Wednesday, March 26, 2014

Dreaming of a CNC Something...

I decided I needed these stepper motors... It started when I saw the instructables page for
Easy-to-Build-Desk-Top-3-Axis-CNC-Milling-Machine nbsp;. I started dreaming of ways to make my own PCBs with SMD components. I am not a very patient guy when it come to hobbies and experiments, so mailing off eagle files and expecting the boards to find me on the boat just doesn't sound plausible.

I started the search for a CNC mill with a resolution for SMD parts, then I discovered the inkjet transparencies and photo-resist so there was no need for a mill.... but I keep forgetting that part.

I found the steppers in an electronics place accross on the mainland from Penang, in Butterworth, Destiny Electronics. Prices are not that good, but he has lots of stuff in stock! When I went for the photo sensitive board stock (booth single sided and double sided Kinsten- from Tiawan) I picked up a couple stepper motors too.
So this is the test setup.

Stepper is a 17PM-J319 fairly popular in the surplus market but very little in the way of documentation. I believe it is a 1.8 degree, 8 volt, 6 wire bi-polar (could wire it as uni-polar I think too)
Ardunio UNO clone, davilino from Seeed Studios
L298 motor driver fresh from China
And last a LM317 based 3 amp variable power supply also fresh from China surplus
These I picked up at the parts store here in Penang so no great price there either. Places like and e-bay have lots of these making me wish Paypal would ship to Malaysia. But then I'd be broke.

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