So, you have a design for an amazing circuit, perhaps even tested it on a breadboard, and you’re thinking of building a more permanent solution on a custom printed circuit board… but wait! Before you open up your favourite PCB design software, read these crucial tips on board layout – it will save you some major headaches later!
A quick disclaimer: I’m going to assume that if you’re reading this article, you have some knowledge on how PCBs work, and that you’re familiar with some of the basic terminology (like what pads, traces and vias are).
#1: Component placement is the key!
When laying out a board, your success will be mostly determined by how well you lay out your board. Don’t stick the pull-up resistor across the board from the button! Allocate sections of your board for different functions (i.e. power, buttons, motor drivers, etc.). This will make the job much easier when it comes to actually drawing traces between pads.
#2: Work to a 50mil grid!
Make sure that your components and traces snap to a 50mil grid. For routing fine traces, there’s often an option to change the grid when you hold the “alt” key; make sure this number is an easy fraction of 50mil (i.e. 25mil, 12.5mil, 6.25mil, etc.)! When I say “mil”, I DON’T mean millimetres; I mean thousandths of an inch! It’s the standard unit when it comes to electronic components and PCBs.
#3: Always use 45。 angles!
You’ll notice in the image above that there isn’t a single right angle in the traces (except in junctions, where they diverge). Why, you may ask? Well for starters, it looks nicer. Much nicer. However, there is a reason for this beyond the aesthetic: it makes routing easier. Angles of 45。 let you duck and weave between pads and other traces, whilst still maintaining some sense of an orderly grid. Also, NEVER use arbitrary angles by drawing the shortest possible trace between two pads with a single line. Apart from looking horrifying, you’ll soon get stuck when you find that your traces collide. Remember when you learnt, in your primary school days, that parallel lines never meet? If you stick to using the same angles throughout, you’ll be able to stretch traces across kilometres of board without having them collide!
#4: Don’t autoroute, do it manually!
Most PCB software packages come with an “autorouter”, which basically does all the routing of the board for you. Unless you have a ridiculously high density of components, it’s best to do the routing manually. Although programs are getting smarter every day, a human mind can currently do a cleaner job most of the time. It’s easy to tell the difference between a board that’s been routed automatically and manually; the autorouted ones usually have seemingly random twists and turns where the software conforms exactly to the routing grid.
#5: Use silkscreen (but don’t go overboard)!
The silkscreen layer, which is basically a layer of white paint that is printed on top of your board, is invaluable (and in most cases, free)! Use it! However, here are some dos and don’ts of silkscreen:
Do:
•Label the IC names, so you know which chip to use.
•Put a pin1 indicator so you don’t solder things the wrong way around.
•Label headers, jacks or terminals (i.e. “Motor Output 1″).
•Label any LEDs, unless it’s self-explanatory, such as a power indicator being the only LED on a very simple board.
•Label the function of switches, as well as which position is “on” or “off” (or whatever else).
•Print the revision number (and date) of the board, if you intend to improve the design in the future.
•Use “vector” fonts – your PCB package should support this. Don’t use any “proportional” fonts.
Don’t:
•Label resistor and capacitor values – that’s what the schematic is for! Too much text clutters up your board.
•make your text too small or thin – it simply won’t print properly if you do. I usually don’t go below 40mil text, and I stick to a “ratio” of 15%.
•Print over exposed copper pads.
Optional:
•Print a logo on the board!
•Draw the component outlines, which makes placement a little easier.
•Label resistor and capacitor names (not values), such as R1, R2, C1, C2, etc.
•Print any important messages or warnings, if you want to emphasise it.
Here’s an example of some silkscreen text, printed in a “vector” font:
Some board manufacturers let you print silkscreen on both sides for free, whereas others charge a fee for printing on the bottom, because it’s an extra manufacturing step. It’s up to you to decide; printing on the bottom gives you more room for a large logo, or some extra information!
#6: Use a ground fill, or “copper pour”!
You’ll notice that in the image above, large areas of the board are covered in copper. These are all connected to ground. This helps you in two main ways: first, it aids in heat dissipation. Second, it makes routing easier, as you’ll likely have a lot of connections to ground, even in a fairly simple board. Ground fills can be created by drawing a polygon around the edge of the board, then renaming it to “GND”. However, there should be a setting to “isolate” the polygon – this draws it a little further away from other tracks, to prevent shorts. I usually leave the isolation setting at 10-12mil or so.
You’ll also want to make sure you’re using “thermal” pads. This is when a ground pad is routed to main fill by little traces, instead of just being connected from everywhere. This helps make sure that when you apply heat from your soldering iron, it doesn’t all get sucked away into the rest of the board, leaving your pad too cold to melt the solder.
If you follow these tips, your board should come out looking a lot cleaner, as well as being much easier to use! However, nothing comes short of experience, so go out there and design a board!
