POSITIV 20 photo-resist - out of date?

John_1981 wrote:Ivan, 

I did not find temperature or concentration to be critical, if anything this is probably too gentle a developer as it took around 20 mins with occassional rubbing with a sponge to remove unwanted etch resist. I dare say that sodium hydroxide would work as well, but the sodium carbonate is less aggressive and I don't mind getting my hands in it.

Hi, John,
these are perfect results!

I found a recipe for processing dry resist film. It used Na2CO3 (sodium carbonate) for development and NaOH (sodium hydroxide) to remove the remaining resist after etching. The concentrations were given very exactly and the temperatures within +-2 degC. The recipe is obviously over-tight.

What light (type and intensity/power) do you use when you cover the PCB with the film and when you develop the pattern? I expect full darkness is not necessary.

VBR from Ivan


VBR Ivan

Ivan, 

I used sodium carbonate, sold as 'washing soda' here in the UK. From memory, i used something like a teaspoon in 200ml of warm (30-40 deg.C) water. I did not find temperature or concentration to be critical, if anything this is probably too gentle a developer as it took around 20 mins with occassional rubbing with a sponge to remove unwanted etch resist. I dare say that sodium hydroxide would work as well, but the sodium carbonate is less aggressive and I don't mind getting my hands in it. I didn't have any problems with over-development, but did under develop some boards. As soon as i dipped it in the ferric chloride it was obvious if the board was not developed enough. 
The etch resist turns soapy after developing and does need light rubbing with a sponge to completely remove it. With the dry film, you do get a good contrast between the exposed and non-exposed parts of the board as it turns from pale to dark blue once exposed to light so you can tell if you have a good exposure before developing.

Hi John,
what developing solution did you use? Is the temperature of solution critical?

VBR from Ivan

Hello John,
Thank you very much for the information. I must confess that I have never heard of the dry-film photo-resist. I have been doing a bit of googling following your reply and there are loads of links and interesting descriptions. I may well invest in a roll and see how it performs (after I have exhausted my stocks of POSITIV-20).

Noted your comment about the image reversal.

Do you know if it will also work for silk-screen printing, for example T-shirts? or could there be issues getting it to stick to the screen material?

I once used the POSITIVE-20 for silk-screening on materials and found it to work very well, but was not so durable. After printing 15 or so it began to break up, even though I was gentle with it.

Thank you again for the information. Very best regards from Harry - SM0VPO

Last time I made a PCB about 12 months ago I used the dry film photoresist that is available in rolls from the usual Chinese suppliers. It works well and is very cheap. The only thing that caught me out was that it is negative type rather than the positive pre-sensitised boards I used to use. I used a laminator after applying the film to the board. If you get any air bubbles trapped underneath, the film is cheap enough to just strip off with sodium hydroxide and start again. I found a 150W incandescent light bulb worked better than the UV lamp I previously used, and for the transparency I use ordinary copier paper soaked in baby oil to increase the transparency, with the printed side against the copper clad board to avoid blurring. I found this system to be less fussy on factors such as exposure time and developer strength, etc. 
I used an old photo frame to hold the transparency against the PCB before exposing. 
Big Clive has a few videos on YouTube using this system.

...board etched ok.

Slightly under-developed at one corner, otherwise 100%. Not bad for 17-years out-of-date spray 

Hi Ivan,
Thank you, I will have posted a picture of the makeshift version as soon as I can here ... I will also be knocking together a 3D printed version - basically three bits of wood with 3D flexible-joints and pulleys.

PS - Here is is "Spinning Jenny" ...



To the right is the fan, the shaft is 2mm Dia piano wire antenna for VHF, the red and white bits on the rod are plastic spacers. The blue bits are makeshift washers to stop the rubber bands from wandering on the shaft. The grey-ish thing to the left of the tin is one leg of my workbench upper shelf, and the skewiff white box-thingy under the fan is a cardboard box with a cell-phone battery. The table leg and the battery box stop the tin from wandering. 

BR Harry

Hi Harry,
congrats to your success. The lacquer is a solution and some separation probably took place due to the frost or long time storage. Intense mixing made the components to dissolve again.

What about the sensitivity to the UV light? Is it O.K.?

I am not sure how you "super lacquer mixer" looks like. Can you attach a photo, please?


VBR from Ivan

Good morning gentlemen gentlepeople,
I had a bit of fun this weekend and thought I would share my findings with you all (both of you )

I believe I have finished the design of my little transmitting converter from 144MHz to 14MHz(10MHz to 19.999MHz - no gaps) for my FT-290R on the HF bands. The PCB is laid out and now it is time to print it and transfer all the rats-nest onto a printed circuit board. I added a few little finesses where there are three holes for ceramic capacitors. I wanted 2.54mm leg spacing, but I have 1000s of capacitors of various sizes that I bought in bulk, cheap. Some of them have 5.09mm leg spacing. This makes them a pain to fit on the board, especially if the board is somewhat crowded. Now I have two choices for every ceramic cap    All I need do now is make the PCB (and make any corrections) before I publish the article.

I came to spray the board with photo-resist. Over the years I have done some work in Spain, some in Lunda and some PCB work in Vänortsringen. I found that I had three tins of the spray. The "new" spray expired in 2018, but it works fine. The one that expired in 2015 is also 1/2 full, but I also had one that expired in 2005. Now this stuff is only an aerosol paint, and I thought that paint cannot "go off" that easily, so I tried using the one that expired 17 years ago. Only god knows when I originally bought it, probably in the late 90's?

It sprayed, sort of ok, but there were 100s small lumps, or blobs of lacquer about 1mm diameter. I shook the can for 30 minutes and tried again. Same thing, but the little clumps were a bit smaller. I can't spend SEK 350 ($40) to buy a new tin if I still have three of them, so I tried an experiment:

I glued a wooden block (2cm x 2cm x 2cm) to the middle of a computer fan, then fired up the fan to mark the dead-center. I drilled it and stuffed in the hole a bit of stainless-steel mobile antenna rod. On the rod I added two elastic bands and suspended the aerosol on the two elastic bands - horizontal. I ran the 12V fan at 7V, so that it rotated the aerosol canister continuously from Friday evening until yesterday - a total of about 48 hours.

When I sprayed the board yesterday evening it was 100% perfect. Maybe 48 hours was a bit of overkill, but the result was exactly as expected. Now I can use up all the old POSITIV 20 I have, even if the expiry date was 17 years ago. For the past two years this particular cannister was stored in a packing box, in the outside shed, where the winter temperature has been down to -25°C during two winters. To be honest I was prepared to order a fresh $40 canister of the stuff but I don't need to do that.

The moral here is that the expiry date on a tin of POSITIV 20 photo-resist lacquer is not necessarily written in concrete. You can probably "renew" it. Maj-Lis had a couple of funny comments about an old man getting the most out of old materials. Today we live in a "throw and buy" world and I am sure that there is a ridiculously large amount of good or recoverable stuff is thrown away needlessly.

I though I would just share this little experience with you all and hope that it can save you some of your well earned money.

Very best regards from Harry - SM0VPO