PCB Etching

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Description

A logical extension to the Kicad workshop (electronics design) is to realize the designed PCB. There are 3 ways to produce PCBs at FIXME (with the required components):

  • PCB Milling with a CNC Router
  • PCB Etching using toner transfer method
  • PCB Etching using the photosensitive positive method

Goal

  • Make custom PCBs for FIXME members and projects.

Obstacles

PCB Milling with a CNC Router:

  • Cost
  • Make a lot of dust

PCB Etching using toner transfer method:

  • Use chemicals requiring precautions for use

PCB Etching using the photosensitive positive method:

  • Use chemicals requiring precautions for use
  • Need an insolation box
  • Number of step for the process

Components

  • Iron(III) chloride / Perchlorure de fer Corrosive!

or

"Toner transfer" PCB etching

Photoresist PCB Etching

Mainly inspired from [2]

Parts

  • Uv light (UV LED strip or DIY UV LED fixture?)
  • Pre-coated photoresist positive pcb
  • Lye (NaOH)
  • Ammonium persulfate (or Ferric Chloride, available at Fixme hackerspace)

PCB Layout

The pcb layout should be the positive (black parts are where copper will stay) and *mirrored* image, full size (1:1) of the layout.

This layout is printed on standard paper with a laser printer (or copier) and set the toner density to the maximum value. This will make the black part really dark so the light can not pass trough.

The paper is then drenched in a 'drying' oil (like sunflower seed oil). This will make the white parts of the paper transparent. Remove the extra oil carefully.

Exposition

Remove the plastic protection on the PCB. Place the printed side of the layout against the copper side of the PCB. Remove the air bubbles captured under the greased paper

Place the PCB & Layout inside the exposure box. Then let the UV light do the work for 2 minutes 30 seconds This is a completely arbitrary timing, we should test different values

Remove the PCB & Layout from the exposure box

Remove the the layout paper (if you want to reuse it, you can dry it)

Development

We use a 1% Lye (Sodium Hydroxide = NaOH) solution for development. (Add 10 grams of Lye to a liter of water)

To know if the development process is finished, take a look to the traces. They should be clearly distinguishable.

Etching

The etching solution is a 200g/L ammonium persulfate mix (or Ferric Chloride).

The etching process can take to over an hour. To speed up this shit, you can add heat and/or bubbles to the solution.

The solution should take a blue to deep blue hue.

The reaction is (NH_4)_2 S_ 2 O_8 -> CuSO4 + (NH4)_2 SO. The resultant liquid is less harmful than ferric chloride but should still be treated with other chemical waste

Photoresist etching with Sodium Persulfate

  1. Print inversed PCB on two layers of transparencies, and make sure that there is visible text to be able to identify the side later
  2. Place glossy side of transparency on the glass of the UV exposure box, and the matt side (with the toner) face up - you should be able to read the text inverted now
  3. Peel back the protective film from the pre-sensitised PCB, and carefully place it over the transparency
  4. Expose for ~3 minutes (time needs to be experimented with still), with a weight (such as one of the bottles) on the cover for pressure
    1. Whilst the PCB is getting exposed, pour 1cm worth of soda (french: soude) into a container
    2. Prepare another container with just water, to rinse of the soda
  5. Remove PCB from UV exposure box, and place into soda solution. Stir and agitate until the soda and PCB until there are no more violet traces that leave
  6. Remove PCB from soda solution, and rince off with water in other container - use plastic tweezers to handle it
  7. Attach PCB to support wire, and place the PCB into the Sodium Persulfate solution, and wait until you can see the resin (should take ~20 minutes)
    1. Whilst waiting, pour the soda back into its bottle, and rinse the containers
  8. After you can see the resin, remove the PCB before the traces get etched away, and rinse it off
  9. Pour Sodium Persulfate back into its container and clean up

(photos to come)

Safety

Equipment:

  • Gloves
  • Goggles
  • Lab coat

Docs:

Schedule

  • 13 décembre 2012: First workshop [Doodle ]

See Also

Participant

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