Dry-film Developing

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Questions about:

1. In the Reference Stack you mention the importance of "breakpoints". I am not quite sure if I understand this. Please provide a brief explanation of this concept.

"Breakpoint" is defined as the point at which the circuit image first appears to be totally developed to the naked eye. Even though the image appears to be finished, an examination with a microscope would reveal that partially developed photopolymer is still clinging to the edges of traces and between closely spaced circuit elements. If these are not removed by further developing, you will end up with numerous shorts and ragged circuit features. If you have a microscope handy , a properly developed image should display sharp edges with near vertical sidewalls.

Note: Visual breakpoints can be very misleading. A more accurate evaluation can be made by carefully running your ungloved fingers over the surface of the panel. If you can feel a distinctly slimy or slippery coating in areas that appear to be free of photopolymer, the image needs more time in the developer.

2. I am using a spray on photoresist to sensitize my PCBs. I followed your instructions on development. I have made my own developer and can see my circuit on the board, I then attempt to etch this board but nothing happens. No etching occurs. I am using Ferric Chloride as a solution. The etchant works because on an simple untreated clad board so I do not believe that this is the problem. Do you have any idea what is going wrong with my process

Even if it looks like you circuit pattern is totally developed, it is quite likely that a VERY thin layer of resist is remaining in the so called "developed areas" preventing your etchant from attacking the copper.

The obvious questions that come to mind are:

a. Are you using the developing solution recommended by the manufacturer of the spray photoresist?

Spray resists (negative and positive) tend to be very particular in their response to developing solutions. Make sure that your is made up exactly as the manufacturer recommends and that you use as fresh a solution as possible. Dry-film photopolymers tend to be MUCH more forgiving of solution makeup.

b. Have you calibrated you exposure source to insure that you are getting adequate illumination to insure reliable image transfer?

If you are using a positive resist (contrast preserving), this is especially critical since the UV light makes the exposed photopolymer easier to remove. If you do not have enough light, you might not be able to develop off the areas that you intend to etch.

If you are using a negative resist (contrast reversing), the UV exposed areas might not be sufficiently cured to resist the developing solution long enough to allow you to totally clear the unexposed regions.

c. Can you feel a slimy coating in the developed areas after developing?

If the apparently clear copper areas have a slimy feel (much like the feel of a layer of liquid soap), some residual resist is left on the board. Increase you developing time or try lightly scrubbing these areas with a kitchen sponge as indicated in the Developing section if the Green CirKit manual.

d. Are you heating your ferric chloride prior to trying to etch?

Heating to about 120F always helps to increase the activity of this material.

 

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