From a Computer's Hands to Yours
This is a continuation of a series on where printed circuit boards (PCBs) come from, the first post can be read here, and now we're going to be diving deeper into how people create PCBs. A PCB is the end product of a multistage development cycle. Before an engineer gets the physical board, they need to design a circuit, simulate the circuit, turn the circuit into schematics, layout the computer aided design (CAD) version of the board, and then generate some files that a manufacturer can use to build their dream. While this might sound complex, it's miles ahead of how the process used to look. Before the advent of computer aided design, engineers hand drew schematics and then wrapped wire around pegs to make solid connections, as pictured below. Today, computers greatly simplify this process, and it starts with a circuit in simulation.
Left: Wire wrapped PCB, source. Right: Hand drawn PCB schematics from the 1960s, source
Circuit simulators are a great first step towards creating a PCB once an engineer has an idea of the circuit they want to build. Simulators like LTSpice or TI's TINA allow designers to use a drag and drop interface to build their circuits, and then the program runs the complicated electrodynamics equations in the background to verify the design. This enables the designer to easily catch and fix issues before going through all of the trouble of having the board made. It's also the design phase that requires the most electrical engineering knowledge, because the process involves directly tweaking your circuit to find the optimal behavior. The more thorough a designer is when representing the circuit in the simulator, the better the end product will be. At FANTM, we have an open source EMG circuit that amplifies signals off your muscles to control your computer. We have a simulation of the circuit we use that we're constantly adjusting; we model the muscle and ensure the output is suitable for Arduino (giving us the FANTM DEVLPR). Once an engineer has a simulation that they're happy with, they're ready to move forward with the design.
A screen capture of a TINA simulation we run. You can see the resistors (squiggly lines), capacitors (two parallel lines), and ICs (the triangles) that we talked about in the last post.
Next, it's time for schematic capture. There is an important distinction between the logical and physical representation of a circuit. The simulation and schematics use the logical representation, allowing a designer to easily see which parts are connected and keep a design clean. However, if you have seen a picture of a PCB you might notice that the ICs don't look like boxes and triangles and the resistors aren't squiggly little lines. Schematic capture begins to bridge this gap: it is when the designer redraws their circuit in the same format as the simulation, but as cleanly as possible. They do this so that other engineers (and even they themselves) can easily review and improve their designs. Another important addition is details on the physical parts; this includes their size, links to where they can be purchased, and their data sheets. Schematic capture gives the computer the information it needs to complete the translation of the logical into the physical in the next phase: layout.
Schematic capture in KiCAD. Notice that the design and style is very similar to the simulator.
The layout phase is where the digital PCB is created. It is when the engineer manually places each footprint (the pads that encompass a single component) on a digital version of the final PCB. Usually footprints that will be connected to each other or serve a similar purpose are placed together. Next, an engineer will use a routing tool to connect the footprints with traces according to the schematic. It's the engineering version of connect-the-dots, and it's more fun considering you're left with a completed PCB. All that's left is exporting a couple files, known as gerbers, and sending them off for production. With the PCB production infrastructure of today the board is usually back in a week or two. Once it's back, all that's left is placing the components to create a completed PCB.
Layout in KiCAD. This FANTM board has been completely routed, and if you have a DEVLPR you'll notice that it looks very similar to the picture.
There are many more challenges and design considerations when taking a design from circuit to PCB, but it's not as complex as it might seem. There are tons of other great tutorials about how to get started, how to work your way through each of these phases, and how to use the different tools you'll need. This is the technology behind the consumer electronics we rely on, and now hopefully you have a better idea of how the sausage gets made. Please leave any questions or comments, and check back in two weeks for our next post! 💪🤖