Sketches of copper
by Antony Chazapis
It has been a couple of years since I have bought all things necessary to make my own PCBs: copper-clad boards – both plain and photosensitive, UV lamps, and chemicals for developing and etching. Somehow, I continuously avoided using them, as I was a bit intimidated by the thought of messing with hazardous chemicals and trying to figure out the process for each stage through trial and error.
For small circuits, it always seemed simpler to just put it all together on a piece of perforated board. However, for more complex constructions, a PCB helps significantly in implementing the schematic faster and with much less – if any – errors. I am midway through building a tube guitar amplifier, which uses a PCB. Getting the electronics part done was so easy, that it convinced me to start making my own circuit boards.
I thought I should experiment with etching first. This is the final step, common to whatever process is used to print the tracks on the copper. Etching removes all exposed copper from the board. So, before etching, you must somehow cover and protect the parts you want to keep. You can draw the tracks, using a permanent marker. It is also common to laser-print them on paper, and then iron on the toner. The photosensitive boards are coated with a special layer, that dissolves in the developer, when previously exposed to UV light. During exposing, light is blocked from reaching the tracks, so the copper underneath can survive the etchant.
I searched around for an old design that could be drawn by hand. The Big Muff Pi is a legendary guitar effects pedal, whose design and board layout have changed numerous times, since its introduction in the late ’60s (a fairly complete history can be found here). I decided to replicate the PCB used for its most popular incarnation – the “ram’s head” version, produced in the mid ’70s (beware of the error where C4 meets R9).
I printed the layout on a piece of paper, and – as suggested by a friend who used to practice this technique a long time ago – I made the holes first. Then, I roughly transferred the tracks to the board using carbon paper, so I could trace them with the marker. I made at least three passes, after waiting for each previous one to dry out.
The etchant used was sodium persulfate (Kemo A100). I dipped the board in and waited for the magic to happen. It did. After a while, any scratches on the exposed copper disappeared and it became shiny and even. This continued, until the copper layer completely dissolved – first at the sides of the board, and then gradually up to the center. Approximately half an hour later, I took the PCB out and washed the etchant away with plenty of water. I dried it, stripped off the ink with acetone, and started soldering.
I completed the circuit in no time. I got excited. In the remaining part of the original 10×16 cm board, I drew up an old-school audio amplifier, that uses two 2N3055 output transistors (found here). It felt that I could build anything, by just making a PCB. Buying and soldering on all the components was the easiest part anyway.
It was time to bring out the UV lights and the photosensitive boards. The marker method was simple, but tracks came out a bit fuzzy and I imagined it being tedious and inefficient for larger and more complex layouts. I had already built the exposure chamber – a simple box made from MDF, housing two Sylvania BL350 8W fluorescent lamps, placed approximately 5 cm from each other and 6 cm from where the board should go.
I needed a small layout to experiment with and test different exposure times. The Micro M+ charge controller, presented in the 2007 ARRL Handbook was the perfect candidate (also found here). I remember reading the article some years ago, struggling to comprehend how the circuit operates. Now, it seemed simple to understand. I copied the schematic over to Eagle and started placing components and routing tracks. I managed to pack it all in a layout measuring 5×6 cm, which meant I could make four chargers from one board.
While the lamps were warming up, I cut the board into four equal parts and taped the transparencies onto the glass. I used two transparencies for greater contrast. The printed tracks on a single sheet were not opaque enough, even when printing at the highest quality. I pulled the protective coating off the first board and exposed it for 4 minutes. I had decided to start from there and see how it goes, having read the experience of others, using similar equipment. After the alarm went off, I sunk it in the sodium hydroxide solution (Kemo E100). For the first couple of minutes, nothing happened. I comforted myself, by thinking that I could still use the board anyway, even by drawing tracks with a marker… Then, the circuit started to appear. Initially, as a negative image of slightly purple hue. This should be the sensitized part of the topmost layer, reacting with the developer. I stirred the chemical a bit, and the purple particles mixed up with the liquid. I did not wait long. I took the board out, washed it and put it into the etchant. Nearly 30 minutes later, nothing happened.
I guessed that the photosensitive layer was still there. In my hurry, I hadn’t measured the exact time the board stayed in the developer. Nevertheless, I decided to move on with the next one. This time, I exposed for 8 minutes and developed for 20. I left the second board at least 10 minutes more in the developer. It may had been too much, but the etching did now proceed as expected. I tried to put the previous one a little longer in the developer and then back to the etchant. It worked. I now had two boards ready. Close comparison revealed that the 8 minute exposure was significantly better, although the tracks were a little slimmer than expected in some places. The third board was exposed for 6 minutes and developed for 15. Just right.
I didn’t make the fourth one. Instead, I printed a layout I had designed in Eagle some time ago – a simple audio amplifier, with one PCB for the power supply and one for each channel (this one). Put one next to each other, the overall size was about 14.5×10 cm. With this, I would test how the exposure chamber performed for an entire board. If the exposure was uneven, the parts directly under each lamp, would have been overexposed, in contrast to the rest of the layout. It came out perfect. To celebrate, I printed one more audio amplifier board I found on the Internet (here). Each of these circuits will be presented in more detail, when complete.
Overall, a big leap forward, an extremely satisfying process, and – hopefully – a strong boost in the direction of materializing ideas and an ever-growing collection of electronic parts into useful devices.