PCB Design Rules
When you design a circuit board, you need to follow the PCB design rules of your manufacturer. These rules are the limits of the machine that makes the circuit boards.
The first time I looked at the rules I got really bummed out because it seemed so freakin’ complicated.
“Why can’t they just make my PCB???”
After a while I calmed down and had a look at what I was up against. And it wasn’t that bad after all
Example PCB Design Rules
The above picture shows the PCB design rules of Seeed Studio.
Well, actually some of it are just specifications. For example board thickness says something about the PCB thickness they can manufacture. That doesn’t really concern you in the PCB design process.
Which rules are important to us as designers?
The rules we need to pay attention to are these:
- Minimum trace width
- Minimum trace/vias/pads space
- Minimum silkscreen width
- Minimum silkscreen text size
- Drilling hole
- Finish hole
- SMT min solder mask width
- Min solder mask clearance
Minimum trace width
The minimum trace width is the minimum width of your traces. Hmmm.. Sounds pretty obvious right?
And it is. You have to make sure you draw your circuit board “wires” with a width of more than 0.1524 mm.
Minimum trace/vias/pads space
Outside each trace, via or pad there needs to be some space. In this case it has to be at least 0.1524 mm.
Minimum silkscreen width
The silkscreen is what we use to draw or write text on a circuit board. The width of the silkscreen has to be at least 0.1524 mm.
Minimum silkscreen text size
And we can see that the text size has to be a minimum of 0.8128 mm. Not sure what they mean about that. But I have experienced several times that my PCB silkscreen text has been increased.
Drilling hole and finish hole
The drilling hole has to be between 0.3 mm and 6.35 mm. And the finish hole between 0.8 mm and 6.35 mm.
Actually, I have to admit that I am not 100% sure about the difference between drilling hole and finish hole. But I have a theory. I believe the drilling hole is just a hole drilled in the board. And the finish hole is a hole with metal coating on the inside. Since the metal coating takes some space it makes sense that this hole has to be drilled bigger.
SMT minimum solder mask width
This is the minimum width of the solder mask. Solder mask is something that the solder doesn’t like to stick to. So it is around all pads to keep the solder on the pad.
And in this case it has to be at least 0.1 mm.
Min solder mask clearance
And the solder mask clearance is the space that is needed around the solder mask. 0.13 mm.
A Simple Way Of Keeping The Rules
To make sure you follow all the PCB design rules, Eagle has a tool called Design Rule Check (DRC).
In the layout editor click on the “Tools” menu and select “DRC..”
Here you can either specify the rules by hand (which is a little bit cumbersome) or you can load a design rule file by clicking the “Load…” button.
Many manufacturers has a design rule file for Eagle ready to be downloaded so that you only need to load it into Eagle.
Click on “Check” when you are ready. If there are no errors, nothing happens except for a text in the bottom of the screen saying “no errors/warnings”.