Introduction: Photoresist Dry Film - a New Method of Applying It to Copper Clad
I recently had a go at using this film to make my own PCBs. It is ideal if you don't have a laser printer or photocopier and therefore can't use the Press'n'Peel (and similar) products. But optimism quickly turned to frustration.
The problem is that it virtually impossible to apply the film to the copper using a laminator without getting wrinkles, bubbles, fibres and dust particles preventing adhesion of film to the copper.
When you develop the copper clad, after exposure to UV, you always get small chunks of exposed polymer lifting off the copper where it has not strongly adhered to the copper.
If you are making crude PCB with large pads and thick traces this won't matter that much because you can always go back over it with a permanent marker and cover up the inappropriately exposed copper.
But this is impractical if you have designed your circuit layout with DipTrace, or similar PCB design software, and the traces and pads are small and spaced close together.
So I initially gave up in disgust after wasting 5m of film with no success.
But then I drew inspiration from my days as a medical scientist which involved preparation of paraffin sections of human and animal tissues.
PREPARATION OF PARAFFIN SECTIONS
Briefly, this involves 'fixing' or embalming the tissue samples in formaldehyde solution.
Then it is necessary to replace all the water in the tissues with paraffin wax so that the tissue is rigidly supported and it is therefore possible to cut 6 micron thick sections that can be stained and examined under a microscope. Consider how difficult it can be to thinly slice fresh beef, compared to if you partially froze it first and then sliced it. The tissues samples are soaked for some hours in a series of graded ethanol solutions since ethanol and water are miscible. Upon soaking in a few changes of pure or 100% ethanol, the water in the tissue samples has been replaced entirely with ethanol. Then the tissue samples are soaked in a few changes of xylene or paint thinners. Since pure ethanol and xylene are miscible, all ethanol in the tissue samples is replaced by xylene. Since xylene will dissolve paraffin wax, soaking the tissue samples in molten paraffin wax will result in all the xylene in the tissue samples being replaced by paraffin wax. After this the tissues samples are embedded in blocks of paraffin wax from which VERY thin sections can be cut using a micro-tome. The thin sections are then floated out on warm water which soften the wax, flattens out the paraffin sections and removes any dust and fibres from the underside. The paraffin sections are then picked up on a microscope slide as depicted in the photo.
So I thought, since this works so well for delicate paraffin sections, why not try adapting the last step in this technique for photoresist dry film.
Step 1: Water Application of Dry Film
Quite simply you cut your pieces of dry film to size as normal.
Then submerge your copper clad in a tub of water. The water depth has to be sufficient to provide you enough room for your hands to work in without introducing air bubble under the film.
Before you dunk the photoresist film, use two bits of tape to remove backing plastic at one corner.
Then immediately dunk the film in the water and continue removing the backing plastic.
You will find that the film floats out nice and flat - it can't stick to itself and it doesn't roll up.
Then using a similar technique depicted in the photo above, raise the copper clad up at and angle and catch the edge of the dry film.
Continue raising the copper clad until it is free of the water and ensure that no air bubbles get trapped between the copper and the film as you do so.
If you get it wrong just dunk the lot back under the water and re-position the dry film.
Once you are happy with the positioning of the film on your copper clad, lay it on a firm surface and use a roller to squeeze out as much water from between the copper and the film as possible.
Be sure not to accidentally separate the dry film from the copper - if you do you can put it back in the water, carefully peel off the dry film and repeat the process.
Now you need to place the copper clad in a warm place to dry overnight.
Step 2: Drying
An oven on 50 degrees Celsius or so works well, however the combination of heat and light from the incandescent bulb is enough to partially expose the dry film. So you will need to wrap your board in a thick towel.
A radiant heater also works - I used the type pictured. These do not get too hot and I hung the board a few centimeters away from it. If you use the type that glows red then you might have to hang the board further away so you don't scorch the dry film. Also the room obviously needs to remain dark.
As the water evaporates away it creates a vacuum under the dry film which 'sucks' the film tightly against the copper.
Any where that water remains between the film and the copper then the film will appear opaque pale light blue as in the photo.
When it is fully dry the copper clad will look as if you have passed it through a laminator. Except there are no bubbles, fibres or dust particles at all.
Step 3: Exposing
Indirect sunlight at around midday works as good as a bench top UV source when exposing the sensitised copper clad board.
Place the board outside in the shadow of a building for example.
I am located in Melbourne Australia and I exposed the board, depicted by the first photo, in winter when the sun is relatively low in the northern sky at midday and the UV index is similarly minimal. Under these conditions 15-20 minutes was sufficient to produce the results in the photo.
A previous attempt at exposing one of my boards out the front in direct midday sunlight, in winter, hugely over exposed the dry film.
If you are located closer to the equator or carrying out this procedure during summer, then you will almost certainly need to reduce the exposure time. It will be a matter of experimenting to find an optimal exposure time for your location and time of year.
Also creating your circuit layout as trace outlines, as depicted in the second photo, will help greatly because there is the maximum amount of dry film surface area bound to the copper. And this will minimise detachment of chunks of film that might mess up your traces.
I also used Vaseline to adhere the transparency tightly to the board. A sheet of glass was not much use since the board was not perfectly flat and the sheet of glass I had was not heavy enough.
Step 4: Cleaning the Traces
Sodium hydroxide is required to clean off the resist after etching
however this does tend to corrode the copper a bit. But a useful way to clean off any corrosion is to then soak the board in household ammonia. If you remember your high school chemistry, ammonia molecules form a deep blue ligand with copper II ions that is highly soluble in water. If you pour ammonia solution into copper II sulfate solution, then first green copper II hydroxide will precipitate. But if you keep adding ammonia solution the copper hydroxide will react with the excess ammonia to form the soluble copper II tetraminesulfate, and thus turn the solution deep clear blue. In the same way, ammonia will dissolve any copper oxide corrosion on the surface of your traces. A bit of gentle scrubbing, while in the ammonia solution, will help dislodge stubborn spots. Ammonia also dissolves copper metal slowly, so don't leave your circuit board in ammonia solution too long.
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