Applying positive photo-resist - Page 1
Forum section: electronics / projects

author: akis
author_rank: Frequent Contributor
timestamp: December 13, 2013, 01:52:15 pm
content: Has anyone got any experience with applying positive photo resist on existing boards? For example trying to reuse a board that was exposed or developed badly, or simply making use of plain copper boards? I am thinking of sprays or maybe there is another way to transfer the image. But I do not like the laser printer - household iron approach because I have tried and could never get it right.

author: Zepnat
author_rank: Regular Contributor
timestamp: December 13, 2013, 08:46:12 pm
content: You mean something like this?http://www.ebay.co.uk/itm/10pcs-6x8in-Dry-Film-Photoresist-and-Developer-Kit-for-PCB-Dupont-Photo-Resist-/301022530107?pt=Vintage_Electronics_R2&hash=item4616574a3bAre links allowed here some forums don't allow them?I LOVE this stuff its brilliant you can use it for etching and then permanent solder resist usually apply onto a wet board and squeegee dry.

author: sleemanj
author_rank: Super Contributor
timestamp: December 13, 2013, 10:08:27 pm
content: For positive resists,"Positiv 20" spray-can is pretty much the defacto standard there I think. Clean board, spray on, allow to dry.There is also positive resist in a bottle on ebay,http://www.ebay.com.au/itm/Photoresist-for-PCB-Copper-Laminate-Photo-Resist-/291026822304?pt=LH_DefaultDomain_0&hash=item43c28ce4a0&_uhb=1Trouble with any of these methods is getting an even coating (and the drying time), which is why negative dry film may be a better choice IMHO, especially if you need to attain any decent resolution. It's also easier to ship :-)

author: Whales
author_rank: Super Contributor
timestamp: December 13, 2013, 11:32:15 pm
content: Quote from: Zepnat on December 13, 2013, 08:46:12 pmFrom the 'About us' page (link at the top of every forum page, between "Profile" and "My Messages":QuoteThis is very open forum

author: akis
author_rank: Frequent Contributor
timestamp: March 05, 2014, 03:38:49 pm
content: Quote from: sleemanj on December 13, 2013, 10:08:27 pmI got a bottle of photo resist. The instructions say to spray on a horizontally placed board, so I did, and yes, impossible to get an even coating, plus there are "spots" of concentrated coating here and there. I dried one in the 50C oven as instructed, exposed it and then could not develop it properly, I am not sure if it was because of the differences in coating thickness or because of me using the wrong developer?I stripped it all clean and sprayed it again, and again cannot possibly get an even coating, it is light in some places and dark in other places, maybe it does not really matter?Has anyone used one of these sprays with success? My spray is the Electrolube Positive Photoresist

author: M4trix
author_rank: Frequent Contributor
timestamp: March 05, 2014, 05:04:40 pm
content: Ditch the spraying business. You'll never get an even coating. Besides, you have to spray it in a dust free environment or otherwise tiny little threads of cloth fiber or dust will be trapped under the coating. Sprays also have a shelf life around one year if kept at room temperature. Did you check the manufacturing date? Sometimes an expired spray cause problems when coating and developing. Well, like sleemanj already mentioned, I recommend using an already presensitized positive photoresist printed circuit boards (less hassle) or using dry film photoresist and a laminator. Of course, it doesn't make sense to buy some expensive laminator for few boardsEdit: what kind of a developer you're using? I recommend, high grade caustic soda bought in a drug store and mixed, 10 grams with 1 liter destilled water, not tap water! The best developing temperature is around 20-21 °C

author: akis
author_rank: Frequent Contributor
timestamp: March 05, 2014, 05:17:43 pm
content: I have just bought 2 cans, 12 pounds each! I mean if they are crap why do they even sell them...Anyway here is a picture of my second attempt, you can see the patches of colour it is not uniform at all.

author: M4trix
author_rank: Frequent Contributor
timestamp: March 05, 2014, 05:33:58 pm
content: Hm, can't really say, the picture is blurry. Try to coat the board and leave it for some time in a dark place (cardboard box) to settle down and then dry it.

author: nessatse
author_rank: Regular Contributor
timestamp: March 05, 2014, 06:55:32 pm
content: Are you cleaning the boards properly before spraying? Cleaning is crucial, any oil or dirt and the resist will tend to bead up or form uneven spots. The way to check is to run some water over your board after cleaning and if it does not form a smooth film, it is not clean. Also allow a few extra millimeters all around as the resist tends to form a thicker layer around the edges. Another technique I have heard of but not personally tried, is to spin coat by sticking the board to e.g. a PC fan and give it a quick spin after spraying to spread the resist.

author: akis
author_rank: Frequent Contributor
timestamp: March 05, 2014, 07:12:13 pm
content: Actually this is not that bad, my second attempt after etching. Not as good as the pre-laminated boards but not bad either.

author: akis
author_rank: Frequent Contributor
timestamp: March 05, 2014, 10:47:12 pm
content: So for whoever cares I would like to describe the process as it seems to have sort-of-worked.1) take PCB, clean and dry thoroughly2) place PCB flat on some A4 paper and inside a deep tray (I used a ceramic baking tray)3) spray PCB outdoors from 20-30cm and hope you get a good uniform coating. I have done it twice and the coating was not uniform and I seriously doubt it can be made any better4) immediately cover the deep tray to stop light getting in (I used a precut sheet of aluminium foil over the tray)5) place in oven heated to 50C (this is very low heat as far as ovens go) - leave windows open in the kitchen for ventilation because it puts out a lot of fumes6) set the timer for 30 minutes and remove at 30 minutes7) expose with pre-warmed UV light for 8 minutesdevelop with normal positive developer. Temperature strictly between 20-22C, no more and no less. Hold PCB from the sides and shake vigorously in developer solution for a number of minutes, maybe as many as 4 minutes. Do not touch board with gloves while in solution in case you scrape the photo resist off.9) if successful the exposed copper will be very light green in colour, and the positive copper will be very dark green. There could be patches of different shades of green on the board but no spots of high contrast colour changes.10) rinse with water and etch in ammonium persulphate, place the tray inside another tray with boiling water (from the kettle) so as to keep the ammonium persulphate at around 45-50C. Every so often reboil the water of the outer tray in the kettle. The whole etching process takes ages especially as the "exposed" copper is still covered by a thin film of resist. In my case it took about 2 hours and had to boil the kettle 3-4 times.I hope the above helps someone make home-made boards. One possible use of this is when you are working with a pre-sensitised board, and something in the process fails, eg the UV exposure or the development. Instead of binning the board, you can use the above technique.

author: akis
author_rank: Frequent Contributor
timestamp: March 05, 2014, 10:54:05 pm
content: Quote from: M4trix on March 05, 2014, 05:04:40 pmI use the "universal developer" as sold from Mega:http://www.megauk.com/pcb_chemicals.php, "4007S005 Seno Universal Developer 500g makes 10 Ltrs - 20ltrs" at 20-22C.I think the trick is to apply very little coating with the spray. I already use pre-sensitised boards, but in this case I wanted to utilise plain FR4s or re-utilise boards that have gone wrong in exposing/development.

author: akis
author_rank: Frequent Contributor
timestamp: March 05, 2014, 10:57:32 pm
content: Quote from: nessatse on March 05, 2014, 06:55:32 pmOK that is a good idea - somehow spin the board so as to ensure a more uniform coating. I presume it will have to spin in multiple ways else the coating will simply be pushed to one side? Maybe I can use one of those cheapo desktop fans and attach the board on one of the blades, give it a spin and then turn it around and give it another spin

author: sleemanj
author_rank: Super Contributor
timestamp: March 06, 2014, 12:58:53 am
content: Quote from: akis on March 05, 2014, 10:57:32 pmYou spin the board flat at high speed and drop the coating in the center (or paint it on, whatever, depends on the viscosity I suppose), the coating is then spun out to the edges for a uniform coating.Here is a fellow who spin coats the (negative acting) UV ink type resist.See:http://hackaday.com/2013/03/09/hard-drive-centrifuge-for-sensitizing-copper-clad-boards/

author: akis
author_rank: Frequent Contributor
timestamp: March 06, 2014, 08:13:04 am
content: Fantastic. I have plenty of old hard disks I could use for the purpose. Just a bit of double sticky tape is all that's needed. Since I have managed to get average results without spinning, I hope that with spinning I might even be able to create intricate layouts on leftover pieces of PCB that I would throw away otherwise.