Wednesday, March 26, 2014

Inkjet and Positive Resist Photo Artwork

I just discovered that it is possible to print on transparency film with an inkjet and use photo-sensitive pre-sensitized boards for custom PCBs. This is my first try.

I begged a few scraps from the local parts store here in Penang, Malaysia. I searched all over for inkjet transparencies then discovered I have a pack (20 sheets) that are at least 5 years old already on the boat! So the only thing left is the developer. It should be about 6% sodium hydroxide, or caustic soda. After a few failed attempts to purchase it in grocery stores and pharmacies, I headed for "The Chemical Man". Sure enough he had it by the kilo. since I only wanted enough for a few trial boards he finally sold me 15 grams for RM1, about $0.32 USD!

The board is much better than the focus of my phone camera

A better photo with the artwork

Now to the process. I wasn't sure my new HP inkjet printed dark enough, so I printed two copied of the board. This LEDTester was the quickest board to get ready that I had. I was in a hurry to find out if it really worked. I pulled one of the glass plates from the oil lamp in the fore-peak and taped the printout down. Then taped the second copy directly over it. Next came the pre-sensitized PCB trimmed to fit so as not to waste much if it did work. I exposed it in the sun, 5:00PM a little cloudy. As a test I covered thirds at a time and exposed 1 minute each so the result was one end got 3 minutes, the middle 2 and the other end only 1 minute exposure. I could hardly tell the difference, but maybe the 2 minute was the best.

Next to the developer. I forgot to mix it ahead, so I covered the exposed board and got to mixing. Since I bought 15 grams and the suggested ratio is 6 grams in a litre of water (6% solution) I estimated a little less than one 6th of what I had and put in an old pickle jar about 700ml. I stirred and stirred! There must be NO particles or crystals of caustic soda floating around or it will disolve everything off the board they say.

OK in goes the board. 2 minutes and it was done. Looked great. I had a little muriatic acid (HCL from the grocery store in the Philippines) and hydrogen peroxide left from last year so I went for it as before. 2 parts peroxide (3%) and add one part HCL(23%). into a zip-lock baggie with the board and another 3 minutes or so I had a board!!

I re-exposed the whole board to the sun for 2 more minutes and back in the caustic soda to remove the layer protecting the traces. One wonders if it would work to print a solder mask and only remove the stuff from the pads and leave the rest of the photo resist to protect the traces?

Oh, by the way the board is a replacement for the earlier post my-first-etched-pcb-for-smd

No comments:

Post a Comment