Printed circuit board (PCB) etching

14 May 2010

Scope

This post assumes some knowledge of PCB etching methods and is an account of my experience with the toner transfer method. If this is new to you, you may have to get Googling on a few of things mentioned here. That said, I’ve linked to the sites I found most useful during the process.

Intro

Up until now, I’ve been hand wiring all the the circuits for my Electric Window series.

It’s a fairly straightforward process to build the circuit on a piece of protoboard, and for an Electric Window 3 circuit, it takes about two hours. The bottleneck of this process is preparing all the connecting wires (shown in white in the above photo) and ensuring all the interconnections are correct, ie, connecting the right components to the right pins of the microprocessor and the power and ground points.

While I find this process efficient/adequate for one-off circuits and prototypes, it gets monotonous when making multiple copies of the same circuit.

For our recent contribution to the Chinatown Remixed exhibition, Deb and I (working as The Latest Artists) envisioned a series of Electric Window 3 light boxes, displaying various animations along with text from Twitter via a wireless internet connection. I decied to use this as an opportunity to finally learn how to etch PCBs.

I’d previously been intimidated by the PCB etching process for similar reasons as Collin from Make:

 

There seemed to be a steep learning curve, as I’d yet to use electronic CAD software such as Eagle, and the actual etchning process seemed finicky and messy.

After watching Collin’s video, a few others on YouTube, and reading some on-line tutorials (found via Google), I decided to attempt etching with the toner transfer method, since I already have a laser printer and clothes iron.

[ More after the break . . . ]

Schematic capture and circuit layout

The first order of business was to electronically capture the schematic and create the board layout.

I knew I’d be creating a single-sided board for the sake of simplicity, so I created a rough layout by hand — based on my existing hand-drawn schematic (not shown) — to get an idea of where the components should best be and which microprocessor pins were best to avoid crossing wires and/or requring jumpers:

As I said, rough, but it was enough to get going.

While there are plenty of software alternatives, free and otherwise, for schematic capture and PCB layout, I decided to go with Eagle because a) it’s free to use for 3″ x 4″ boards, and b) there’s an abundance of resources/help out there for it.

I used this helpful tutorial and parts library from Sparkfun to get going with the program, and soon enough I had my first CAD schematic:

I then moved onto the PCB layout portion and used the auto-route feature to start the intimidating process of trace layout. With a bit of additional tweaking by hand, I had a layout I was happy with. I originally designed the board to be 3″ x 1.5″ but that resulted in tight traces and traces running really close to microprocessor pin connections. Being unsure of the tolerances/accuracy of the etching process to come, I decided to give the circuit more margin and ended up with a 3″ x 2″ board:

Toner transfer

Although I’d repeatedly read that glossy or photo paper was best to print the circuit for transfer onto the copper board, I didn’t have any of these fancy papers on hand, and didn’t want to make a trip to the store if I didn’t have to. So I decided to try it with regular paper.

I readied my otherwise unused clothses iron:

Naturally I didn’t turn it on until resting it on its back, and I wish more turorials were explicit about whether or not you should have water in there or not. The answer is no.

I cut my copper board to size and used a sponge to clean it with isopropyl alchohol (a 70% solution from the drug store).

I grabbed a scrap piece of wood to do the ironing on:

and it didn’t work. After several attempts to iron the circuit pattern onto the pattern from regular paper, it just wouldn’t stick. Whenever I’d move the iron around, the paper would slip and the result would be a smugged circuit pattern on the copper board.

I really didn’t want to make a run to the store for special paper, so I consulted Google instead. I found this great article explaining how you can use glossy magazine paper, the principle being that the laser toner doesn’t stick very well to it, thus making it easy to transfer onto the copper — the same principle behind using special glossy or photo paper I’d previously read about.

I set to work with the cheaper and more easily available magazine paper I already had around, trimming it to fit my printer before making a few prints of the circuit:

Back to the ironing board:

and it stuck! The paper stuck pretty much immediatly after applying heat with the iron, and I was able to move the iron around and evenly heat the entire circuit. Soon I could see the traces through the paper:

I let the board cool a bit, then put it in a sink of warm water. The magazine paper came off very easily, and the traces didn’t budge or scratch away. I’d read that you need to let the paper soak for 5-10 minutes before it’ll come off the board, but I found with the magainze paper it only needed to soak for 2 or 3.

I’d also read that you need to press really hard while ironing, and iron for 5 or more minutes. I’m not sure so much force and time are required with the magazine paper technique, as I ended up with a few smudged traces:

Specifically, there were a few pads that had shorted together on the transfer:

Using an X-Acto knife, I was easily able to scrape spaces between the pads before etching.

Etching

Now the fun part: etching with chemicals!

I decided to use ferric chloride since I didn’t want to mix my own etchant with ammonium persulphate and water, or the like.

I bought some handy etching trays with corner spouts at the local electronics store, and with some chemical resistant gloves, got set up in the utility sink:

Rubber gloves on from this point: I filled one tray with water for rinsing, and I poured about a centimetre (0.4″) of etchant into the other one. I put the circuit board, traces up (more about this later), in the etchant, and let it soak for about 15 minutes. Not much seemed to be happening, so I decided to try the scrubing technique described in Sonodrome’s great tutorial video:

 

It worked like a charm, the exposed copper started to come off the board, while the circuit traces held up well, even against fairly rigorous scrubbing. Caution needed to be taken at this step to avoid splashing etchant around, though!

Here’s the etched board with the toner still in tact. I think it looks silver/white from remaining fibres of the magazine paper:

Next I used isopropyl alchohol and a sponge (seperate from the one I used to scrub with etchant) to clean off the toner. Some of the pads had smugged edges, faithful to the smudged transfer image, but the board had no short or open circuits:

Drilling and assembly

I trimmed to board to length, and drilled the holes. Since I aready have a Dremel tool, I bought the Dremel drill press attachment and a package of small bits. I also bought the Dremel keyless chuck to accomodate the smaller bits.

Despite not being carbide bits, as recommended by many tutorials for drilling PCBs, they worked fine, and didn’t break or noticeably dull.

Lastly, I populated and soldered the board:

Not the most photogenic, but totally functional :) I wanted to tin the board, but the local shop where I got my supplies didn’t have any tinning materials.

Initial impressions

So, did I save any time!?

Let’s compare. I takes me 2 hours to make this circuit from protoboard. I’m familiar enough with the circuit that I don’t need to keep refering to the schematic, the real time taking tedium of this method is cutting all the connecting wires.

To learn Eagle and create the PCB layout, I spent about 6 hours. Since this was my first time through the process, I’m not sure how much of this time I should count against the protoboard method.

Etching and drilling, including the necessary clean-up of nasty chemicals was about an hour the first time through. Again, part of a learning process.

Soldering the circuit took only around fifteen minutes, though.

For one circuit board, this definately didn’t save any time.

Rinse and repeat

I needed 3 more boards for the Chinatown Remixed installation, so I did all these in a second etching session.

I made a few modifications to the layout, to run the traces clearer of the microprocessor pads, and to move the programming header (bottom centre) away from the LED screen connector (upper left) to make it easier to program the board when attached to the LED screen (glad I didn’t make all 4 like this!).

Here’s the 3 boards after ironing the toner transfer to the copper:

I did the 3 boards seperately instead of together on one larger boards to experiment with the timing of various steps of the process. This time around, some of my traces broke, so I repaired these with a black Sharpie marker.

All the boards etched successfully, including the portions repaired by the Sharpie. Next, I drilled them: all the small holes first, followed by large holes; I did this in case I accidentally drilled a large hole with a small bit, allowing me to enlarge it if necessary.

Next impressions

So, did I save any time across multiple boards?

The answer is yes! Although, for this circuit I only saved about 15 minutes per board compared to the protoboarding method (not including the time spent learning Eagle). However, I assume there would be more time savings with more complex boards.

That said, I do feel that I saved labour. Soldering each circuit together on a protoboard, making sure to correctly wire each connection requires considerable mental attention, in contrast to the “dumb labour” required to etch and drill the boards, followed by the comparatively easy task of populating the boards without having to handwire all the interconnections.

Another advantage to the PCB method was that Deb and I could work in parallel to solder the boards together.

I also imagine I’ll shave some time off as I become more proficient with the process, so I’ll definately continue to etch my own PCBs!

Some tips

I would highty recommend the use of glossy magazine paper for printing your circuit designs on: it’s cheap/free; you probably have some already; and it comes off in water very easily and quickly.

If you use glossy magazine paper, I found that a lot of ironing pressure wasn’t required, just be firm. I also found it only took about 3-4 minutes (for a 3″ x 2″ board) to do the transfer. More time can actually start to smudge the transfer, as discussed above. You’ll have to experient with this.

High speed steel (HSS) bits seem to work fine. I ordered some additional bits from Digikey for drilling the PCBs, since I’ve read so many times that anything other than carbide bits will dull very quickly when drilling PCBs. I’ve found the HSS bits to work well so far. Bare in mind I’ve only done about a half dozen boards. However, I have yet to break a bit and the bits are much cheaper than carbide. I just bought a bunch of bits to compensate for any dulling. For the price of one carbide bit, I was able to buy 3 HSS bits.

If you are not agitating (ie, constantly moving) the board in etchant, place the board traces up. I’ve heard that putting the traces face down enlists gravity to help remove the etched copper, but I found the ridges on the bottom of my etching tray started to leave lines on my circuit when I didn’t agitate.

The finished product

If you’ve read this far, you’re probably curious about the final product these circuit boards belong to!

They’re used to drive LED displays from Sure Electronics, for a series of Electric Window 3 light boxes in a public art installation.

The following picture shows 3 of the Sure displays cascaded, and the driver circuit with an XBee radio attached:

The XBee is used to wirelessly query a WiFi-enabled master controller (not shown) for strings of text to display, from Twitter; this post has a clear explanation of the final installation.

The black, rectangular connector on the bottom-left of the driver circuit mates directly with the LED display, as shown below:

The dangly bit on the bottom left is the power connector.

Next these units were installed in custom-made, acrylic enclosures:

Here’s some “still on the bench” shots of the finished boxes:

The deadline for this project didn’t permit time for better photos of the final boxes before installation on-site, so stay tuned for more pics and videos.

Thanks for checkin’ all this out, all the best with your circuit etching endeavors!

Comments (9) | Tags: , , , , , | More: Blog

9 Responses to “Printed circuit board (PCB) etching”

  1. Alex Mizell says:

    Awesome documentation, dude! I am definitely going to try the magazine transfer method when I do some boards with larger traces. I’m going to stick with the presensitized boards for now because my art has a lot of fairly close traces to fit in the board size I wanted and I’m pretty sure that I’d have a lot of shorts.

    I think I will also try the HSS bits, cause I’m breaking a ton of bits, at least once or twice per board. I’m using carbide resharps, which at least are cheap. They punch right through the board when you line it up just right, but if they catch on the copper they tend to break off right in the hole which is a major pain. If you’re getting six boards without breaking a bit then I need to pick up on that.

    I’m finding that my production time goes down with every run, but I’m still plagued with regular failures. The complicated driver boards have a tendency to etch faster on one side than the other, with some traces etched too thing and others still shorted after hours in the bath with constant agitation. Today I picked up an “economy” etching tank from Fry’s with an air pump to constantly agitate the etchant. I’m hoping this will improve my yield.

    Did you say you were working on something with the TLC5940? How’s that going?

  2. Alex Massaad says:

    I’ve been making PCBs like this since I was in high school, I was surprised at how easy it is to achieve pro results!

  3. ao says:

    @Alex Mizell — I haven’t had any board failures w/ the magazine method, but that may be a result of using thicker traces. As I get more experienced with the process I’m sure I’ll start to test the limits of trace width and spacing, which I’ll eventually have to compare to the photo-etching method.

    Strange that you’ve had to soak your boards for so long, even w/ agitation. Is the copper clad on the boards you’re using really thick?

    As for drilling, def. try the HSS bits, they even have a bit of “give” in that they’ll bend a wee bit to center in the pad. Carbide bits sound frustrating.

    The TLC5940 project is coming up next-ish!

  4. ao says:

    @Alex Massaad — I’m really happy with the results and glad I’ve finally started to etch!

  5. Alex Mizell says:

    Yeah I had another driver board etch badly the other day even with the tank and bubbles, so it’s definitely something I’m doing wrong earlier in the process. Since the traces show up fine I’m guessing they are exposed properly but perhaps I’m not developing the boards long enough. Like maybe the etch resist (though invisible) is still too thick in some of the exposed areas, so it takes too long to eat that off before it gets to the copper. I dunno but I have to figure it out soon. Weird that it’s only the driver boards with lots of parallel traces that have this problem.

    I just found out that it’s possible to get a spray-on photoresist coating. If it’s much cheaper than getting the presensitized boards I might go that way. I will try to do at least one piece soon with the magazine method and see how it goes. Sure would be nice to get boards for half the price I’m paying now.

    Did you happen to look at the proposal? : )

    http://alexmizell.net/The%20Magnificent%20Mood%20Machine%20-%20Proposal%20for%20Transformus%20Creativity%20Grant.pdf

  6. ao says:

    @Alex — good luck w/ the grant! Looks good, nice photos! Without having seen the requirements for the grant, it’s hard for me to comment in more detail. Fingers crossed :)

    You may have a point about not developing/exposing the boards for long enough. Unfortunately I haven’t tried the photoresist method yet, so I can’t comment too much.

    Let me know how the spray-on version works, sounds interesting.

  7. Alex Mizell says:

    I gather that the spray-on photoresist used to work ok in the olden days but the formula was a carcinogen, so they changed the formulas and now it doesn’t work as well. Plus you have to dry the coated boards in a dust-free zone, which I’m pretty sure my house doesn’t have.

    A big part of my etching quality control issue was insufficient development. Now I leave em in there a little longer than I think I should and they come out near perfect. I still have a few broken traces that I think have to do with my transparencies not being completely opaque, but they are relatively easy to fix.

  8. Monty says:

    This is a superb article. I guess for more complex boards it is better to contact manufacturers like http://www.protoexpress.com.

  9. penny says:

    We are a pcb manufacture,wish for the cooperation.

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