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post	PCB photo transfer	2024-11-24 15:00:00 +0200	true

Specific options for manual PCB making

Minimum with / Minimum trace separation: 0.3 mm (12 mil)

The following settings may be changed to make Gerbers for fabrication

Vias the same size as pads
Solder mask: do not tent vias

Photo-transfer tests

All tests with 8 W/m UV LED strip (4x25cm) @ 100%

Test 1

Double layer tracing paper 10 zones from 30 s to 5 min in 30 s increments

Make sure to have enough margin to pull the corner of the protective layer after exposure
Gunk really goes away when rinsing the developer

Best result: 2 min

Test 2

Single or double layer tracing paper 5 zones: from 1 min 30 to 2 min 30 in 15 s increments

Single layer blacks are not opaque enough, so do not block UV, so resin hardens in places where it should go
Develop with solution for at least 3 min
Good results at 2 min exposure

Test 3

4 layers transparency film 10 zones: from 1 min 35 to 3 min 05 in 10 s increments Best results in zones 4-5: 2 min 05 / 2 min 15

Develop 5 min
Rinse developer under running water to clear the gunk
Develop again a few minutes
Wipe with isopropyl alcool to remove last traces of gunk

Actually etching the board

Try 1

I printed several stencils but they would not line up: it appears that my printer (HP MFP 1217nfw) expands in the Y direction significantly and in a non-uniform way. To get good registration of the stencil, I had to print on 3 separate sheets of transparency film, keeping the design centered on the sheet (not at the top of the page, which would have allowed me to print two designs on the same sheet).

Exposed 2 min 10 with 3 layers of stacked transparency film.
Black stencil areas were overexposed, resulting in gunk remaining on the board. (Also, the developing solution was not new and possibly a bit weak).
I did not do a hardening exposure after rinsing
Bad result at etching with lots of copper remaining
Also quite a few cut traces

Lessons learned:

spend more time doing a very thorough lamination to avoid raised traces
my printer deforms designs depending on their position on the page
reduce exposure time for 3 layers of transparency (1 m 55?)
rinse more carefully, scrub gently, watch for remaining gunk
let the board air dry before touching up
do a hardening exposure (also serving as a double check that there's no leftover resist)
use fresh etching solution (2 parts H2O2 / 1 part HCl)

Try 2

Exposed 1 min 30 with 3 layers of stacked transparency film. => this was plenty for hard traces protection
Rinse+++ the unexposed film => gently swipe with tooth brush to really eliminate all gunk
Etched with a softer solution
- 100 mL water / 50 mL HCl / 25 mL H2O2 (12%)
- Ecthing is much slower, allowing better control over the process, less Cl2 vapours
Removed resist with NaOH (a few grams in hot water)
Fixed broken tracks by soldering pieces of resistor legs