Introduction: Photoresist Dry Film - a New Method of Applying It to Copper Clad

Picture of 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

Picture of 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

Picture of 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

Picture of 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.

Comments

baecker03 (author)2017-07-18

I use a hair dryer on low or med setting to dry. there is a good video on YouTube about it as well.

boylesg (author)baecker032017-07-18

Less convenient to have to stand there directing the hair dryer though.

gm280 (author)2017-07-16

What about typical alcohol as in rubbing alcohol to apply the resist to the copper clad board? I have made so many PC boards and tried so many methods. I presently use P-N-P (Press aNd Peel) Blue transfer sheets. And it seems to work very well with everything I've done. But I am always looking for different ways to make them. Thanks for the post.

boylesg (author)gm2802017-07-18

Alcohol certainly gets rid of muck from your fingers and improves the ability of the film to adhere to the copper as a result. But it doesn't get rid of fibres and dust particles, in fact you will add more fibres to the copper from the cloth you rub it with. I would suggest rub the copper down with alcohol or what ever to remove grease etc and then apply the film under water as well as that.

boylesg (author)gm2802017-07-18

Press'n'Peel is worse than bloody UV film if used as instructed. I tried it and the amount of touching up I had to then do was just impractical.

So I also applied the second lot of press'n'peel via water. The stiffer plastics meant that it started lifting off the surface of the copper around the edges before it was 100% dry. So I broke out the iron regardless. And it actually worked very well despite the fact it was still damp. The amount of touching up I had to do on this second attempt was actually manageable.

Press'n'Peel seems to have a particular problem with the copper not being mirror smooth, i.e. the weave of the fibreglass cloth is embossed in the copper layer.

boylesg (author)gm2802017-07-18

If ethanol dissolves the photosensitive polymer then you can bet that all alcohols will do the same. I have not tried xylene or other liquid hydrocarbons. You could also try mineral turpentine and what ever other volatile hydrocarbons you can get from your local hardware store.

boylesg (author)2017-07-16

Further areas of experimentation would be
to try different fluids, preferably one with a lower boiling point to
speed up the drying process. I tried metho/ethanol (boiling point around
60 degrees Celsius) but sadly it dissolves the photoresist polymer.

Xylene
might be worth a try but you would have to be careful about that
because (from my days in the Department of Anatomical Pathology at the
Austin Hospital) it does carry some risk of being toxic and carcinogenic
- it is a benzene derivative after all. So if you try this latex gloves
and a chemical fume extractor system is recommended if you intend to
prepare a lot of boards.

Mineral turpentine might also be worth a
try - that would be cheaper than xylene. Not sure about it biological
toxicity though. Latex gloves would be minimum protection.

Acetone is out because, if ethanol dissolves the UV polymer, acetone definitely will.

acolomitchi (author)boylesg2017-07-18

Evaporation rates: http://www.ilpi.com/msds/ref/evaporationrate.html

a. Xylene - twice as fast as water. Not much of a tradeoff - inhale xylene fumes for 20 mins and it's a blinding headache almost guaranteed.

b. Mineral turpentine (mineral spirit) - 3 times slower to evaporate than water

If you think a VOC should work, try gasoline (non-polar, cheap)

My advice? Stick with water if it ... well... sticks.

boylesg (author)acolomitchi2017-07-18

Gasoline might be problematic because there is more than just hydrocarbons in it - antioxidants and who knows what else!

boylesg (author)boylesg2017-07-18

Also don't bother with gasoline that contains added ethanol

boylesg (author)acolomitchi2017-07-18

From my histopathology days xylene, toluene and many other benzene derivatives are toxic and carcinogenic. That is why, if you intend to try these as an alternative to water, that you wear latex gloves and carry out the technique under a fume extraction system of some sort

boylesg (author)boylesg2017-07-16

Xylene is sold as 'paint thinners' at Bunnings in Australia and what ever hardware chains are in your own country.

eburman (author)2017-07-17

As a pathologist this really hits the spot! Love it!

tytower (author)2017-07-16

Step 1 was very interesting . I felt sure you would end up with a metal of some sort infused in there eventually.

MekkMan (author)2017-07-16

great idea hadn't thought about this method since grad school.

boylesg (author)MekkMan2017-07-16

Are you or were you involved with histopathology also?

DIY Hacks and How Tos (author)2017-07-16

Interesting. I might need to try using this method some time.

I have repeated the process 3 times and it works reliably.

It also works with Press'n'Peel but the stiffer plastic means that it starts lifting off the copper around the edges. So I applied the iron despite it not being 100% dry and the it worked a lot better than not applying it via water. But there are still parts of the circuit I will have to touch up with a permanent marker. But no where near as bad as the one I did without water.

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