After drilling my board (and finally getting the right holes in the right order), I took it back home to etch. I wanted to print an etch-resist mask on my laser printer and iron it onto the board, and I wanted to do it using plain, glossy paper.
Pads and Holes
I had originally laid out the board using default sizes of 28 mil holes and 50 mil pads. Since I broke too many drill bits and ended up drilling with a 39 mil bit, that would have left me only 5.5 mils of copper surrounding each hole. That’s not nearly enough to solder to, much less enough to consistently align with an iron-on transfer. I went back into FreePCB and increased all the pad sizes to 70 mil, to give about 15 mil of copper around each hole.
Lesson: Keep a good supply of small drill bits.
Lesson: Don’t bump up to larger drill bits if you run out of small ones. Wait for more of the right size.
Lesson: Use larger pads than default when laying out a board for iron-on transfer.
Ironing in Pieces
Once I had increased the pad size, I exported the bottom copper layer as an image file. (FreePCB doesn’t seem to have a working Print function.) Because the bottom layer is drawn from the perspective of the top of the board, one would normally mirror it to view how it would look from the bottom. But the iron-on process reverses the image, so I needed to print it unmirrored.
Lesson: Export/print the bottom copper layer unmirrored when making iron-on transfers.
Because the 16.8″ length of my PCB was greater than the size of paper my printer accepts, and because the printer doesn’t seem to print at exactly accurate size, I used the GIMP to chop the image into three pieces, to iron on in sections. I split the image in areas with no holes and all horizontal traces so it’d be easy to line up, and I left about 1/4″ of overlap at each joint so I’d double up toner on the board rather than taking any chance of having a gap.
I ironed the two outer sections onto the board using our household iron set at 350°F and no steam, then soaked the board for half an hour to soften the paper and carefully peeled it off.
A lot of the traces came off with the paper. It didn’t feel like I had peeled the traces off the PCB — it felt like they had never adhered in the first place. Reinforcing that suspicion was how much effort it took to clean the traces off the board and start over — the ones still on the board were well stuck. I ended up using acetone to clean them off the board.
I printed another copy and ironed it on, using the maximum temperature on the iron and the Giles Corey method (more weight). This time I soaked the board for over an hour. Once again, large sections of traces peeled away.
Press ‘n’ Peel Blue and Successful Ironing
I have a few sheets of Press ‘n’ Peel Blue that Joel bought and encouraged me to test drive, and I’d had reasonably good luck the previous time I tried them, so I got them out. To avoid wasting an entire sheet of Blue, I printed the design onto a carrier sheet of plain paper, then cut appropriate-sized sections of Blue and masking-taped them to the paper over the printed areas and printed again.
You can see across the top where my printer went nuts printing PostScript source code on the first try, but I got the design printed onto the Blue on the second try. I ironed a section of Blue onto the freshly-scrubbed PCB board, and . . .
Excellent adhesion most places; terrible adhesion around the holes.
So what’s different about the holes? I think they’re dimpled from when the drill hits the copper and before it starts to bite. The surface is slightly lower, and there’s not enough pressure to adhere the Blue (or the plain-paper transfer) to the copper.
I needed something between the iron and the transfer to help distribute the pressure, even down into slight surface variations. I considered a sheet of felt, but I was afraid that it would be too soft to distribute the pressure well. I settled on a kitchen paper towel folded in half, and voila!
Good adhesion everywhere!
Lesson: Use a thin pad between the iron and the toner transfer to help distribute pressure through surface irregularities.
I had struggled for a couple of weeks to think of an appropriate etching tank for a board of this size. I needed a plastic or glass tank long enough for the board — but preferably also narrow, so as not to require a huge amount of etchant to cover the board. Finally I thought of a section of PVC pipe, capped and slit in half. $20 and a trip to the bandsaw later, I had an etching tank. (Two, actually.)
Lesson: It’s surprisingly difficult to fully seat a cap on 3″ PVC before the cement sets.
I set the tank over the bathroom sink with the ends on paper towels, in case my PVC glue joints weren’t watertight. Good thing, too, since they weren’t. By the end of the etch, one of the paper towels was stained, and I was particularly glad I’d used it.
Lesson: Use lots and lots of PVC cement to get watertight joints for odd configurations and applications.
At first, the etchant appeared to be removing quite a bit of copper, as evidenced by its increasing opacity. Half an hour in, though, not much was happening, even though I dropped by every few minutes to agitate the tank.
After a while, I got the bright idea to heat the etchant with my heat gun, and the etching really took off at that point, proceeding at a nearly visible pace.
Lesson: Heat the etchant to at least 100°F.
Once all the unwanted copper was etched away, I used a plastic fork to remove the board from the tank and rinsed it under lots of running water. Then I went to the kitchen sink to filter the used etchant into an empty plastic bottle for later reuse.
Lesson: Coffee filters pass about 1/4 cup of liquid etchant before becoming strangely impermeable.
Lesson: Impermeable coffee filters do not make particularly good funnels.
Lesson: Poor funnels tend to spill liquid over the edge into the white porcelain sink.
Lesson: Poor funnels tend to slip down inside the plastic bottle.
Lesson: The nooks at each end of capped PVC hold quite a bit of etchant that spills all over when you’re trying to pour it out.
Lesson: Start the etchant recovery process a couple of hours before your wife comes home so you have plenty of time to scrub the sink.
After cleaning the toner off the board with acetone, the traces really look nice. They have reasonably crisp edges and not too bad dropouts. This ended up being one of the nicer boards I’ve made by home etching.
Lesson: Try plain-paper toner transfer next time, using the paper towel pad trick.