Before you can even begin laying out your PCB design it is important to have completed an accurate schematic diagram of the circuit you plan to create. PCB design is a lot easier if you start with a clean, neat and logical layout of the circuit. Signals should flow from input to output in an organized layout with all of the components, their respective capacitors and pins clearly labeled. Notes can make your job much easier in the future when it comes time to actually layout the PCB. By incorporating good design practices into your electrical schematic you will be well on your way to creating PCB designs that are both organized and efficient.
Know Your Mechanical Prerequisites
Next you need to be aware of any mechanical prerequisites required for the PCB layout. Does the board have to be in a specific shape? The dimensions of the board become a mechanical constraint. The shape will also determine where mechanical components like the potentiometer or switch have to be located. Understanding how the signal is going to flow, where the inputs and outputs are going to be, and how the power is going to get in, is crucial in this step. At this stage it is important to draft the locations of any USB jacks, barrel jacks, slide switches, RF jacks or the RPSMA. This will define the constraints of your build. The layout will help you understand where the different circuit sections need to be located.
Once you understand your constraints, copper weight is the next factor to consider when creating your PCB. When purchasing copper it is usually designated by weight. The weight is defined as how thick the copper would be if it were pounded flat to cover one square foot. The resulting thickness for one ounce copper is 1.37 mil, or a thousandth of an inch. One ounce copper is universally less expensive and acceptable for most jobs, making it the material of choice.
Two layer boards are inexpensive and adequate for most PCB applications. Sometimes it is worth investing in a four layer board which provides a dedicated VCC plane and ground plane in the middle. This allows you to place traces more efficiently. Four layer boards also provide additional capacitance between the two layers, which acts as a filter that provides decoupling across the board.
Traces are the conductive tracks and other features etched from copper sheets and soldered onto a non-conductive substrate. They are the lines you see completing the circuit between the different components lying on a circuit board. A useful heuristic is to use 10 mil traces for low current paths 20 milliamps or less. A ten mil trace becomes resistant at 30 milliamps. You can use circuitcalculator.com to figure out how thick your traces need to be for a given copper weight and current.
As a general rule, try to avoid auto-routers since they tend to create messy trace lines, it is better to keep your traces clean and simple. Another useful rule of thumb is to avoid right angles in your traces. This can round your edges which will increase the probability of issues in programs like Logic which need to be able to read edges. Finally, you want to keep tracks as large as possible, only using thinner tracks to meet space requirements. The process of alternating between different trace thicknesses is called “necking”, this gives you the flexibility to use large low impedance tracks when necessary.
Ground Plane and Design Rule Check (DRC)
The ground plane on a PCB is a large layer of copper foil connected to the circuit’s ground point. The ground point is typically one terminal of the power supply which serves as the return path for current from various components. In multilayer_ circuit boards, a separate layer is often entirely covered with copper allowing the designer to punch down from a component to complete the circuit. When laying down your traces it is advised to lay down all of your signal traces first.
Since you would ideally want to keep the bottom ground layer clean, some traces will inevitably end up needing to be placed on this level. This is why it is important to run the Design Rule Check (DRC) function which will tell you if you have any dimensional problems or error wires. The DRC will not tell you if your return path is inefficient. If you have any traces on the ground layer, make sure that the path from a given component to ground is as straight a line as possible. If it is a low current return, you may be able to get away with simply punching down to ground as the current will follow the path of least resistance. However, keep in mind that for higher currents, any traces within the path from the component to power supply can interfere with the return path. You want the path to be as clear and as short as possible.
When you have completed your PCB design your next task is to handle the aesthetics or silk. It is important to remember to use labels and headers on all your components, signals, pins, inputs and outputs since it will take time for a manufacturer to create your board. Many programs like EAGLE will label components like resistors by default. This can be benefit if you ever need to get in there with probes or a soldering iron to re-work the circuit. Once you have your prototype and are ready to test it, make sure to set a limit on your power supply slightly above your maximum current but not enough to cause anything to burn. 100 milliamps is an acceptable standard, not too high above the typical 30-40 milliamps of a circuit board. This will let you know that your circuit is robust enough to handle any rail or limiting situations. You may also use a meter and put it from ground to Vcc and make sure the currents run as expected. If things do not perform as expected you may have to redesign the layout of your board.