So far, I’ve been able to successfully avoid making my own printed circuit boards (PCBs) for all the electronics projects I’ve done.
Between Protoboards (letting someone else do the etching), perfboard (which I hate, but end up using), and “deadbug” construction, I have been able to bob and weave my way out of learning this valuable skill.
Well, it’s a set of skills, really, isn’t it?
To create a PCB, you have to execute the following steps:
1) design the circuit
2) lay out the circuit onto a board
3) create the copper traces on the board
4) drill the through holes
5) solder the board together
Steps 1 and 2 require a deeper understanding of Eagle, which I’ve not really bothered to do, because I haven’t been serious about creating boards.
Step 3 (and possibly 4) are what I see as The Giant Leap. You can lay down traces on a board in essentially one of 2 ways. You can either chemically etch away some of the copper, or you can mechanically carve away some of the copper. There are other, less-interesting-to-me, ways of doing it (drawing the traces with a silver pen, carving out traces by hand, etc), but I’m going to ignore those.
Let me start with the mechanical method first.
One of the reasons that I built a CNC machine was to use it to make PCBs. The idea of building the circuit in Eagle, then sending the graphics to the CNC, seems like a perfect use for the tool. You can also drill the through holes at the same time as cutting the traces! Having said that, I’ve not done it yet. There are several reasons why, but let’s start with just two. First, I haven’t really figured out how to use small drill bits with the CNC yet; speed control and collet size are both contributing factors. At the moment, I can only use 1/4″ endmills. It seems like the precision endmills are mostly 1/8″ shank. Second, I have doubts about the accuracy of my CNC; it seems like every wood project I build ends up getting cut 2 or 3 times, because the machine freaks out in the middle of the cut; or I end up having to do a lot of sanding of the workpiece at the end, because it has lots of rough edges or scalloping or similar.
Both of these problems have made me shy away from doing CNC circuit boards. I could probably overcome the problems, but I haven’t bothered, yet.
This takes us to the chemical method. The idea, here, is that you take a copper board, lay down some kind of mask to protect the copper you want to keep, then use some kind of chemical soup to etch away all the copper that’s not covered by the mask. Once you’re done, you wash off the mask material, and you’ve got a board that’s ready to be drilled and soldered.
Let me stop at this moment to say that, because of my very odd educational career, I made it all the way through high school and college without taking a single chemistry class.
So, let’s break this down into “creating the mask”, and “etching the board”.
There are essentially 2 ways to create the mask. You can either deposit some kind of material onto the copper (like the toner transfer or Sharpie methods), or you can use a photosensitive copper board, and “develop” the traces onto the board. The toner transfer method always seemed kind of grungy to me; there was a lot of “you need a laser printer” and “print onto shiny magazine paper” and “you have to rinse off the board in water to get the stuck paper off” sorts of instructions. I’m sure that it’s a fine method of doing things, but it bummed me out somehow. I admit that I had some problems with the photo-mask method, too — the etching process already includes a lot of chemical mumbo jumbo that I’m not necessarily comfortable with, and now I need to add another layer of that into this process? Yikes.
Having said all that, I at least understand what the concept of this part of the process is; take a fully-coppered board, and put the circuit traces on it. Accurately.
Now we get into the actual etching.
So the idea, here, is that you have a bunch of copper on the board, and you need some of it to go away. So you need to use some kind of acid solution to remove the unwanted copper. There are a multitude of acids that will do the trick; this is where various religious arguments start to take place.
The time-honored method, used worldwide, probably by professional board-making houses and everything, is using Ferric Chloride. I don’t know what the stuff is; I stop reading as soon as they talk about how toxic the stuff is, and how much *more* toxic it gets when you add copper to it (the stuff that etches off the board goes into the etchant). There are a lot of warnings about “don’t let this spill on your clothes” and “definitely don’t breathe this stuff” and “use the real thick kitchen gloves for this” … no thanks. I have little kids in the house. The last thing I need is one more hugely dangerous chemical lying around.
The next-most-widely-suggested method (from my informal survey of reading and dismissing the idea of doing this over the years) is using HCl (hydrochloric acid or muriatic acid). Now, this is the stuff that’s in your stomach, so it’s not necessarily the most evil stuff ever, but again, the posts talk about “you use it to maintain the pH in your swimming pool”. I guess if I had a pool, I’d have the stuff, and I wouldn’t think twice about it. But I’m not bringing extra stomach acid into the house if I can help it.
Then I saw this post by Quinn Dunki (who is a LadyAda-caliber electronics demigoddess in her own right) about etching PCBs using an etchant that is, essentially, safe to drink. I’m not drinking it, especially not after it’s got copper in it, but the stuff in it (vinegar and Hydrogen Peroxide and table salt) really is pretty tame. Seeing the resulting boards and knowing that the input chemicals are safer, really made me think this one over. There’s still the question of what to do with the used etchant (can it be reused? can it de dumped down the drain? send it to a landfill?), but I’ve been pretty convinced by the various debates I’ve heard, that this stuff is actually kind of good for your septic system (in small quantities) — hardware stores sell copper sulphate as a root killer for use in the garden.
OK, OK, I’m convinced.
So, what do I need to have on-hand in order to make boards? I’m going to parse through the post and collect a materials list.
for Safety:
– nitrile gloves
– eye protection
For the mask:
– acetate sheet (Kinko’s?)
– photo-sensitive PCB
– blue painter’s tape
– old picture frame
– daylight-spectrum CFL bulb (or regular desk lamp)
To develop:
– Tupperware tub (sized to fit the board)
– positive developer
To etch:
– something to do for 20 minutes
– Tupperware
– vinegar
– hydrogen peroxide
– salt
– foam brush
– acetone (nail polish remover)
To drill:
– tiny, tiny drill bits (#69, #67)
– lighted, head-mounted magnifier
Now, my understanding is that this is all for single-sided boards. You can apparently do double-sided boards (or a smaller 2-sided board can be gotten locally) by developing one side, then sticking in some registration pins, and working the other side. That sounds complicated.
A few personal reminders:
– If you’re doing through-hole components, put all your traces on the “bottom” layer. If you’re doing SMT components, put all your traces on the “top” layer
– The “top” layer needs to be *mirrored* before printing! The “bottom” layer must *not* be mirrored.
– Don’t add too much salt to the etchant.
– Try to keep everything at room temperature.
– Possibly fresh ingredients work better than stale ones.
– Can etchant be regenerated/reused?
– max density on the transparency
– only 2 layers of transparency
– make sure all the photoresist is gone