PCB Design Best Practices

● The first thing you should do before you place any components or traces is think about where your PCB will be manufactured. Will you mill the board in the Maker-E or will you send it out of house to be professionally manufactured? This decision will affect the set of design rules that you use. Design rules are limits set up within the PCB design software to make sure the PCB conforms to the abilities of the manufacturing process. These rules include things like trace thickness, via size, lines spacing, etc. Below are a set of design rules you should use whenever you intend to create a PCB using the PCB mill in the Maker-E.

● Set your design environment dimensional units to metric or imperial, depending on your preference.

● Consider the environment where your PCB will be used. Is this a demo or evaluation board that will not be part of a larger system? Or will your PCB be connected to other devices or sit inside of an enclosure? The answers to these questions should affect your PCB layout, shape, dimension, etc.

● Consider using power planes. These are large copper areas of your PCB to which multiple connections need to be made. This will minimize the number of traces needed for the PCB and may help with radio frequency interference.

● When you begin placing components, place inputs and outputs first, things like buttons, switches, displays, connectors, etc. Think about if and how someone will interact with the PCB and place components accordingly.

● Next, place integrated circuits (ICs). If one IC has many connections to another IC it is wise to place them physically close together so that you can keep connecting trace length to a minimum.

● If your ICs have supporting components (things like resistors, capacitors, crystals, etc.) keep these components very close to the ICs. Again, this will minimize trace length and keep via count to a minimum.

● Keep components close together. When having PCBs professionally manufactured you pay by the square inch so it is cost effective to minimize space use, unless certain components have specific rules about clearance to other components and traces.

● Keep trace length to a minimum. When positioning components, do it in such a way that trace length can be minimized. Keep an eye on the ratsnest when moving components around to get an idea of how long traces will need to be.

● When running thin traces (typically traces less than 1 mm in width) avoid 90° turns. These sharp turns can create breaks in narrow traces which may cause your circuit to malfunction. Instead, use two 45° turns to accomplish a 90° turn.

● Try to minimize the number of times the ratsnest intersects or crosses over itself. Every time there is an intersection between two lines in the ratsnest there is potential for the use of a via.

● Minimize the number of vias used in a PCB design. If milling a board in the Maker-E, more vias add more manual processing time to your board. If your board will be professionally manufactured there may be limits on the number of vias you are allowed to have in your design.

● When designing PCBs that will be fabricated in the Maker-E using the PCB mill, there is a limited set of bits available for use with the mill. You should limit hole sizes in your PCB to the following diameters:

* Indicates that the tool is normally in the mill, for the firefly project

** Requires permission from the lab director to use

• If you have a large board, try laying things out in blocks. Lay the power supply items out as a block (outside of the board outline), then the inputs, and then the outpts, etc. This allows you to arrange smaller pieces at a time. This makes the process a little less intimidating, and also allows for neater boards with less crossovers. After you have laid out a block to your satisfaction, drag it into the board outline and route traces.
• PCB design is an art, it takes a long time. If you think your first try is perfect, you have done something wrong. You will likely have to start over many times when designing a board. It is not possible to visualize all of the connections at once. When you start connecting things, you will usually find a better way to do it and want to start over. This is where the block idea comes in handy. If you design in blocks, you usually start off with a better design, and usually end up making small modifications to blocks to make them fit better.
• Have a layout order. Place IO first, then large things like chips, or connectors that have a lot of connections.
• Route signals and interconnections first. Power and ground should be run last. Usually, you can take care of most of the ground connections with a power plane, and the power connections you can usually find a way that works. It is usually easier to snake power connections around to parts than it is to place power connections first and then have to snake everything else around that.