I put together a short video of how I assemble a die, for anyone who's interested!
Here are the steps shown in the video, and some notes to go along:
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1. Receive boards (and components) from PCB factory
Soldering components on double side flexible PCBs is a real pain! For the previous version of the dice, I did it all myself, and it was dreadful. This time around though, I had the PCB factory (PCBWay in this case) do it for me. I'm pretty sure that they also did it by hand, but they are obviously much more skilled than I am. :)
2. Solder Charging coil
A step I didn't show in the video is that I also needed to prepare the coil leads and remove most of the enamel before soldering. My hope is that for large quantities, I'll be able to order coils with the lead lengths that I need and not have to strip or maybe even cut off the extras.
3. Solder Battery
I'm pretty proud of this one: up until recently, I had wires connecting the battery to the PCB, because, well, that's what you do, right? Only I finally realized that I could shape the flex PCB just right so that I could solder the battery directly to the flex circuit, saving a ton of time, and a little bit of material (the wires).
4. Program and Test
Plug the board in, program the bootloader, run some tests, make sure the LEDs light up, etc...
I imagine that in a production setting, this will be a lot more involved. I think I will end up using a standalone Raspberry Pi that can take care of everything at the press of a button, no HMD needed. It'll be fun to build!
Right now, this is my setup:
5. Paint / Clear coat
While I am experimenting with colors and materials, I use this step to change the color of the PCB (to white in the video). Once I have settled on what color(s) to use, I'll just get the PCBs made with *those* color solder masks. However, the coat of paint actually serve a secondary purpose, and that is to cover the exposed metal contacts (for instance on the battery) ahead of the next step, the folding. Initially I placed a strip of Kapton tape over the battery, but I think a couple layers of clear coats will be just enough. The intent is to prevent shorts during the folding, the coating doesn't need to be resistant. Once the resin gets poured everything will stay in place!
6. Fold and Glue
This is right now the longest step and the one I am trying to improve. Mainly the issue is that I have to wait for the glue to dry :) over and over.
I had tried adding little 'solder tabs' to the edges, thinking I could use get the edges to stick together with a little touch of heat, but the solder wouldn't bridge. Eventually the tabs came off the flex substrate, probably from the repeated heating of me trying to get the solder to wick.
I am thinking of redesigning the PCBs to have some sort of hooks on the edges, so that they can latch together as I fold them:
I already have registration tabs, but with the hooks, I'd only need to add a bit of glue once the whole thing is folded. They would hold together long enough to fold the whole thing up.
We'll have to see, I have a few more things I need to tweak on the PCB design before the next revision anyway...
7. Prepare Molds and Resin
For the resin, I am using Total Boat, although I have used ArtResin as well. The main property I am looking for is a resin that cures very slowly and hard so that I can try and get the clearest resin as possible.
One of the big thing I am still trying to figure out with the resin is how to add texture without rendering the resin completely opaque to the LED light. I think the two-step process can actually help a lot here, for instance by having a clear first layer and then a more opaque (i.e. diffusing) second layer. Also white resin obviously diffuses better than black, but I really want to have some black dice :)
Anyway, ideas welcome!
8. Place PCB inside mold
The most difficult part right here is to align the PCB correctly. I think I need to add a registration 'pin' to the mold, so that the folded PCB can nestle perfectly in the right spot.
Unfortunately, I'm not quite ready to make more PCBs to have the matching registrations, so for now I just need to take my time, but I think that's something I'll be able to solve for production. Again, the less chances to mess up and the quicker I can make each step, the less it'll cost me! ;)
9. Pour resin
I really need to try and see if I can modify the molds so as to not need the vacuum chamber. Right now the clearances are so tight that the resin doesn't always flow everywhere unless I do that. It does get better with the pressure pot curing, but sometimes it still leaves small bubbles.
10. Cure under pressure
This is the main way to get a good finish! You can see I use a small compressor and a modified paint pressure pot. There are tons of tutorials online if you're interested in building your own too.
11/12. Demold and Clean up
The first demold and clean up doesn't need to be too precise, as I'll be casting the whole thing again. By the way, the reason I cast the dice in two steps is so that the electronics can be properly centered!
13. Place PCB in final mold
For this second casting, I did create registration pins, this really helps line things up quickly. Of course this is something I was able to do without modifying the PCBs, I just had to make new molds.
14. Pour / cure / demold redux
The second pour is pretty much the same as the first one, although in this case, we really want to leave the resin to cure under pressure for as long as possible, in order to get as good a surface finish as we can.
15. Clean up
You see me use an exacto blade to cut the sprue off here for a good reason. I want to make sure and create a proper tip where the prue was, rather than a very rounded corner. You may also notice that the mold cut lines are along the edges of the die, rather than in the middle of the faces. I'm hopeful that production versions of the molds will also be able to do this to reduce the amount of cleanup and sanding necessary.
16. Sand and Polish
This step can take a long time if the casting and initial molds weren't up to par. I also would like to investigate alternative ways to cut the sprue cleanly, leaving really sharp and smooth surfaces. I don't know if that will be possible, but if it was, then sanding wouldn't be needed at all.
17. Paint Numbers
The final step is, of course, to paint the numbers. I used a simple acrylic paint and, as you can see, just rubbed off the excess paint. I think professional dice manufacturers use a tumbling machine to both remove the excess paint and round off the edges. I tried using a small rock tumbling machine myself, but it didn't quite work. I'm sure I'll be able to sort it out with the factory once I get to that point.
And that's it! I hope this was informative.