Castellated PCB edges are kind of magical. The plated semicircular features are a way to make a solid, low-profile connection from one board to another, and the way solder flows into them is deeply satisfying. But adding them to a PCB design isn’t always cheap. No worries there — you can make your own castellations with this quick and easy hack.
[@CoilProtogen] doesn’t include much information in the Twitter thread about design details, but the pictures make it clear what the idea is here. OEM castellations are really just plated areas at the edge of a board that can be used to tack the board down to another one without any added hardware. The hack here is realizing that lining up a bunch of large-diameter vias and cleaving them in half with a sharp pair of scissors will result in the same profile without the added cost. The comments on the thread range from extolling the brilliance of this idea to cringing over the potential damage to the board, but [@CoilProtogen] insists that the 0.6-mm substrate cuts like butter. We’d worry that the plating on the vias would perhaps tear, but that seems not to be the case here.
The benefits of a zero-profile connection are pretty clear in this case, where castellated PCBs were used to replace bulky header-pin connectors on a larger PCB. We can see this technique being generally useful; we’ve seen them used to good effect before, and this is a technique we’ll keep in mind for later.
Right now, we’ve got artistic PCBs, we’ve got #badgelife, and we have reverse-mounted LEDs that shine through the fiberglass substrate. All of this is great for PCBs that are functional works of art. Artists, though, need to keep pushing boundaries and the next step is obviously a PCB that doesn’t look like it has any components at all. We’re not quite there yet, but [Stephan] sent in a project that’s the closest we’ve seen yet. It’s a PCB where all the components are contained within the board itself. A 2D PCB, if you will.
[Stephen]’s project is somewhat simple as far as a #badgelife project goes. It’s a Christmas ornament, powered by two coin cells, hosting an ATTiny25 and blinking two dozen LEDs via Charlieplexing. The PCB was made in KiCAD, with some help from Inkscape and Gimp. So far, so good.
The trick is mounting all the components in this project so they don’t poke out above the surface of the board. This is done by milling a rectangular hole where every part should go and adding castellated pads to one side of the hole. The parts are then soldered in one at a time against these castellated pads, so the thickness of the completed, populated board is just the thickness of the PCB.
The parts used in this project are standard jellybean parts, but there are a few ways to improve the implementation of this project. The LEDs are standard 0805s, but side-emitting LEDs do exist. If you’d like to take this idea further, it could be possible to create a sandwich of PCBs, with the middle layer full of holes for components. These layers of PCBs can then be soldered or epoxied together to make a PCB that actually does something, but doesn’t look like it does. This technique is done in extremely high-end PCBs, but it’s expensive as all get out.
Still, this is a great example of what can be done with standard PCB processes and boards ordered from a random fab house. It also makes for a great Christmas ornament and pushes the boundaries of what can be done with PCB art.