughout.

However, there are no hard rules, and when you have a digital embedded circuit you will have both types of layout to deal with. This means I will layout each block as it fits each layout profile above. Digital and analogue lines from the micro will follow a more blue/red rule until they reach the analogue type circuits. This allows me to break out the signals from the micro and keep tight and compact analogue circuit’s ring fenced from each other. There is, however, one very important part of the circuit I have not talked about yet, the power rails. Now it’s not hard to remember to connect power rails like your +5 or +3.3 volt lines but people do forget about the ground or 0 volt rail. For each section of circuit or large device, I always check that I

Finally, I will run connection and clearance design rule checks that are built into PADs to flag silly errors before generating a final export to the mechanical engineer. He will, in my case, check that there are no 0603 resistors sitting under an IC when it was meant to be on the back of the board. I only made that mistake once! I then can send off my files for the board to be made. It is a good idea to be certain that all the components will fit on the board. I once worked at a place where we had a PCB mill that could drill and cut the tracks from blank sheets of copper. This was a good way to see if everything fit in the right place before spending money on prototype boards. There are many hard and fast rules you can add to your design approach, but these simple and basic principles allow me to design a board with minimal errors and reduce my chance of a re-spin. ●


Modern Printed Circuits: Sierra Circuits