How Photo Etching works:
The unit is designed to facilitate "photo etching" of copper boards. "Etching" a PCB is done by protecting the copper that represents the traces with an "etch resistant" coating. The board is then placed in an etchant like Ferric Chloride, where the copper that is not covered with etch resist is chemically disolved away. What remains are the copper traces. Qwicktrace works with various "photosensitive copper boards", where the "etch resist" of the traces is made in one of two ways:
- Expose the traces to UV light, which strengthens the etch resist, making the copper underneath the exposed area slower to eat away. This allows the non-trace area to be removed first, leaving the traces behind.
- Hide the traces from UV light, which weakens the etch resist on the rest of the board. The board is placed in a "developer" chemical that eats away UV exposed etch resist, leaving behind the entire copper clad board, but with traces still covered by the resistant. The board is then placed in the same etchant in step #1, where the non protected copper is eaten away.
How Qwicktrace PCB works:
The Raspberry Pi 4 controls the unit, including the small TFT touchscreen used for the user interface. A simple custom "Hat" PCB fits onto the Pi to help it out with controlling the various components. The Qwicktrace software understands how to read and render Gerber files and SVG files, so the process of using the unit is as follows:
- Upload a Gbr or SVG file to the unit's "pcb-files" directory
- Select the file you uploaded on the unit's touch screen. Files are listed in reverse date order (most recent on top).
- Select a "profile" used to expose the photosenstive copper board. The profile determines whether the UV light reaches the traces or the non-traced area, as well as the total exposure time needed.
- The Qwicktrace unit renders the circuit traces as either a negative or positive image, turns on the UV LEDs to expose the board, then turns off the board when exposure is complete.
The LEDs run on 12 volts, but their on/off state is controlled by Pi via a BD-139 transistor on the custom Pi Hat. The Pi and the HDMI interface board both require 5 volts, which is supplied by the small voltage regulator module on the Pi Hat. The Rev 2 board of the Pi Hat uses a "stackable header pin connector", which has extra long pins, and allows the GPIO pins of the Pi to be exposed and connected to even with the PiHat board in place (for future expansion).
Way back when I used to make pcbs, I found that using a UV light box with diffuse or multiple sources e.g. flouro tubes, multiple leds etc, results in poor edge definition. Due to parrallax, any gap between the mask plane (which is up inside the lcd glass in your case, so quite big) and the photo resist plane, means that light can come from multiple directions past the edge of tracks, and end up in different places on the pcb i.e. the edges are blurred. (commercially vacuum printing frames, and putting the emulsion down onto the photoresist somewhat solved this)
I found that having the light source a good distance from the pcb gives much more reliable results, and a point source is best. i.e. have some high power leds with lenses focused on it from some distance away. As a matter of interest, I just used the sun on a clear day quite often.
It is a simple optical lever so
Distance / Wsource = Tlcd / Parallax
If you want Parallax<=0.05mm , and the Tlcd glass is 0.3mm thick, and Wsource is 100mm, then
Distance = 100 * 0.3 / 0.05 = 600mm
WSource is the diameter/diagonal of the diffuse/multi point source. If you put the whole source through a single lens, it will be the source size, not the lens size that counts as WSource.
If instead you used a single lensed led where the die itself was 2mm square
D = 2*sqrt(2) *0.3/0.05 = 17mm
Well thats too close, but using a single lensed led at 100mm is going to give you ~10um parallax. Using the most powerfull single led is probably going to be best