Another project this semester was to design an antenna for our electromagnetics class. I will go into more detail on that antenna in follow-up post, but basically, I chose to design a microstrip patch antenna. The simplest way to do that is with a double-sided copper clad board, turning it into a PCB by etching one side. What is left is a rectangular antenna of specified dimensions to work at a desired frequency (actually, it doesn’t have to be rectangular, but it is the simplest design).
This was my first time etching a PCB, which is a little daunting. It requires use of nasty chemicals, and from everything I read, it’s somewhat of an art that needs time and practice to perfect. That may be true, but I seem to have stumbled onto a method that works fairly well the first time. I was somewhat pressed for time, with finals and multiple projects, so I didn’t have a long time to experiment with this.
The first dilemma was whether to use ferric chloride or some other chemical as the etchant. In fact, there are various alternatives, such as persulphates, or this instructable that mixes hydrochloric acid and hydrogen peroxide. I planned to use the hydrochloric acid and hydrogen peroxide method if ferric chloride didn’t work. It turns out that wasn’t necessary, ferric chloride worked perfectly for me!
At first glance, the list of supplies is fairly standard:
- Copper clad board (I used MG Chemicals 540 from Digikey)
- Ferric chloride solution (I used MG Chemicals 415)
- Laser printer with plenty of toner
- Toner transfer paper (Pulsar 50-1101)
- Recommended: toner reactive foil (Pulsar 50-1225)
- Fine-tipped Sharpie marker
- Acetone (KleanStrip Acetone)
- ScotchBrite industrial strength scour pads
- Microfiber cloth*
- Sealable, shallow container*
- Long chemical resistant gloves* (Nitrile)
- Safety glasses
- Clothes iron (Sunbeam 3956), or laminator if available
- Ironing board or oven mitt
- Optional: thermocouple to measure temperature (Fluke 87-V w/ thermal probe)
CAUTION: Items marked with * will be contaminated with toxic chemicals!
I’m using a laser printer and toner-based method of masking the board (there are also UV light approaches, if you have the equipment). Basically, toner is a kind of plastic and when heated, it will melt. The idea is that it’s possible to print the PCB design on paper, then reverse the process by melting the toner off the paper and onto the copper board. This will then act as a mask, preventing the etchant from removing copper under the mask. In theory.
In practice, it seems a lot trickier, because toner and most papers aren’t exactly designed to work this way. Temperature is important: not enough heat and the toner won’t properly melt, but you don’t want it too hot, either. Clothes irons aren’t designed for precise temperature control, and nobody knows what temperature it should be anyway. Then there is the issue of removing the paper after the toner has melted, you want it to pull away cleanly without removing the toner. Later, the etchant solution might remove some of the toner mask by accident. So there are many variables and potential sources of error.
I think there is a pretty good solution for transferring the toner and protecting the mask during etching, but it’s a little more expensive. Is it worth spending more for better results and to save time? That is completely up to you. A company called Pulsar has made a full line of products for PCB etching, “PCB Fab-in-a-Box”, claiming you can make PCBs in 8 minutes. I didn’t buy the full kit for this one-off project, but I was very intrigued by their toner transfer paper and green reactive foil. Their paper claims to work with any laser printer, more accurately and easily than other types of paper. Their special green foil is then applied on top of the toner mask, to seal it during etching. They even sell an applicator that applies the optimal temperature for their products (it turns out an iron works, if not as effortlessly).
My process of mixed DIY and off-the-shelf products didn’t take 8 minutes, it took much longer, including time to document the process. But, the results of my admittedly simple PCB were almost perfect the first time, so I think there is something to it.
The first step is to prepare the copper clad boards:
- The board may need to be cut to size. I had 3×5″ boards and only half that size was needed, so cutting them in half gave me twice as many to experiment on. My boards were 1/16″ FR-4 substrate, which is fiberglass and not easy to cut. Depending on your application, a thinner board or different material might be preferable. A lot of people will say use a heavy duty paper cutter or something similar to cut the boards, but I don’t have one of those. Instead, I used an X-acto knife and ruler to score the copper surface, followed by a utility knife until there was a deep cut in the board. Then I bent the board over the ruler, so all the stress was on the cut, and it broken cleanly in half. If you have better equipment on hand, by all means use it!
- Next, I used a small file around the edges of the board, especially along the cut edge. The point was to make the board as flat as possible.
- The boards have a smooth copper surface, but creating some roughness can help the the toner stick. That is certainly true in my experience, my first test board had one side left smooth, and the toner would flake off, but it stayed on the scoured side. Some people suggest fine sandpaper, I used ScotchBrite industrial pads, it really doesn’t matter as long as you don’t choose something too coarse that it really damages the copper. I liked the pads, because I put the whole thing in soapy water to simultaneously clean and scour the surface.
- Clean the boards. I started with hot water and dish detergent. The important thing is not to touch the copper surface while or after cleaning, but hold it by the edges. Your hands have oils and stuff that will also interfere with the toner application. I patted them dry with clean microfiber cloth, then wiped them over with acetone for a final cleaning.
Toner and Foil Application
Now you should have nice, scoured but shiny copper clad boards, clean and dry. It’s time for the toner application. My etch masks were simple rectangles, so I actually used Word to create a couple of rectangles (you can specify what size to make them) and pack them onto a single 8.5×11″ sheet. For a more practical PCB design, there should be software that can print the traces. I did a test print on standard copy paper, to make sure I knew which side was “up”, and to verify the printed sizes were correct.
- Print out your PCB mask. Pulsar’s toner transfer paper is one-sided, the blue side is the printing side (thus, the importance of knowing which side is “up” in the printer tray). When you handle it, don’t touch the surface of the paper, but hold it by the edges, again to prevent oil and residue from your fingers. I packed as many masks as I could on one sheet, as it’s not cheap and I didn’t want to waste any. I was able to fit two 3×5″ boards on a single sheet, enough to four test antennas.
- Cut out the masks and place them with the toner side down (against the copper board). I chose to apply both sides at once, because there is a chance of damaging the opposite side due to the applied heat. Once you apply the top mask, the bottom mask should be “stuck” on, so it won’t move if you carefully flip the board.
- Prepare a bowl of clean water to immerse the board after the masks have been applied.
- Place the board and etch masks on an ironing board, oven mitt, or some other temperature resistant surface. I set my clothes iron to the highest setting (“linen”) and waited for it to heat up. A thermocouple probe is useful for measuring the temperature here. The hottest temperatures I measured while applying the iron were in the 180-190°C range, which anecdotally seems to be about optimal. I pressed down firmly on the board. The trick is to get the toner hot enough to stick to the board, without getting so hot that it damages the mask. Since copper is a great conductor, and the surface of the board is relatively thick copper, it acts like a heatsink almost, so it will take some time to melt the toner. I also moved the iron so the heat would be spread more evenly across the entire surface of the board. The paper should not be burned.
- Flip the board and repeat the process on the other side, if necessary.
- Drop the board into the bowl of water, and wait for the paper to loosen up. After some minutes it should lift off with a gentle shaking of the bowl. Now careful wiggle the bottom mask paper to check that it’s also loose, and remove the board from the water. Now you should see a black mask on your board, exactly where you intend the copper circuit to be.
- An optional step now is to use something like Pulsar’s toner reactive foil, which basically melts at a lower temperature and sticks to toner plastic, creating a better seal against the etchant. The application process is similar to the toner masks, only cut a piece of the green foil down to size and lay it on top of the board (both sides if necessary). Notice the foil has a shiny side and a dull side; the dull side faces down, against the board. For this process, I set my clothes iron to a lower temperature (“polyester”). The foil will melt very quickly compared to toner, so you won’t keep the iron on nearly as long, just move it quickly, but smoothly across the surface until the foil appears to be evenly melted on the board.
- Now carefully peel off the foil, and you should see a nice clean mask underneath.
If everything went perfectly, you will have a cleanly masked board with your desired circuit in black toner plastic (or green foil), while everything showing copper will be etched away in the following step. However, it’s likely that there will be small defects in the masking process.
- If you need to remove mask, you could use acetone maybe on a Q-tip, or perhaps an X-acto knife, but you’ll have to be very careful, especially with small mask details. For my patch antenna, I had two large rectangles, which is really the simplest possible case. The foil left quite a bit of residue, so I used soapy water and a scour pad to remove it.
- If you need to fill in missing mask, you can use a sharpie, which is more easily managed.
- I would gently wash, rinse, and dry the board before etching, to remove as many contaminants as possible that might interfere with that process.
Now the nasty part, make sure to apply your safety glasses, gloves, and have a safe, well-ventilated work area (the usual stuff when dealing with chemicals). In this process, ferric chloride will be used and it will contaminate your gloves and whatever container is used for the etching process. Read the label on the ferric chloride, so you can make sure you understand all the risks and necessary precautions.
- I read that heating the etchant will accelerate the process, so what I did was heat a large pan of water on the stove, with the idea of placing the etching container into the water (if it’s plastic, it will float). The water isn’t very hot, but warmer than room temperature (around 45°C according to my meter). This should be safer than trying to heat the etchant directly.
- Now pour enough ferric chloride into the container to just cover the board when tilted, it won’t take a lot. Gently drop the board into the etchant solution, and place the container into the heated water. You can let it sit, or you can gently roll it back and forth, I did both. I also flipped the board to ensure both sides were fully coated.
- Watch carefully to see when the exposed copper has been eaten away, that’s when you want to remove it from the etchant solution. If you wait much longer, you are risking the etchant getting under the mask and ruining your board. Again, my PCB is a very simple design with no fine traces, so I had considerable leeway here. For a complex circuit, I think you would want to take the board out sooner than later, and cut off any excess copper manually. It’s easier to remove a little excess than to fix missing, ruined traces. Even better is probably to use thicker traces on a DIY project like this, because it’s such a rough, experimental process.
- Now I took a disposable microfiber cloth and wiped as much of the etchant off the board as I could, to not contaminate anything else. Then I dropped the board into soapy water to remove the remaining etchant residue.
- Properly dispose of the etchant solution as a hazardous chemical, DO NOT pour it down a drain or anything like that, it’s very toxic to the environment. You can also seal the container and maybe use it for another batch of boards, but I’m sure it has a limited reuse period, before it’s too contaminated to work properly anymore. Also properly dispose of gloves, cloths, anything that touched the etchant.
Toner Removal and Final Cleanup
Now we’re almost done! Time to remove the trusty mask that served us so well. With any luck, you will have a nice, clean-looking board with green/black masks covering your copper traces.
- Use acetone to remove the masks. They should come off quite easily, exposing the bright shiny copper.
- At this point, I did one final wash of the board, so it’s clean and ready for soldering.
I was happy with the results, given that it was my first time etching a PCB, and I didn’t have all the professional quality equipment. For my simple needs, this method was adequate and a fun learning experience. By the third board, I had a fairly clean PCB for my patch antenna. The toner transfer paper and reactive foil added substantially to the cost of the project. The final cost to create my PCB was about $10.
Here’s a preview of the finished antenna, complete with horribly soldered SMA connector 🙂