Tuesday, November 15, 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.

Sunday, November 6, 2011

Creating PCBs - Exposing the board

Now it's time to expose the photosensitive film to the UV light through the negative artwork mask. To get a clear image it's important that the artwork be held tightly against the board.
For this, I've used a cheap $2 plastic photo frame that I've cut down the length to fit neatly into the UV lamp. In this is (from the top)

  1. Glass (which I broke the corner off, cutting it down to fit the re-sized photo frame. OK, I suck at cutting glass)
  2. 1 sheet of rapidraw film, just to diffuse the UV light a little
  3. The artwork
  4. The copper board with photosensitive film
  5. A sheet of A4 paper, folded, to give the sandwich some more thickness
  6. The cut-down original backing plate of the photo frame.
It looks like this.


So this assembly goes into the UV lamp, like so:
And is exposed.

Exposure timing
I'd previously done an exposure timing test with the film, but not with the film on copper board. The exposure test is done by progressively covering sections of the film, and determining which section has been exposed sufficiently where the artwork is clear, but not exposed where the artwork is dark.
Too little exposure, and the film won't harden enough (with negative film, the exposed areas of film harden)
Too much, and the areas that shouldn't have been exposed may start to harden. This is why the artwork contrast is very important.
OK, so prior experimentation indicated that about 2 minutes should be enough. With this test board, I had three identical patterns, so I exposed one section for 2 minutes, the next for 2:30, and the last for 3 minutes.

This is the result:
From left to right, 3 min, 2.5 min and 2 min exposure
The vendor says that the film will darken where it is exposed, and you can see that is the case. In fact, at this point, it all looks quite good. Now to remove the un-exposed areas with the developing solution.
3 minutes on left, 2 minutes on right
Hmm, this shows some problems. The rightmost numerals have etched away in some cases. I'm not sure if it's due to underexposure, or if the film was not bonded sufficiently with the copper. The center top, I don't know WTF happened there - the only explanation I can come up with was that the artwork wasn't pressed well enough against the film, and some light has spilled? I would have expected the actual artwork to be blurry if that were the case, however. The leftmost exposure looks good.
The actual development of the film occurs pretty quickly (it's more of a "stripping" of the un-exposed regions), and I'm not sure yet what occurs if the film is over developed.

Next time, we'll see what the copper etching reveals...

Creating PCBs - Applying the film

The photosensitive film needs to be applied to the blank copper board before it is exposed through the artwork mask. But before that, the copper board needs to thoroughly cleaned - any skin oils or tarnished spots, and either the photosensitive layer will not adhere, or the etchant will not etch the copper protected by the oils. Both these are undesirable.

I've always cleaned copper board with a powder cleaner, like AJAX, and a scouring pad. The cleaner helps remove oils, and the scourer leaves a slightly "brushed" finish to the copper. The powder of the AJAX probably helps with that, too. Clean the board until it is uniformly bright looking. Oh, you should have cut it to the required size, before this step.

The vendor says that the film should be applied to the copper board using a hot roll laminator, but using an iron is an acceptable second option. The first trick though is separating the 3 layers of the film from each other. The photosensitive part is sandwiched between two clear protective plastic films - one of which needs to be removed before the photosensitive part is applied to the copper.

At first, I didn't believe the protective layers were there - they are difficult to separate, and even harder to see. The suggested method of getting them apart is to use a piece of adhesive tape to stick to the protective layers, then peeling them back. This does work, eventually, and you can pull back one of the protective layers.

Once one of the layers is removed, apply the film (unprotected side to the copper) from one end and work the film along with your fingers to ensure there are no air bubbles. Don't worry about working a dark room to do this - the film is not that sensitive. Conversely though, don't do it in direct sunlight.

When you're convinced there are no air bubbles, it's time for a press with the iron. Recommended temperature is 130°C, and maybe obviously, don't use steam! Also, don't use the ironing board as a base, as you want to apply a fair amount of pressure with the heat, and ironing boards aren't generally that firm. A flat piece of wood, like a wooden chopping board on a sturdy table is ideal. Place the board, copper and film side up, on the board, then a layer or two of paper, then apply the iron and press down for about 10 seconds.

That's it. The board now has the photosensitive film bonded to it, and the clear protective layer should still be attached to the top. Keep it somewhere dark and leave the protective layer in place until you're ready to expose the board.