Tuesday, 15 November 2011

Creating PCBs - Etching and Lessons Learnt

Next step, after exposing the photo-resist and developing the un-exposed areas away is to etch away the copper where is has been exposed.
This was done in a solution of ammonium persulphate - it's my first time using this, as in the past I'd always used ferric chloride. It's nice that the ammonium solution is clear, as it makes it easier to see the process occurring.
Something I didn't expect is the requirement for some gentle heating of the ammonium solution for the etching to occur. With ferric chloride, the etching would occur at room temperature; the ammonium required me to place the etching container into a warm water bath (about 60°C) before anything appeared to happen. Agitation during the etching was performed manually with a small, soft-bristled nylon brush.

Here's the result, with the photo-resist still on the board
Copper etched, photo-resist still in place
The middle looks awful, because of what went wrong with the photo-resist. What's odd though is that there are specks of copper that don't seem to be covered by photo-resist, but haven etched away. The left and right areas look acceptable. The right has lost some of the numerals, but that was due to under-exposure at the photo-resist step. Wherever the photo-resist was, the copper has stayed, so that speaks well for the stability of that layer in the ammonium solution.

The next shot is with the photo-resist removed. This can be removed by abrasion, but I chose to use a caustic solution to remove it. I used a cheap oven cleaner product. A small amount sprayed on the board, and the photo-resist just brushed away.
Photo-resist removed
This shows how badly the center section has gone. The right side looks good - the tracks, at least. The left side SSOP pads look a bit "fat" in the top right. The bottom left looks "dirty" between the pads

Etched board - back-lit 
The back-lit shot reveals the true result. The left side does have unwanted bridges between the pins of the SSOP package, and the vertical lines aren't as clean as the centre and right, either.

Lessons Learnt
Overall, I'd say this wasn't a bad result for a first attempt - OK, so the result wouldn't have been usable if it were a real board, but I think that it was a valuable exercise as there are some things to take away from it for next time:

  1. Application of the photo-resist film needs more care.
    I think I rushed this, and there was wrinkling near the centre that has caused that part of the etching to fail. I was also concerned about the light sensitivity of the film, and rushed it. It's not that sensitive.
    Consider buying pre-sensitised board in future.
  2. Artwork sandwich needs more pressure
    There are some areas of the board that look like they aren't as sharp as others. Next time I'll add a piece of 3mm polystyrene foam to the back of the artwork stack to ensure the pressure is sufficient and uniform.
  3. Create artwork in reverse
    The artwork was used printed side UP, which meant that there was the possibility of light bleeding through the thickness of the transparency sheet. The artwork should have been printed in reverse image, and used printed side DOWN.
  4. Best exposure time seems to be around 2:30. More time seems less problematic than shorter, particularly where fine details are involved, but may be causing "fattening" of the traces. This may be alleviated by using reversed artwork (above)
  5. Closer inspection required at the developer stage
    Closer inspection of the board after the photo-resist developing stage may have indicated that some resist had not etched away. Another short dip in the developer may have resolved some of the final blemishes. Need to investigate how critical the development time is and whether more time will damage the exposed resist or not.

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