Whether your resist mask was created by scratch milling, photolithography, or toner transfer, etching will be the same process. It’s a chemical reaction that transforms copper into something that dissolves into the surrounding solution – usually copper chloride. It is important the chemical doesn’t eat through your mask, and ideally etchant works fairly fast, is safe, and is easy to dispose of. The two most common etchants for home PCB’s are Ferric Chloride (FeCl3) and Hydrochloric Acid (HCl) + Hydrogen Peroxide (H2O2). A big advantage or scratch milling is you are only etching the scratched areas, which means it’s fast, uses minimal chemical, and emits minimal fumes.
Ferric Chloride is the standard, reasonably cheap, and reasonably safe. You’ll have to order online, or get at a specialty electronics store if there is one in your area. The main drawback is that it is very messy and very hard on clothes. I’ve spilled tiny drops on jeans, and not only are they stained forever, it will eat holes in them over the weeks. So if you do use this, be very neat and careful. The reaction is FeCl3 + Cu → FeCl2 + CuCl, and then the CuCl does some further reacting. You end up with a black sludge that, like any etchant, you can’t just pour down the sink. It will etch your copper pipes and cause harm to whatever is at the end of them.
Hydrochloric acid sounds crazy dangerous, but in fact it is pretty common — it’s even available at a 0.5% concentration in your stomach. No need to vomit though, as it’s also commonly available at a 30% concentration as the driveway cleaner Muriatic acid. It needs to be mixed with Hydrogen Peroxide, that is, one part Muriatic Acid and two parts Hydrogen Peroxide, due to the various dilutions. This can be a very fast reaction, especially if it is warm and agitated — I’ve etched scratch milled boards in three minutes! It creates Copper Chloride and water, using the formula H2O2 + Cu + 2HCl = CuCl2 + 2H2O. As a bonus it creates a beautiful clear green color, which makes it easy to monitor the etching process. For disposal, apparently you can get back copper by adding aluminium, but be careful! This vigorously releases chlorine gas (you can blow a PET bottle to smithereens with tin foil and Muriatic acid). Mostly just you want to neutralize the remaining acid with a base, which can be done with baking soda, lye or even limestone.
Whatever you end up using, it helps to experiment with test strips. Control for chemical concentration, time, temperature, volume, copper area, trace size, scratch depth, resist type, agitation, brushing, and anything else you can think of. Record your results, and before long you’ll have a process that works consistently.
You can use the HCl etchant for many boards, but I find it is fastest just using a small fresh amount and changing it once it starts getting a deep green. You know it is working well when you see little bubbles forming on the copper, however these bubbles also prevent more etchant from reaching the copper. This is why people agitate the solution. You can rock it back and forth, remove and flip the boards, brush it with a soft sponge or even make a little vertical bubbling machine. Brushing really helps with etching if your mask is durable enough and the chemical process is fast. Heating the solution also helps, but never boil it as that releases chlorine gas. If you have a lot of exposed copper, you’ll want to do this in a well ventilated area, but for the small scratches the fumes are negligible.
You’ll want to watch the board carefully, as over etching could eat through the finer traces. Visually, you don’t want to see any shiny copper, just the mask and the substrate. Removing and rinsing the board for a close inspection helps with dialing in – you can always put it back in and etch longer. Keep track of the concentrations, temperatures, how you agitated, and the etch times. After a few boards you should have a very repeatable process.
I usually etch first, and then put the board back on the mill for drilling, but realistically you can drill before etching. I just imagine the etchant slightly eroding the drill holes, making things slightly harder to solder. Need to test that.
Once the board is etched with no bridged traces, you can remove rinse and then clean off the mask with acetone. It works best to put the acetone on a paper towel rather than directly on the board, otherwise is just evaporates. Technically, you don’t really need to remove the mask unless you are tinning the board, as the solder will melt right through it. Tinning is really helpful with long term durability though, and super easy to do, so I tin everything.
For tinning, I use Liquid Tin. It is something you need to order specially, but a small amount lasts a long time. This may make your holes slightly smaller, so be sure to test that if you have tight tolerances. The key to tinning is having a clean board, so after removing the mask with acetone, wash it with alcohol. The board should start turning grey immediately — if not, remove it and clean it with alcohol again. It two or three minutes your board with be protected from oxidation forever, and that’s a mighty long time.