# Chaitan

An Adom molecule LED nameplate. The name is etched out of the soldermask over the ground plane, lit by 14 side-view LEDs firing inward across the board.

The guided walkthrough starts on its own once loaded. Open full screen instead.

The one idea: the name is the ground plane. A rectangular GND copper pour covers the top layer and the soldermask is opened in the shape of the letters, so bare plated copper shows through. There is deliberately no second ground path, no bottom ground pour, so every milliamp those 14 LEDs draw physically crosses the letters on its way back to the connector. The line you get to say to Chaitan, "your name is part of the electrical path that lights up the LEDs", is a statement about the netlist rather than a slogan.

## The board

Board | 236 x 76 mm, 2 layer, 4 mm corner radius
Pin box | 224 x 64 mm = 7 x 2 on the 32 mm scaffold grid
Name | 194.0 x 40 mm, Familjen Grotesk Bold
Mask openings | 362 letter polygons
GND plate | 229.6 x 69.6 mm, top layer
LEDs | 14 x Wurth WL-SMSW `155060GS75300`, 515 nm side-view
Current | 168 mA at 5 V (14 x 12 mA)
Parts / traces / vias | 38 / 60 / 36

## Molecule compliance

- A large machine pin at every corner: a 3.6 mm body press-fit into a 3.45 mm plated hole, 0.15 mm of interference, solderless.
- The pin box is 224 x 64 mm, both exact multiples of 32 mm, the base scaffold's large-contact pitch, so it drops straight onto 8 x 3 contact positions of the 18 x 18 field across the 576 x 576 mm workspace.
- Minimal wings: the board stops 6 mm outside each corner pin. That is as tight as the pin allows, since a 5.2 mm annular ring eats 2.6 mm of it and a press-fit needs roughly 2x board thickness of laminate outboard of the ring.
- MP1 and MP4 are GND, MP2 and MP3 are +5 V, so the plate can run off a workcell control panel instead of USB-C. The ground pins land inside the plate, so the scaffold's ground return also passes through the name.

## Why side-view LEDs

A normal chip LED is a top-view emitter: it fires straight up, away from the board, and the copper you are trying to light sees nothing. This board uses a true side-view part. The die sits on its side behind a window in the long face of the package, so the beam leaves parallel to the board about 0.5 mm above the soldermask. Ringed around the edge facing inward, the light rakes across the plate at a grazing angle and washes the exposed-copper letters instead of escaping to the sky.

Worth knowing if you go sourcing: Kingbright's `APTR3216` family is reverse-mount, firing down through a hole in the PCB, and Broadcom's `HSMx-C265` is bottom-entry. Both come back in a "right angle SMD LED" search and neither does this job.

## Power

USB-C VBUS at 5 V through 150 R into a 3.2 V LED gives 12 mA each, 168 mA for the board, well inside the 500 mA a charger gives an unnegotiated sink, so any phone brick works. `R_CC1` and `R_CC2` are the mandatory Rd pulldowns: without them on both CC lines a USB-C source never enables VBUS and the board stays dark.

The layer plan is inverted from the usual convention on purpose. Top is GND, one rectangular pour, with the name cut out of it; bottom is the +5 V plane. Everything is placed top side only, because the line does single-sided pick and place, so each LED string runs LED, then 150 R, then a via down to the +5 V plane, while the cathode bonds straight into the plate.

## Fab notes

- Specify ENIG, not HASL. The letters are 194 x 40 mm of deliberately bare copper and are the entire point of the board. HASL leaves them lumpy; ENIG leaves them mirror flat.
- Dark soldermask maximises contrast against the exposed letters.
- The four 3.45 mm machine-pin holes are an interference fit and are meant to be undersized. Do not let a fab house "correct" them to a clearance fit.
- Tented or plugged vias: there are vias whose rings lap onto LED resistor pads.

## Verifying the mask opens

Do not trust the render. Read the gerber. `F_Mask.gbr` is negative polarity (`%TF.FilePolarity,Negative*%`), so what is drawn is where mask is removed:

- 362 region polygons, bounding box 194.04 x 40.00 mm, the letters
- the plate rectangle absent from F_Mask; present would mean inverted
- zero letter regions in `F_Paste.gbr`. The letters are decoration, not solder joints, and must never be stencilled

## Reproducing it

Built with adom-tsci from parametric tscircuit source. Board size, pin box, LED count and silkscreen are all derived from the name's ink box, so a new name is one command:

`scripts/make-nameplate.sh "Chaitan"`