Since I last posted pictures of my Roomba controller board, I’ve created several other PCBs (not Roomba-related) and improved my design, toner transfer, etching, and soldering techniques. I was particularly nervous about putting the board into my robot, considering the amount of vibration and shock, so I decided to redo it.
First problem: the copper traces were much to fine. During the toner transfer (TT), a couple of these tiny wires were missing or incomplete. Even worse, once I tried to solder the complete wires, the heat of my soldering iron burned up the wire if held there too long. I’ve discovered it’s favorable to make wires as wide as possible, even at the expense of breaking clearance rules (i.e., I changed my DRC clearance setting from 10mil to 5mil). It’s much easier to break shorts between traces after toner transfer by scraping off the toner with a razor blade. Even if a short is overlooked before etching, it’s possible to break the connection by scratching away excess copper.
Secondly, I read online that Brother printers (which I have) are inadequate for TT because the toner has a much higher “melting point,” which is hard to reach with an ordinary electric iron. Apparently a portion of toner is made up of plastic, and the goal of TT is to melt it so it bonds to the copper board. The toner is stuck to both the board and the paper, so you remove the paper by soaking it in hot water. For this new board, I printed on an HP printer with much better TT results. Further, I read that glossy magazine covers or magazine pages can be used instead of expensive photo paper. I achieved a nearly flawless transfer using _free_ spring break vacation brochures I found posted around school.
Thirdly, the iron doesn’t always transfer toner evenly and there’s no way to know for sure before you remove the paper. My girlfriend spotted a $15(!) personal laminator for sale at Aldi. Hobbyists mention their laminators frequently online but I never planned to buy one because they commonly cost between $30 and $120… I cautiously assumed you get what you pay for, but a $15 loss would not ruin me. Fortunately, it works great and it’s faster and easier than an iron, giving far better results. As before, I place the layout images on freshly scrubbed and alcohol-cleaned copper board, but instead of ironing, I pass it through the laminator at least 12 times (probably overkill, but one pass seems insufficient… I’ll experiment a bit later), and then soak the board. It’s not actually laminating, so there’s no need to use any special plastic pouches–it’s just applying heat and pressure. To increase the pressure, I taped the board to a folded paper six sheets thick. The extra thickness sometimes causes a jam, but I simply push the board through.
Etching started as a very messy process, but I’ve worked out a clean and effective procedure. I fill up a plastic container with hot tap water. Then I put on rubber gloves, put the board into a reliable ziploc sandwich bag, and carefully pour a meager amount of ferric chloride acid into the bag. Seal the bag completely and place it in the hot water. Now the etching is contained, it’s being warmed, and it can be continuously agitated without touching the acid. If the water cools, I replace it as necessary until the board is etched. When I remove the board, I go out over the grass in my yard and carefully take the board out of the bag, then immediately pour water over the board and my gloved hand to rinse off the acid clinging to the board. Then I place the now-clean board directly into the water to dilute any remaining acid. Next, I liberally pour backing soda into the bag to neutralize the acid and hopefully leave the byproducts more basic than neutral. At this point, the residue seems to be “liquid rust” so it’s important not to spill it anywhere. I pull off my gloves and seal them in the bag, then throw out the bag. I’m not sure this is the best way to dispose of the byproducts, but I do try to use as little acid as possible, it should be less harmful once neutralized and it’s probably somewhat like throwing away rusty metal and copper. I’ve read some sites that suggest pouring the neutralized liquid down the drain and taking the precipitate to a waste management center, but I’m not sure this is much better.
Finally, I learned one more thing through error with this new Roomba board. When I removed the toner, I used nail polish remover as suggested on the web. As you can see in the pictures, the toner from the HP printer smudged all over the bare PCB, although it did clean off the copper traces. Using both acetone-based and acetone-free nail polish remover, the toner clung to the exposed board. Later I tried using my scrubber to mechanically remove the toner without any chemicals… this worked well, as is apparent in the upper-right portion of the board.
Regarding the tiny Roomba graphic in the corner of the board: I opened the official Roomba manual PDF in Inkscape and found a sketch of the Roomba itself as vector art. Before printing the layout, I scaled the graphic to fit the corner, placed it, and tiled the entire layout over the 8.5″x11″ canvas. It etched extremely well =)
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