Turtles all the way down: Fabbing circuit boards

14 February 2014

This brings us right along to designing and fabricating the circuit boards that our bright, shiny new open source chips will plug into. This level of complexity is probably one of the best understood parts of the development process. Arguably electrical engineering has been around since the discovery of electricity, because a circuit of some kind is required to guide an electrical current to do useful work. You could make the case that the wet string that Benjamin Franklin's kite was tied to was one of the first electrical conductors (because the Baghdad battery hypothesis has too many holes in it for my liking (I know, I know, Mythbusters disproved Ben Franklin in the thunderstorm, I need an alusion of some kind to make this work)). The advent of EDA software has accelerated the design process by at least several orders of magnetude compared the days of drafting circuitry on sheets of paper.

In recent memory I've experimented with two open source packages, KiCAD and GEDA. These applications can handle even the design of multi-layer circuit boards, but I will warn you that some messing around with them is required to get used to their user interfaces and toolsets. Hypothetically speaking, if you start by loading up the proof of concept circuit schematics in your EDA package of choice you can start the process of manufacturing a mainboard for our hypothetical trusted and open source computer. Many open hardware projects make KiCAD or GEDA files available along with their source code, so it is certainly worth messing around with them to get a feel for things. More experienced engineers no doubt have some ideas for what they want to accomplish, by definition know what they are doing and are probably inclined to get right to work. Rather than attempt to go into detail that I don't understand well enough, I'll leave it as an exercise to the reader (don't you hate it when people say that?) and push on to the topic of circuitry manufacture. Having a set of schematics for a mainboard is all well and good but they have to be fabricated if they're going to be useful. That means transferring, etching and drilling at least one printed circuit board for our trusted and open computing platform. Hobbyists and hackers have been etching their own circuit boards for decades employing a variety of techniques. For example, here's a fairly simple technique that requires chemicals no more exotic than vinegar and hydrogen peroxide, both of which can be acquired from the corner grocery store. There is a more complex technique which involves ferric chloride, isopropyl alcohol, and baking soda, the first reagent of which can be purchased at a well stocked hardware store or Radio Shack.

The layout of the circuitry must be transferred onto the copper clad board somehow, and then the copper that isn't part of the circuitry must be removed. There are many ways to accomplish both tasks:

I'll be honest with you, I don't have any first hand experience with working with them, aside from tearing stuff apart that used them once in a while. I've hung out at HacDC when folks were doing it and I've read about it.

It also seems feasible that flexible circuits could be used in this project instead of rigid plastic, metal, or fibreglass boards. They're certainly more lightweight than copper-clad boards. There is a highly rated tutorial on how one may make their own flexible circuit boards that may or may not be of interest in the context of this discussion. There are flexible circuit construction kits available on the open market (much to my surprise). It is also possible to cut and lay out your own flexible circuit boards if one has access to a vinyl cutter. If your friendly neighborhood hackerspace or workplace has a laser cutter a slightly different method can be used to make flexible circuit boards. Or if you have the cash (and a spare printer laying around) one can acquire inkjet printer tanks that have conductive ink in them and print out circuit boards at home. Unfortunately the boards then need to be cured, and so far as I know nobody's got a way to do that at home.

The circuitry in much of the computing hardware we use today is comprised of multiple layers, because there are so many traces connecting so many components that putting them on the top and bottom of the board only isn't enough. Even if the traces are creatively packed there's only so much surface area available on a given board. The solution is to make each layer of the circuit board as thin as possible, make them interconnect by any means necessary, stack the layers in the correct order, and then laminate them together. Circuit traces on different layers of the board are connected with vias - contact pads that bridge layers together. Rik te Winkle wrote a tutorial on how to use EAGLE to draft multi-layer boards, which should be of use, though the exact methods won't apply to our aforementioned F/OSS CAD software the design techniques will.

Some hobbyists have painstakingly etched and cut each layer and soldered them together, which certainly gets the job done. If you've got some spare cash you might want to consider acquiring or building a 3D printer capable of making circuitry, similar to the EX1 from Cartesian. The EX1 is also open source, so at some point hackers are going to start building their own... alternatively, you could print your boards on a more conventional 3D printer and use a conductive material to form the circuit traces.

The reason I mentioned flexible circuitry first is because multiple double sided flexible circuit boards could be stacked and laminated together to create multi-layer circuit boards. Flexible circuitry is very thin and seems amenable to the process. It should be possible to do the same thing with home-etched PCBs but the resulting boards would tend to be thick, heavy, and you might have heat dissipation problems. Nothing that couldn't be mitigated, but those are factors to keep in mind. Again, I have no first hand experience with this, so if anybody knows better please leave a comment. I'm aiming for as practical a solution as possible. After the boards are etched and stuck together, holes for legs of discrete components will need to be drilled, and then the components - from the most basic resistors, diodes, and capacitors to sockets for the integrated circuits - must be placed on the board in the correct locations. Depending on the size of the components used (traditional discrete components through commercially available SMT), the amount of difficulty involved in positioning them on the board is variable. Personally, I would say the process ranges in laboriousness from 'tricky' to 'fuck it'.

Then everything needs to be soldered down, and the circuits need to be tested and probably debugged. This is not unusual for experienced hackers or electrical engineers, it's all a part of the process. There are probably other dynamics that I'm unaware of - my experience with electronics isn't quite rudimentary, but neither am I an expert. Let's just say that I know enough to know that some pretty weird stuff can happen on the bench, but not enough to know what it might be or how to fix it. Good luck.

This work by The Doctor [412/724/301/703][ZS] is published under a Creative Commons By Attribution / Noncommercial / Share Alike v3.0 License.