One of [CNLohr]’s bigger claims to fame is his process for making glass PCBs. They’re pretty much identical to regular, fiberglass-based PCBs, but [CNLohr] is building circuits on microscope slides. We’ve seen him build a glass PCB LED clock and a Linux Minecraft Ethernet thing, but until now, [CNLohr]’s process of building these glass PCBs hasn’t been covered in the depth required to duplicate these projects.
This last weekend, [CNLohr] put together a series of videos on how he turns tiny pieces of glass into functional circuits.
At the highest level of understanding, [CNLohr]’s glass PCBs really aren’t any different from traditional homebrew PCBs made on copper clad board. There’s a substrate, and a film of copper that is etched away to produce traces and circuits. The devil is in the details, and there are a lot of details for this build. Let’s dig deeper.
To bond the copper to the glass slide, [CNLohr] first preps the materials by degreasing with acetone, and rubbing the copper with steel wool. The glue is Loctite 3301, a low-viscosity light-curing glue that can be squeegeed between the copper and glass, then exposed with a grow lamp.
That’s enough to get the copper on the glass, but several steps away from a circuit board. To etch the copper, [CNLohr] is using Riston film photoresist. This is applied by floating it onto the copper with a few drops of water and putting it through a laminator. The laser printer photomask is floated onto the Riston with some 3-in-one oil, the photoetch exposed, and then removed.
The next few steps – washing off the uncured photoetch, etching the copper, and washing off the cured photoetch – are the parts of this process that have given [CNLohr] the hardest time. He needs to use chemicals that don’t interact with the bond between the glass and copper. He’s found sodium carbonate works well for removing uncured photomask, ferric chloride for etching copper (although we’ve had good experience with cupric chloride and peroxide), and potassium hydroxide to remove the cured photomask. With the cured photomask removed, [CNLohr] has a perfect glass circuit board.
Of course, building a glass PCB is useless without soldering components to it. For that, [CNLohr] is using bismuth solder paste for low-temp hot plate soldering. If you’re only doing single-sided SMD work, this is a great way to produce beautiful circuit boards made of glass, but now we’re wondering if a small enough diamond drill bit would allow for double-sided construction.