mandag 16. november 2015

Dry film photo resist

I tried a new technique for making circuit boards yesterday - dry film photo resist.

I already own a (huge) UV-light box that I haven't used for a number of years. My previous UV experiments have all been under a large bulb, so I didn't what to expect.

The way the film works is this:

It has three layers. A bottom plastic layer that must be peeled away (this is on the inside of the curl when the plastic comes on a roll), a middle layer that will form the resist, and a top plastic layer that is peeled away once the film has been exposed.

The film is negative, meaning that any parts not covered by something when it is exposed to light will be "cured" and turn into resist. The remaining parts that were covered during exposure can be washed away during developing.

A friend of mine had already printed some templates on transparent plastic. Though they are meant for positive resist, they work well for the experiment. I put two copies of the same print on top of each other to better block the light.

Start off by sanding the copper clad board with a very fine sanding paper or a scotchbrite sponge to make stuff stick a bit better. Clean off with some alcohol or acetone or something similar to remove any grease.

To remove the bottom plastic, I used a trick I found here - put a bit of sticky tape on each side and rapidly rip the sides apart (it is important to do this quickly). I turned down the lights in the room while doing this, remember, you're working with photo sensitive film.

Then cover the copper side of the board with the film, protective plastic up.Try not to touch the film with your fingers to prevent greasing it up.

The film must be fastened to the board using an iron at a very low temperature (or the plastic will melt and warp) or idealy a laminator. I tried an iron with ok but not extremely great results.

Once the film sticks, put the transparent foil face down on the UV light box, put the board on top and something heavy like an old mathematics book on top of that to keep everything nice and flat.

I tried various exposure times from 2 to 6 minutes. 2 minutes turned out best, the others had cases of light leakage - ragged edges, thinner lines than on the foil etc. My UV light box has 10 powerful tubes and is very powerful so times will be different with other equipment.

After exposure I put the cards into what I think is a 1:100 mix of sodium carbonate and water (but I got it in the same package as the dry film from a vendor in china, so I'm not really sure). The development time was three minutes. After two minutes I started brushing the non-exposed parts with a coarse paint brush to rub off the film.

I haven't tried etching the boards yet so I don't know how well the film sticks. It looks and feels very sturdy though.

Exposed for 4 minutes

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