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How to do your own Photoresist PCBs on the cheap

How to do your own Photoresist PCBs on the cheap


26.06.09
My usual method of transferring a circuit from breadboard to stripboard is tedious work, and if I wanted to use "just enough" board space, I'd have to redo the layout several times. Since even mediocre electronics hobbyists do their own PCBs, so I figured it's about time I started too. Not only would this allow me to design circuit layouts on a PC where mistakes are free, but I could also use layouts designed by other people. And what tops off an electronics project more than soldering a PCB you designed and created yourself? So I researched different PCB production techniques, checked what my supplier had, and researched some more. Toner transfer seemed to be the most commonly used method among hobbyists, but I didn't have easy access to a laser printer, and definitely not one I can stuff magazine pages into without getting in trouble. The results seems to vary greatly depending on all sorts of factors, and in the end you might not even get excellent trace resolution. With toner transfer ruled out, the other options I had were to draw my own traces with a marker (no thank you) or use the photoresist method. Alright, so what are the downsides?

Start-up costs.

550 EUR for a UV box? 340 for a PCB guillotine? Another 500 for an etching tank? Fat chance. It's time to DIY.

 Pro UV Box Pro guilotine Pro etch tank

The following materials are necessary for making a PCB:

Copper-clad board, with pre-sensitized photoresist    -    Buy this from your electronics supplier, there's no shortcuts here. There may be different copper thicknesses and board materials available. I use FR4 board, 35 Ám copper layer with positive photoresist. It has worked well for all my projects so far.

Developer    -    The photoresist will most likely be developed by sodium hydroxide (NaOH). This is the main ingredient in most drain cleaners, so instead of paying for a 50g bag from your electronics supplier, go to the grocery store and pick up a 500g bottle. In my case both cost the same.

Etchant    -    There are many choices available here. The most common are ferric chloride, copper chloride and sodium persulphate. I only have experience using sodium persulphate, but it hasn't left me wishing for something else yet. It rather clean, easy to store, and only fumes small amounts of oxygen while decomposing. The drawbacks are slow etching speed and relatively high price.

In addition the following tools are also needed:

Means of UV exposure    -    Check out my UV LED exposure box to see what I did to create a means of exposing boards to UV. Otherwise if you live close to the equator or under an ozone hole, I've heard of people using sunlight. Not nearly as fast or reliable though.

Means of cutting copper-clad board    -    The ideal solution here is to buy a PCB guillotine. I haven't been able to convince myself it's worth the cost however, so I use a common Stanley knife. What you do is score the board on both sides first. I make sure to cut deeply, but this might not be necessary. Once you've scored the board, simply put it in a vise, and snap it!

Drillbits    -    I also needed some new supplies to drill holes for through-hole components. FR4 board is made of fiberglass, so it will dull common steel drill bits. Tungsten carbide must be used if you want to get a decent lifetime out of your drillbits. The stripboard I "grew up on" used 1mm holes, which fit for almost any component in my experience. So I purchased 1mm or #61 drill bits, and some 1.5mm or #53 bits for wire connections and such. What you want to look for when buying bits is resharpened carbide, these are cheap, and often come with a standard 3mm shank so even the tiny 1mm bits can be used by a standard drill chuck. Holes can be drilled by hand, but I really recommend using a drill press. Keep in mind that glass dust will be created when drilling fiberglass, so wear a dust mask. Running a small shop-vac near your PCB while drilling is a good idea.

Once I had acquired the bare-bones minimum of tools and materials, it was time to test my setup. I created a small PCB using (god-forbid) SMT. I wasn't happy with my previous travel charger, and with my new PCB setup I suddenly had access to the SMT chips, which almost all of the newer chips are sold as. For making the trace mask/PCB-design I use ExpressPCB, which is free. Make sure to adjust the component's hole size according to the drill bits you have at hand, or you'll drill all the copper away when making a hole. Also check that the component's lead spacing is correct, the datasheet will always give exact measurements.

mask applied to PCB  Exposing PCB

My mom was so kind as to print the PCB mask on a sheet of overhead transparency, which would considerably lower the exposure time compared to paper. The trace mask was taped to the PCB with clear scotch tape. For SMT boards place the ink side up, and for through-hole the ink side towards the board. Always double check that the mask is the right way, otherwise your PCB will be mirrored. I set the timer for 2min and 30s on my first attempt. I was unsure of the exact time required, and didn't want to over-do it. At the same I had heard of exposures taking 10 minutes or more with certain UV bulbs, so I wasn't sure what to expect. From later exposures I've had best results with exposure times near 6 minutes with transparencies, and 40 min and up with standard printer paper. Though printer paper takes a long time, it turns out very nicely, and the ink is dark enough to stop almost all of the UV so you don't need to worry about overexposure. Some people soak the paper in oil first to make it clearer, thus reducing exposure times. I weighted the PCB down with some scrap iron to keep the mask pressed tight to the PCB.

developed PCB
Picture is of the PCB after being developed in sodium hydroxide.

After exposing the PCB, the unexposed traces will be slightly greenish compared to the exposed photoresist when viewed under bright light at an angle. This is hard to see and requires optimal lighting, so don't despair if you don't see anything. The traces should become clearly visible after development however, and the UV exposed photoresist should be removed entirely, revealing fully exposed copper. To develop the exposed PCB I placed it in a shallow bath of warm water, and slowly added drops of concentrated NaOH solution. You'll need to add quite a lot of NaOH before anything happens, but once it does can can overdevelop the board it you're not careful. The exposed photoresist will begin to dissolve once enough NaOH is added, thus giving a brownish color to the water. Depending on the quality of the photoresist your board was coated with, the developer may not dissolve the unexposed resist, or strip it off as soon as it's done with the exposed resist. So watch this stage carefully.

When mixing dry NaOH crystals remember dissolving NaOH is an exothermic reaction, so heat will be generated and can crack glass if too much is mixed in at once. When mixing a batch of NaOH solution, I use precooled water, and add a few crystals at a time. The first time I was mixing up my solution I had a single drop of water fall into the NaOH bag whilst there were some crystals left in it. I only noticed because I felt some extreme heat by my finger (which was holding th bag). A tiny drop of water had dissolved a few crystals and created a supersaturated NaOH solution, which had become almost hot enough to melt the thick plastic bag. So mix the crystals into water, and always use gloves and eye protection!

Semi etched PCB

The next step is to place the board in the etchant. Sodium persulfate etchant works best at 50C, so you need to keep the solution warm by some means. I simply placed the etchant tub in a hot water bath, in the bathroom sink. Exposed copper should go rosy/light pink within 2 minutes, indicating the etchant is tarnishing it. If there is no change in color, remove the PCB, wash it, and develop it some more. Reaction speed is very dependent on temperature, and also dependent on how fresh the etchant solution is. At room temperature expect at least 40 minutes to etch a board, while when closer to 50C it only takes 10-15 minutes. Stir occasionally, keeping bubbles off- and fresh solution on the PCB. As the sodium persulfate is exhausted the solution will gradually become deep blue with copper sulfate (CuSO4).

After etching, holes should be drilled since the photoresist will protect the copper. Leave the resist on until you're ready to solder. To remove the resist, you can either scrub it off with steel wool, or if you can acquire NaOH cheaply, simply expose the PCB to UV (no mask this time) and redevelop.

completed PCB

Here's the completed travel charger PCB. Notice even the tiny traces outlining the PCB turned out, and this was the first run without optimization! I can definitely recommend this method to anyone who wishes to make their own PCBs. It's clean, cheap, and fairly quick. And above all the PCBs turn out great!

For further reading, check these sites out:

PCBs Fabrication Methods
Etching In House
PCB Etching

PCB Bubble Etch Tank
Update 25.04.11

The worst part of my PCB making process has been the etching. It works so slowly, and the solution has to be heated using a hot water bath which takes all eternity. Not only that, but I had to sit and stir the solution by hand while the copper was being etched. To remedy this I whipped up a little bubble tank, using some cheap aquarium components, a 2$ container from the dollar store, a few zip ties and some odds and ends. It's not handsome, but it is handy.

   

And the results? The solution can now be heated to working temperature all by itself while I expose the PCB. Not only that, but thanks to the bubbles mixing the solution etch times have been halved! If you plan on making more than two PCBs I demand you build something similar to save yourself all the hassle of doing it by hand.




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Disclaimer: I do not take responsibility for any injury, death, hurt ego, or other forms of personal damage which may result from recreating these experiments. Projects are merely presented as a source of inspiration, and should only be conducted by responsible individuals, or under the supervision of responsible individuals. It is your own life, so proceed at your own risk! All projects are for noncommercial use only.