Spring 2016 3DOT Goliath, PCB Layout
By: Jerry Lui ( Manufacturing Engineer)
The general layout of the PCB was determined by the how sensitive the signal was, the specific wiring of IC’s to surface mounted components, and mounted surface area. Power input was placed away from sensitive signals and had a trace width of 32mm @ 1oz/ft^2 copper thickness (determined by the PCB house/fabricator) with a total temperature shift of 10C which allows total current of 1A. The trace thicknesses can be quickly determined by the following chart:
While laying out the board make sure to perform a DRC error check frequently. The included .DRU error checking file has generic limitations which may or may not adequately represent your fabrication houses’ design requirements (EagleCAD will always flag drill and VIA holes). The fabrication house will generally supply a .DRU file that can be used in place and will often allow traces to be placed closer together.
All of the signals were moved as far from the power input as possible in this case I have placed on the top left corner. The large power supply capacitor was also placed as close as possible to the power input and the rest of the circuit was laid out to take power from that capacitor. Also, components were placed similarly to how they were wired in the schematic as much as possible so that they are generally easier to follow and understand.
Note that there are red dotted lines (and blue underneath) along the perimeter that represent the ground polygon which is needed to create the ground plane (red for top layer, blue for bottom layer). The isolation width (free space between the traces and ground plane) was set to 12mils by default but was changed to 16mils as a precaution. This value is mainly determined by the fabrication house limits.
To place a trace on the other side of the board while you’re currently laying a trace (top to bottom) click on the middle mouse button. This will automatically create a VIA (electrically connected through hole) that connects both traces.
TIP: If you don’t create a ground plane first you can right-click on any GND signal and hide it. This will reduce the air-wires shown so that laying out the PCB will be easier. All GND signals will be hidden with this option but can be restored by typing “ratsnest *” in the command line.
TIP: Component values can be hidden by disabling the tValues layer. This makes it significantly less cluttered.
In the picture below, the ground plane has been created by hitting the “ratsnest” command. Pay close attention to capacitors during the ground plane creation. The “relief” option should be enabled so that the negative terminal can actually be soldered easily; this option is set on by default.
The mounting holes seen above were created using VIA’s. The size is determined by what screw or mounting system that’s planned on being used. In this case the size was set to 86.6mils or the standard inner drill size of a M2 screw.
Text such as the label “Goliath spr’16” can be placed using the “text tool” and should be placed on the tPlace or bPlace plane. Make sure to properly label components and the positive terminal, negative/ground terminal or both.
This is what the board will look like after being fabricated from OshPark.
Conclusion
Laying out a PCB can be time consuming depending on the amount of components on your board. Never use auto routing as the algorithm used is not that great. Remember to account for trace thicknesses, pin locations, and temperature limits for components.