The board and parts finally arrived! Here, I’ll talk about building the boards.
PCB Fixture
I didn’t want to have a exposed PCB on my work station since they’re pretty fragile, and there’s a risk that you can short things out. I therefore decided to 3D print a little fixture to hold the PCB:
I designed the fixture in OnShape. The board will be installed, top side face down, onto the top of the fixture. As you can see, it’s basically a cup that will cover one side of the PCB, which means the microcontroller, ADCs and DAC, as well as almost all of the passives, will be covered. The PCB’s connectors and test points, though, will be exposed, so testing and debugging the PCB while it is in the fixture shouldn’t be a problem.
The PCB is held up by the supports at the edges as well as two cylinders near the middle of the fixture. One of the cylinders has a hole in it; that’ll be tapped by an M3 tap after printing, which will allow me to put a M3 screw into it. This will keep the PCB from coming out of the fixture. Since the tapping, as well as the screwing and unscrewing, may stress that support, I added ribs for extra stability. I originally didn’t want to have the edge supports, but because the PCB is half as thick as typical PCBs, the board is pretty flimsy, so supports are necessary if I want to easily hook up connectors to the headers. The edge supports need to have slots cut out of them to make sure that the supports do not interfere with the through hole connectors, which would prevent the PCB from sitting flush inside of the fixture. The fixture also has a slot cut out of one of its walls to expose the USB connector.
Built PCB
I built two PCBs at once. I seriously underestimated the amount of work that building these boards was; it looks simple, since there are so few ICs, but the large number of passives make the work tedious and time consuming. The process was the same as building of the Stampduino: I used a stencil to put solder paste on the PCB, put the components onto the PCB using tweezers, and then I put the whole thing in my reflow oven. Afterwards, I examined the soldered PCB and cleaned up any failed solder joints, shorts or components that shifted / tombstoned during reflow.
In addition to soldering all of the components on the top side, I had to install the through hole connectors as well as populate the resistors and LEDs on the back side. Not difficult, but again, more time consuming than one may think. The hardest part was getting the LEDs right; they all look the same unpowered but have different colored lights when powered, so making sure that the right LED was on the right pads for each board took some time. Also, the polarity markings are on the bottom of the LEDs, so flipping and then reflipping each tiny component was pretty annoying.
I used 0402 components since that’s what I used for my Stampduino, but in that application space was a premium, and there were relatively few parts. In this case, space was not an issue, and the board was much more complex, so 0603 components probably would have made the assembly a lot less painful.
One thing to note is that I made an error ins the schematic; for the LDO, one of the feedback resistor values is incorrect. R7 should be 1.87 kΩ, but the schematic says 18.7 kΩ. That’ll be fixed when I publish my schematic.
My grumbling aside, it was definitely worth the effort; the PCBs look great, and they look even greater on the fixture! The PCB fits very well in the fixture, and the whole thing feels very secure, so I’m not worried about the fixture falling apart or the board falling out of the fixture or anything like that. One thing to note is that I found a polarized version of the 4 pin male header connector that the fan hooks up to, so I decided to use it to prevent accidents. The connector sticks out a bit, and interferes with the fixture a little, but it should be fine.
I’ve done minimal testing so far. First, I checked the resistance from ground to the power rails and made sure there were no shorts. Then, I supplied 6 volts to the 12 volt input to test the LDO. Since the regulator is hooked up to a bunch of chips, I was worried that the LDO would output a voltage over 5 volts and potentially damage the chips. By providing a 6 volt input, rather than 12 volts, this ensures that even if the LDO isn’t regulating its output correctly, the chips that the LDO power aren’t seriously over-driven. Fortunately, since I fixed R7, the LDO output the correct voltage (4.95 V, which is fine). I also saw that there were no large currents, which means that there was no short to ground on the power rails. After seeing that the regulator worked properly, I raised the input to the expected 12 volts, and the LDO still output the correct value. This testing shows that none of the power rails are shorted to ground, and the main voltage regulator functions properly, so I can move on to functional testing. Next I’ll have to confirm that I can program the microcontroller, and then start writing code to check all of the ICs. One step at a time!
EE Diary 