Grade my PCB Layout

Thread Starter


Joined Nov 5, 2015
Hi, over the years I've made a number of PCB's but this is my first 4 layer board and hoping I can get some opinions on it. I know you can't say how well it will work, just based off general layout rules.

I have some 2 layer boards based on the same design (different layout) however I think there are ground issues and appears to be mid levels of noise. For example some relays connect to a DB9 connector on board, when the relays are turned on/off rapidly the isolated circuit for pH will be affected a little bit. I think this happens because these traces run under the pH circuit (other side of board) but also chop up the isolated ground plane for pH. Other than that the 2 layer board isn't too bad but I think with so much on the board I should be using a 4 layer as the ground plane is a mess, I'm sure sections are cut off from each other somewhere which probably isn't good even if I am getting away with it.

I spent about 30-40 hours on this layout, I didn't want the board size to change from current so needless to say it was a puzzle. I feel this is my best layout to date as I have signal types separated better and a lot less moving traces from top to bottom and vice versa.

Here's what the board is about, the first image below shows where the various circuits sit. If anything sounds wrong please let me know.

  • Board is a HAT for Raspberry Pi, it plugs on top of board and all components are on bottom. Board is 102mm x 140mm.
  • Layers
    • Top layer - mainly outputs, no copper pour however bottom right corner has an isolated ground plane for pH circuit. I didn't want to share with main ground plane. On left side of board under Pi is a +12v power plane for the DC ports. I put a +12v power plane on bottom as well in same area.
    • 2nd layer - main ground plane, no traces.
    • 3rd layer - +5v power plane, no traces.
    • Bottom layer - Right side of Pi are mainly inputs and PWM signals and under Pi is small +12v power plane for the DC ports, no copper pour.
  • I'm using the I2C Bus for a PCA9685 and the pH circuit, Pi is 3.3v so using BSS138 as logic level converter to bump up to 5v as there's an external port as well, yes I know... All traces are on bottom of board, no vias, and short run.
  • The most sensitive circuit is for pH as it reads voltage at a high resolution. This is fully isolated on I2C bus using ISO1540 and a RFM-0505S for 5v and ground. It has an ATtiny85 which sends the data via I2C bus to Pi software. In image you can see only half of each isolation part are covered with isolated ground plain.
  • Pi is configured for 16 - 3.3v outputs (LOW/HIGH) and 8 digital inputs. All Pi outputs go to "Black" box at top of picture.
  • PCA9685 has 16, 0-5v PWM outputs, 8 go to PWM/Analog converter and 7 go to DC ports (N mosfet's) on left and 1 to another N mosfet circled in blue near the top.
  • Added 0.6mm isolation on all planes.
  • Signal traces are .254mm wide, a few short power traces .3048mm - .4096mm unless obvious high current.
  • Signal vias are .4mm drill .8128 diameter.
  • Trace spacing .4mm or greater.


  • Am I correct to not have a copper pour on top and bottom layer as I have dedicated ground plane? Should one be added but not connected to anything?
  • Should I keep a +12v power plain on top and bottom of board for the 12v DC ports on left side? Thought was more copper for heat and current although overkill for latter.
  • For this layout is it worth the extra money for 4 layer vs 2 layer?
  • In the 4th image I highlighted an L in red, this is a 1mm wide cutout in board, is this a good idea or not? Thought is to keep any noise from board away from pH circuit. I was also thinking of adding one along the top with a bridge to keep board firm but going back and forth on this idea as it cuts up the main ground plain.
  • When I connect my main +5v output to the power plane should I fill the small pour with vias? Same with the +12v power planes?

Thanks for reading and any comments or tips you might have.
Top layer copper.

Bottom layer copper

Top view

Bottom View

I won't post pictures for 2nd and 3rd layer as they are just blank obviously with a lot of holes in them. :)
Last edited:

Thread Starter


Joined Nov 5, 2015
One more question.

There are 7 of these TO-220 mosfets, you can see circled in red is the ground, is that sufficient for possible 3 amps? The trace is .8128mm and via .8mm drill and 1.4224 diameter.

I added some green dots on the +12v planes, if both should be used should vias like that be added? I assume yes.