Making a good circuit board layout is a skill that as a professional or as a hobbyist you might want to acquire. Most of the time, people tend to spend a lot of time on their circuit design but then cut short when it comes to the PCB layout process. This is a mistake, PCB should be considered as a whole component, in fact, the most complex component, which can truly affect the functionality and the quality of your circuit design.
PCB design is both an art and a science.
That said, little disclaimer, there is no perfect layout design, everything is a compromise between your objectives and constraints such as electrical requirements, mechanical placement, component sourcing, and time, to name a few.
This article will highlight practical layout design tips that I feel can be helpful for people starting to make their own PCBs and that I discovered both as a Hobbyist and as a professional engineer. Here is a list of the subjects we will cover:
- Know your manufacturer’s capabilities
- Place your components to make your routing easier
- Calculate your track width and spacing
- Always use solid Ground planes
- Avoid using 90-degree trace angles
- Bypass capacitors
Know your manufacturer’s capabilities
Even if it may not seem related to PCB design, knowing which manufacturer will make your boards is necessary as they all have different capabilities and this can affect your design. Go on their website and check their recommendations. They will give you a lot of information such as the minimum track width, spacing between tracks, drill sizes, or the minimum character size they can print on the silkscreen.
Place your components to make your routing easier
Placing your components is where PCB design becomes an Art. There are certainly worse ways to do it but there is not a single “right” way to place your parts. Everybody with his own experiences and skills will make a different layout. Again, this is a matter of compromise (you will hear me say that a lot).
First, place your mechanical parts: Potentiometers, jacks, switches, and LEDs are placed according to your front panel design. This is why I like to develop my front panel at the same time as the circuit, that way I know what I want my module to look like.
Then, try to place your electrical parts to reduce the overlapping of rastnests. Take your time to do it as it will make your life easier during routing. Too many overlapping rastnests can cause you a bit of a headache trying to route that with only 2 layers.
Those 2 tips are pretty straight forward but here is a list of ideas to keep in mind to make other steps easier such as board assembly or troubleshooting:
Orientation: Try to place all your components in the same orientation. For all ICs, keep your pin1 in the same orientation, for groups of capacitors and resistors put them in the same orientation arranged in rows or columns, besides having a good-looking board, you will ease your life during the assembly process.
SnapGrid: Use the snap grid for easy geometrical perfection (I do like symmetry).
Components Side: Keep your components on the same side of the board: For TH board, it might seem obvious as there is no gain of space putting components on both sides. But for SMD, you might be tempted to put them everywhere. The manufacturing process will be easier (and probably cheaper).
Groups: Keep your components in functional groups, if you can separate them into small blocks on your schematics, then place your components accordingly. This will shorten the lengths of traces since the parts you’re placing are already logically grouped together.
Bypass Capacitors: Keep in mind the position of your decoupling capacitors, we will specifically cover this topic shortly after.
Calculate your track width and spacing
The track size you will use depend upon a few parameters:
- Electrical requirements
- Routing space
- Your manufacturer’s capabilities
- Your own personal preference
In practice, the track width is calculated by the current flowing through it and the temperature rise you tolerate in your board. You can find a lot of online width calculators. But unless you absolutely need to go small I would recommend using the bigger track width, the bigger, the better. For example, in my design, I always start with 10mils for signal tracks and 20mils for power tracks.
Remember that bigger tracks have lower DC resistance, and lower inductance.
One thing I want to highlight is that you can change the size of the track along the way, this is called necking down, and can be useful to go between components pads or for routing a BGA.
Another aspect to consider is the space between tracks. This is determined by your manufacturer’s capabilities (again) and the maximum peak voltage difference between the two tracks. For that matter, I recommend checking the IPC-2221 which defines the clearance required depending on voltage and on whether the tracks are on internal layers or external layers. I won’t develop about the effect of altitude since most synths will be played on earth (I guess).
Always use solid Ground planes
Use solid ground planes, a large area of copper, on both sides of your board to connect every ground rastnests. You have to remember that copper is not an ideal conductor, so using ground planes will provide a low-impedance path for return currents. Ground planes can be “solid” or in grid, I would recommend solid ground fills as they have the lowest impedance and are better for heat dissipation.
Two things to keep in mind while using solid Ground planes:
- On two-layer PCBs, exchange tracks direction between layers. Use only vertical traces on one side and horizontal on the other. That way you won’t “break” your ground plane and thus won’t create a large inductive loop.
- Remember to use thermal relief on ground pads to make soldering easier.
Avoid using 90-degree trace angles
For many years, it has been a common PCB design practice to avoid 90° angles. In fact, most PCB layout tools allow you to draw your track with 45° angles.
There was a simple reason to avoid 90° angles: they can create an acid trap during the etching process on the inside of the angle. This can cause over-etching which can decrease your track width, create necks, and even break your track.
Well, to be honest, modern manufacturing process makes this reason less and less a concern but in my opinion, there are still reasons to avoid using 90° angles :
- Routing at 45° angles often reduces overall track length while reducing current loops so it will improve both EMC emissions and immunity.
- IT LOOKS GOOD! This is a personal consideration but I like the way it looks and I feel that for many years professional PCB layouts were associated with 45° angles.
For “T” junctions, I like to have small pieces of copper to eliminate any 90° angles. This makes the track more physically robust and most importantly, it looks nice.
You’ve probably already heard of this one, bypass capacitors are placed between the power supply pins of integrated circuits and the ground. On schematics, you can usually find them drawn away from the functional circuit near the power supply symbols.
Their main goal is to reduce both power supply noise and spikes that may occur on the supply lines due to other components on the board. They do so by creating a low-impedance path to the ground for high-frequency signals. They also serve as a charge reservoir for switching ICs that might draw a high instantaneous current.
Even if it is a bit off-topic, a typical value for bypass capacitors is between 10nF and 10µF. This value will be highly dependent on the frequencies that you want to bypass. We will talk more about this in a complete article on capacitors. Just remember that you cannot replace multiple decoupling capacitors with only one as they all work at different frequencies.
As you may already know, bypass capacitors must be placed as close as possible to the pins you want to bypass. If they are placed far from those pins, the length of the track between the pins and the capacitor will then create a small inductor and resistor which will ultimately lower the useful bandwidth of the capacitor. In simple terms, placing a bypass capacitor away from the pins is like not putting a capacitor at all.
Where I want to go further is that as close as the capacitor can be, it also needs to be on the current path. That is a common mistake that I see in a lot of Eurorack designs. I the bypass capacitor is not on the current path it is also less efficient in cutting AC signals and on protecting your ICs against spikes.
Bypass capacitors should be place on the current path !
I have made a small simulation on LT Spice to show you how the place of this bypass capacitor can affect its efficiency.
PICTURES COMING SOON