SPI Backpack PCB for Liquid Crystal Displays: Part 2 Print to Finished PCB
In this part I’m showing a method for transferring the laser printouts to double sided PC board material.
Double sided printed circuits must have the top and bottom masks perfectly aligned. I begin by using a sharp scribe to pierce the exact center of each corner hole on both the bottom and top layer printouts.
Next I insert one of these tiny nails (3/4″ #18 gauge) into each corner of the bottom mask. I could use sewing pins but nails have flat heads which keeps them standing up during the next steps.
The next photo shows the bottom layer printout with alignment nails inserted into the mask’s corner holes. If I was making a board without convenient holes, I would add holes to the Eagle drawing outside of the board area and just trim them off after completing alignment.
Now I take the top layer mask and carefully slip it face down onto the alignment nails, smooth it out and tape the top edge. The tape maintains the mask alignment from here on.
Next I removed the four nails. In this photo you can see how the two masks align face to face. Holding the aligned sheets up to a strong light should show any problems.
I’ve scrubbed the blank PC board material with fine steel wool and wiped it down with lacquer thinner. It cannot have dirt or fingerprints. Now that my masks are aligned I can position the blank over the bottom printout. I use bits of Kapton tape, though the board should stay put without tape if the paper is not moved before applying the Iron. I cut the blank about a quarter inch oversize on all sides to make this step easier, the excess will be trimmed off after etching.
It’s time to heat up the Iron. I use the highest setting, which I think is still a little cool. I let it sit on the mask/board/mask sandwich for two minutes to make sure the copper is hot enough to melt the toner. There’s a slab of MDF underneath the paper so my workbench doesn’t get cooked.
I remove the Iron and immediately use a small rubber roller (brayer) to press the mask firmly onto the copper. Not sure this step is necessary but I think it helps even out the transfer when the Iron face is not perfectly flat, or the Iron temperature may not be even across the face. When the board has cooled a bit, turn it over, repeat the heating and rolling process on the other side.
When the board has cooled enough that you can handle it, trim away most of the paper but leave a little bit around each edge. Be careful at this stage, pulling the paper off will ruin the toner transfer.
Next the paper has to be carefully removed from the board in such a way that all the toner is left on the copper. I put the sandwich into a tub of warm water with a small amount of dish soap. In a half hour or so, the paper will begin to disintegrate. Just rub the paper gently with fingers, center towards the outside, and it sloughs off easily. If a bit is stubborn, give it more soak time. You must remove all the paper from the areas that will be etched. Paper left on the masked areas is OK.
This is the transferred board dried and ready for inspection. The toner is somewhat delicate so you can’t touch it. I use a visor magnifier and check all traces for complete coverage. Repairs can be made with a fine point Sharpie pen though I believe a typewriter correction pen would work better. Sharpie ink is too thin.
I painted the quarter inch of excess copper around the board edges with typewriter correction fluid and it stood up well to the etch bath. Also have to watch for smudges in the mask. My old laser printer has a tendency to drop toner in random places, I have to scrape these off with the scribe.
Finally into the etch bath. I’m using the aerated cupric chloride method as learned from Jim Williams. I have a good size aquarium pump feeding four air stones at the bottom of a 4 inch square etch tank. There’s about a pint and a half of Muriatic acid in there. I pull the board out every minute to check progress. In this photo it’s about 2/3 done on one side. At that point I turned the board over so the other face got the bubbles. Took about 15 minutes to do both sides.
It can be difficult to see when all the copper is removed. You have to inspect both sides carefully. Here I placed the board fresh from etching onto a florescent light so I can see through it. The alignment is looking pretty good as evidenced by light through the pad holes, and I’m not seeing any shadows where the copper was removed.
All that is left now is to clean off the toner with Lacquer thinner, trim it to final size with my big garage sale tin snips, and drill holes with a #64 bit in a Dremel tool. This composite photo shows the top and bottom after drilling. You can see the pad alignment around the drilled holes is very good.