Etching Printed Circuit Boards

There are two popular methods of etching PCBs at home: what is known as toner transfer, and UV exposure. The first method relies on transferring the toner from a laser printer to a copper-clad board, which will then form the protective layer. This method is widely used. The pro's are obvious: no UV source is required and there's no developing of the exposed board. The flip-side is that transferring the toner to the board is not trivial. I've no experience with this method.

I use UV sensitive boards. These are exposed through a mask, then developed, leaving a etch-resistant layer on unexposed areas (in case of positive photo-resist, which most pre-coated DIY boards have). The exposed areas are not protected, and will be removed by the etchant.

In this guide, I'll show you how I do it. I'll also try to keep you from making the mistakes I made. So without further ado, let's begin.

Things you will need

a computer program to create PCB artwork: I use Cadsoft Eagle
a laser printer
laser printer transparancies
pre-coated positve photo-resist boards
a UV source: I use my mom's old sun lamp
developer: NaOH, also known as caustic soda, available in ample quantities at the local supermarket
etchant: I use sodium persulfate
chemical-resistant containers: I use plastic bottles to store etchant and developer
chemical-resistant dishes: ceramic or glass dishes are ideal, since they can stand some heat
a source of heat: the etchant works at 40-50 °C, so you'll need some way of warming it up and keeping it warm; I use an old hot plate which I regulate using a variac
safety goggles
plastic tweezers or something similar
patience

PCB artwork

Of course, you'll need something to put on your PCB. I use Cadsoft Eagle to design my PCBs. Once it's finished, print your artwork on a transparent overhead sheet. This will be your mask. If you're producing a PCB with the traces on the bottom, you'll need to mirror the artwork. Think about how often you'll turn things over.

Once you have your printout, inspect it against a bright light. If there are areas you can see light through while they should be black, it's not dark enough. This can already be seen when printing on regular paper. Laser printers often have trouble filling large areas. That it looks good without illumination from behind means nothing. Try setting the toner density higher (in case of a laser printer), or print two copies and carefully align them. This is what I do. If you are careful, you can do this very accurately. The result will be much darker.

UV exposure

When I first started, I experimented with sources of UV light. I tested several fluorescent lights and a regular lamp that was still supposed to give off a lot of UV. Nothing performed to my liking, requiring exceedingly long exposures. The sun is a great source of UV, but not a very reliable one in my country. I finally settled on my mother's old sun lamp, a mean sucker with a nasty mercury vapor bulb. You don't want to look directly at it. I don't keep it on any longer than I have to.

When preparing the PCBs you should be very careful when removing the foil so not to damage the sensitive top layer. Don't touch it with your fingers either! Preferably, you shouldn't even touch it with the tweezers. Handle the board by the sides as much as you can. Soft tweezers are better than hard ones. I use a set of nylon tweezers, that have the added advantage of not dissolving in the etchant.

I use a photo frame to press the PCB against the printed side of the mask. The frame that I got has a 4-mm thick glass plate, which should stop a lot of UV. But it's no match for my sun lamp. I get excellent exposures in 2-3 minutes with the PCB about 30 cm from the lamp. Watch out for dust and dirt on the PCB, the artwork, and the glass plate.

Developing

After the board has been exposed, take it out of the frame, and put it in a bath of caustic soda (NaOH). 4 grams on 200 ml is enough. At 25 °C it should develop in about a minute. The traces will become visible. The protective layer should release in the exposed areas (in case of a positive sensitive PCB). You might have to agitate the bath a bit to make this happen. Rinse with cold water.

Etching

Finally, we have to etch our developed board. Prepare a bath of persulfate at 40-50 °C. How much persulfate you need to etch your board depends on how patient you are and how much copper has to be removed. As a rule of thumb, dissolve 100-200 gr sodium persulfate per liter of water. It can take 20 minutes or longer to completely etch your PCB. Don't be discouraged if it goes slowly. You might not see anything happening until you're almost done. Lower temperatures mean that the process takes longer. Don't leave the board in longer than it needs to. It will etch the traces from the sides under the protective cover.

Finishing Up

Once your board is etched, rinse it off in cold water. I don't clear off the protective coating from the traces until I start soldering. The next step is to drill the holes. I have the luxury of using a precision drill press in the local machine shop, but I have also gotten good results using a Proxxon/Dremel tool in a standard drill press. Get good drills, suitable for drilling in steel, or preferably get tungsten drills. Once I am ready to start soldering, I clean off the coating, and apply a coating of solder flux (Kontakt Chemie Flux SK 10), that also protects the board during storage. When I'm done soldering, I remove the residual flux, and apply a protective, insulating layer (Kontakt Chemie Plastik 70 or Urethan 71).