“I originally imagined the LED Industrial Piercing as a project specifically to make use of 0201 LEDs. In the end, they weren’t even necessary. 0201 LEDs are just too small! Evidently, we needed to go deeper, so the purpose of this next project was to stick as many as possible of them onto the face of a stud earring.
Watch the following youtube video for an extensively narrated journey.
Continue reading this page for an in-depth explanation.
Design
These off-the-shelf LED stud earrings are very cheap. They have a tiny machined battery holder for two LR521 cells. The idea was to replace the plastic gemstone and single LED with our own circuit board, re-using the metalwork.
Electrically, the circuit and firmware would be identical to the badge I recently created using a CH32V003. Read that page for the story behind it and an explanation of how it works.
The challenge here is shrinking the design of the badge by a factor of 3 (or one-ninth the area) by shifting from an LED pitch of 3mm down to 1mm. We also insisted on not having a thick border around the board. This poses quite a challenge for routing the tracks.
Homebrew blind and buried vias
It’s possible to get multilayer boards made with so-called blind and/or buried vias. The limit is far, far beyond what we’re working with here. The “redistribution layer” of BGA packages is normally built as a very small, high-density circuit board. These often have very fine pin pitches and use blind vias, via-in-pad, and so on. The point is we could easily order a circuit board to these tolerances – if we had the cash for it.
A small batch of HDI boards starts at several hundred dollars, and the more complex the stackup the more the price goes up. I would very much like to keep this entire project under $50 total.
I decided to craft my own circuit board sandwich, two two-layer boards held together by an array of solder pads.
I wasn’t sure if this was going to work. I put all the pads around around the perimeter, partly with the idea that I could inspect all the connections, of course that would mean removing the boards from the panel to do so, which I didn’t want to do until the rest of the board is assembled. I suppose we could have started by fitting the components, and sandwiching the boards last, but that sounds like a recipe for disaster.
The drill diameter for the vias is 0.25mm, which puts us just into the second tier of board pricing (almost all of the cheapest PCB offers have minimum 0.3mm). It may have been possible to do a layout of 0201 LEDs within the cheapest tolerance, but I didn’t want to risk it. Remember the distance from one LED to the next is 1mm. At this sort of scale, the copper thickness around a plated hole is noticeably uneven.
The main display is an 8x8 matrix at 1mm pitch. With the corners cut off to fit into the round profile, that adds up to 52 LEDs within a 9mm diameter circle. The panel top and bottom is 5mm wide. The whole panel is 23mm by 28.5mm.
I stencilled paste onto one of the boards, and sat an unpasted board on top of it. The symmetrical panel helps ensure perfect alignment.”