My (limited) experiences with diy UV pcb methods.  - Page 1
Forum section: electronics / general

author: Fflint
author_rank: Frequent Contributor
timestamp: December 27, 2020, 01:02:31 pm
content: Hi,I'm starting this thread to share my experiences with commonly available UV pcb making methods and to compare them with others who use them. I'm getting end results that are good, but it is quite time consuming so I'm looking for improvements to my process.Having discounted pre-sensitised boards as too expensive for my purposes I first I tried using positive photoresist spray (called Positiv20) then negative dry film photoresist from China. I'll start my description with elements common to both methods.ArtworkI only have a Brother HL-1210W so this is what I use set to 1200dpi. I'm printing on transparencies marked "For laser and xero use LX". I believe they are acetate.One thing I found out fairly quickly is that the transparent film has a slippery highly reflective side and a mat side. If it is put in to the printer's tray wrong side up the printer will not be able to feed it in. The roller that picks up paper will slip and the sheet will not move. If this happens one can turn the transparency to the other side and continue printing.Second, the blackness as printed is unsuitable. Under a microscope lots of holes in the cover are visible (100x magnification). Other people reported using a dry erase marker to improve it. I tried that and it works, but I found carefully covering the whole printout with a fairly thin dry erase marker than wiping tedious. So I tried using pigment powder called lamp black or carbon black (it is pretty much soot in a jar). I put a little of the powder o the printout, rub it in with small cotton bud, then blow away excess and clean the printout by rubbing it with clean cotton disks. This process takes me a minute or two and the results are great.Exposure boxI'm using a cheap device used in beauty salons for curing nail lacquers. It has 4 9W 365nm tubes. I put the pcb on the bottom tray (around 6-7cm from the tubes) and I cover the artwork with 6mm clear PCV.positiv20I tried few methods of spraying including gluing the pcb to a spinning pc fan. The spinning method results with coat that is way too thin or if spinning at less rpm the result is uneven. I found only one method that works well. Spraying manually in a zig-zag pattern as per the instructions, but with very important note. One has to start spraying away from the board, then using constant pressure on the spraying nozzle start the "zig-zag" pattern with maybe 50% overlap and only a single pass. This is very important and requires many attempts before the coat will be really even and thick enough. Then I put the board into a small toaster oven preheated to 70degC for an hour. If making dual side pcb. After first side dries the second side can be coated and cured in the oven for an hour again. This is the time consuming part of the process I really would love to eliminate somehow. Does anyone have any tips how to dry/cure it quicker. The instructions claim an hour is required for good adhesion so I followed that. I'm curious if others got acceptable results with shorter times.One tip I can offer here is that if you're having trouble with this part, make your board bigger as I was having most difficulties with the edges. Also try to have the pcb as level as possible.I found exposing for between 5min30 and 6 minutes works best for me. I develop with 10g of NaOH per liter of water for 30-45s (14degC).Then etch in sodium persulfate in 40degC. The best that can be done with this method (for me) is 0.2 trace and 0.2 space although the printer seems to be the limiting factor, not the uv process.Dry filmI cut the dry film off the roll with scissors. I removed one of protective layers with cello tape. Once one protective layer is removed the film ceizes to roll itself into a small tube and becomes flat. It can be lied onto a pcb then (cleaned and scrubbed as before). Excess can be cut off with scissors. Then I sent it through a cheap laminator with its plastic enclosure open. The laminator has to be open to avoid snagging the film on internal plastic before it hits the rollers. The roller enclosure is hot so take care if anyone does it the same way. I send it through the rollers 5-6 times then leave to cool down.Then I expose it for40 minutes per sidewith negative artwork. This is my main gripe with dry film. 40 minutes seems ridiculously long, but this is required or the thinnest traces do not harden. Also negative artwork uses many times more laser printer toner than negative (I guess it depends on your board).I then develop it in potassium carbonate 3% by weight in water at 20degC for 3min 30s. Again best results are 0.2mm trace and space. However, if I could print with higher dpi I think I could do better with even longer exposure time.Again etch in sodium persulfate.So in summar both processes take same amount of time: 2-3h. Dry film is easier to use, is cleaner and doesn't smell while positive spray is cheaper (regarding printer toner use - can of spray is quite expensive, but lasts a long time).Any tips to improve the efficiency of the process?I attach a view of a test board I used. Both methods produce similar results. Trace Width and spaces are:first row:0.1/0.05(bad),0.1/01(bad),0.2/0.2(ok),0.3/0.3,0.4/0.4Then one per row:0.5/0.5,0.8/0.8,1.2/1.2Edit: I forgot to mention all my tests were at pretty cool ambient temperature (around 5-7degC). Maybe this was the reason for long exposure times of the dry film? Does anyone know if it is temperature dependent?

author: jmelson
author_rank: Super Contributor
timestamp: December 27, 2020, 07:25:43 pm
content: I have been using dry film for about 30 years. I now get it from Think&Tinker. I clean the PC board with fine 600+ grit sandpaper, then wash with water and dry carefully, so as not to get water spots, dust or fingerprints on it. I use a Kepro laminator, but the hobby laminators are probably about as good.I make the master artwork with a laser photoplotter that I built. It gives much more accurate and high contrast images that you can get with a printer, but this is probably more involved than most people want to deal with. It exposed litho film made for newspaper typesetting.I have an exposure frame that has 4 24" fluorescent black light bulbs. These are 405 nm lamps, NOT 365 nm. I have both wavelengths, and foundthe 405 nm works much better for the dry film. The inside of the box is lined with crinkled aluminum foil as a reflector. The exposure is ONE MINUTE! You get greater hardening of the resist with more exposure, but you also get "bleed" of the light through the black part of the photomaster, and that results in partial hardening of all the resist, including what is supposed to develop away. It also makes the edges of lines gradual instead of sharp. Expose the dry film with the thick PVC layer removed but the very thin, crinkly acetate layer still attached. After exposure, allow the cross-linking of the polymer to continue in darkness for 10-15 minutes BEFORE peeling the acetate and devloping.I develop in warm sodium carbonate and rub the PC board with my fingers to fully dissolve the resist that is supposed to be dissolved. Development is no more than 3 minutes. You can tell by the feel when all the soft (unexposed) resist has dissolved.I have a Plexiglas exposure frame to hold the artwork tight against the PC board with vacuum.Jon

author: sleemanj
author_rank: Super Contributor
timestamp: December 27, 2020, 07:43:57 pm
content: In all the UV led boxes I have built, dry film exposure has never been more than a couple minutes befire developing, and then a few minutes after developing (in sodium carbonate) to harden. My current box is 22 seconds before developing from memory although I have not used it for a while.

author: ace1903
author_rank: Regular Contributor
timestamp: December 27, 2020, 08:52:34 pm
content: With standard fluorescent white tube and 3mm glass to cover film over PCB it takes 6min for me.Long times sometimes are related to too high temperature during the curing process.This creates a thin layer that acts as a filter and doesn't allow photopolymer under it to receive UV radiation.70 degrees is to evaporate solvent quickly not to bake the photopolymer film.Toaster oven thermostat at 70 degrees is totally unprecise.

author: Fflint
author_rank: Frequent Contributor
timestamp: December 28, 2020, 01:43:58 am
content: Quote from: jmelson on December 27, 2020, 07:25:43 pmQuote from: sleemanj on December 27, 2020, 07:43:57 pmCould you let me know, what is the thinnest trace width/space between traces you normally do, please?When I was testing the dry film exposure time I started with 2 minutes and went up from there. At 3-4 minutes tracks 0.8mm (30mil) were showing up (but quite soft) so if this was all I needed I could stop there and once the background dissolved give them more time to harden. However, 0.2mm (8mil) tracks appeared only around 30 minute mark and got firm enough not to lift during etching at 40 min. I wonder if those long exposure times are a result of both film's sensitivity being closer to 405nm as suggested by jmelson's post above and me testing for the limits of resolution I can get.Quote from: ace1903 on December 27, 2020, 08:52:34 pmI only used a toaster oven for the photoresist spray, not for the dry film(for the spray I used a digital thermometer to verify temps-once I found the correct setting and the oven warmed up it stayed around 70-80 degC) . The film was laminated with a cheap laminator. The laminator has no temperature controls I estimate the rollers to be 70-100 deg tops. Perhaps I'll try sending it through the laminator less number of times tomorrow.Quote from: jmelson on December 27, 2020, 07:25:43 pmThis is interesting, I didn't notice my film's protective layers to be any different. Both (inside and outside) appear the same thickness (I haven't measured them, this is judged how easy they are to tear etc). You mentioned a pvc layer and a acetate layer. Perhaps I'm removing the wrong one if this is the same film. I have few pieces of scrap film leftover. I'll try measuring the thickness of the whole thing and the protective layers on their own tomorrow. I hope a microcator(1um resolution) will be an adequate tool for the job.

author: sleemanj
author_rank: Super Contributor
timestamp: December 28, 2020, 01:54:03 am
content: Quote from: Fflint on December 28, 2020, 01:43:58 amI don't go below 8 mil, I use a default of 12 milLong ago I wrote this documenthttps://sparks.gogo.co.nz/dry-film-tips.pdf

author: Fflint
author_rank: Frequent Contributor
timestamp: December 28, 2020, 05:34:46 pm
content: Quote from: sleemanj on December 28, 2020, 01:54:03 amThanks for writing it. It is one of best resources regarding dry film available so far.

author: jmelson
author_rank: Super Contributor
timestamp: December 30, 2020, 01:53:34 am
content: Quote from: Fflint on December 28, 2020, 01:43:58 amI think I have done 10mil traces just fine. Now, my photoplotter does 1000 DPI precisely, so I can make artwork that is quite accurate, and put the film emulsion facing the photoresist, so the light can't go sideways. The dry film resist is the type where UV light hardens it. So, the film would have a clear spot where a trace was intended on the board.QuoteThe developer does NOT harden the resist. The UV hardens it, the developer washes away the UN-hardened resist.QuoteWell, you may not be using the same resist I am. My resist is (I think) DuPont Riston 4615, sold by Think & Tinker as MM 115i.It comes on a roll, and has a thick PVC or similar soft plastic film on one side to prevent the resist from sticking to itself,and a thinner acetate film that protects the film after lamination to the board. The resist is really sticky, as it is basically cyanoacrylate glue without a curing agent. They tell you to leave the crinkly acetate layer on for 15 minutes until ready to develop.Jon

author: Fflint
author_rank: Frequent Contributor
timestamp: December 30, 2020, 01:11:03 pm
content: Guess what... It turns out the 6mm piece of pvc I used to press the artwork to the pcb blocks a lot of uv... This explains my very long exposure times. A 3mm piece of glass seems to work much better. I've ordered a bunch of different transparent plastics and I'll see what works best. Allegedly PCV is transparent to uv wavelengths longer than 300nm. I guess the piece I have has been treated to block it.Quote from: jmelson on December 30, 2020, 01:53:34 amYes, that's obvious. If my description seemed contrary that wasn't my intention. Thanks for pointing it out for others who'll read it.I didn't have a chance to measure individual foils, but the whole thing is approximately 35um thick. The inside foil is soft, the outside foil is a bit crinkly. The film is green/transparent when new and becomes dark blue when exposed.I read somewhere there are two kinds of film commonly available. Dupont Riston and some Hitachi film. I wonder what the difference is.

author: jmelson
author_rank: Super Contributor
timestamp: December 30, 2020, 05:07:23 pm
content: Quote from: Fflint on December 30, 2020, 01:11:03 pmOK, all the dry film I have ever used is a purplish-blue color, and surprisingly dark. But, then, it is SUPPOSED to absorb the deep blue illumination,so maybe that is needed. If your film is green, that must be a different sort. And, the Riston doesn't change COLOR when exposed, but it does become slightly darker. You can definitely see the exposed pattern on the laminated film. And, it seems that the darkness change become more noticeable after the 10 minute post-exposure waiting period.I use something like 3 mm Plexiglas (acrylic) for my vacuum exposure frame, and the lights are 405 nm "blacklight" bulbs, that look almost black when off. I have 365 nm lamps also for a front panel label film, and they do NOT WORK well on the PCB resist.Jon

author: HB9EVI
author_rank: Frequent Contributor
timestamp: December 30, 2020, 07:19:32 pm
content: never got good results with Positiv20; it seems impossible to get the layer thickness everywhere equally, so I use posiive-layered Bungard PCBs, works ufb.UV illumination works fine with fluoresent UV tubes for tanning, about 2min. then develope with NaOH and etching with Na-persulfate or FeCl3

author: ozcar
author_rank: Frequent Contributor
timestamp: December 30, 2020, 08:04:52 pm
content: Quote from: HB9EVI on December 30, 2020, 07:19:32 pmDecades ago (!) I used to use Positiv20, and had no problems with it - even when it was way past the expiry date on the can (kept in the fridge though). I even used stuff that you had to paint onto the board using a brush, which was a bit more tricky. Also used unbranded pre-coated boards sold here in Oz by the now long-gone DSE, Kinsten boards, and most recently Bungard - they all seemed much the same to me.Have not tried any of the negative-acting stuff.

author: sleemanj
author_rank: Super Contributor
timestamp: December 30, 2020, 11:48:24 pm
content: Quote from: jmelson on December 30, 2020, 05:07:23 pmEven Riston is not a single product, it's a range of them with different properties, including colour.Buying from China of course you'll never know what you are getting, my assumption has always been that the sellers of small lengths of it in China probably just get end of rolls or time-expired stuff from the factories, or stuff that walked out the back door of factories.At various times I've used the darker blue/purple stuff, light blue/cyan stuff, and stuff somewhere in between, I don't recall using green stuff but it's out there. For a hobbiest, doesn't matter, all works about the same.

author: Fflint
author_rank: Frequent Contributor
timestamp: December 31, 2020, 08:58:03 am
content: I had another weird issue when using the film. This time on actual pcb rather than test one. Based on previous testing I was expecting 0.2mm spacing to be achievable so I printed negatives for a dual sided board with that spacing between 0.5mm traces and copper fill. I used the 6mm PVC for exposure same as before because I already knew how long to expose for with it.After exposure one side looked great, the other side had a small area (less than a quarter of a cm sq) that looked like there was some bleed through the mask. Under red light few thin lines looked like they were slightly darker than the rest on few mm of their length.I thought that perhaps the mask was lifted on an air bubble to cause this or the exposure was too long. I stripped the film from the board and tried again making sure there are no air bubbles at all and shortening the exposure from 40 to 30 min. This time I got this "defect" on both sides of the board (on both sides in a very small area).This is quite puzzling. I have to assume the PCV I use to cover the board is not flat enough. I'll be trying using a very thin piece of cd case and I'll remove 2 out of 4 lamps from my exposure lamp. It is arranged so that two cfls are on the top and two are on side walls of the unit. I imagine that maybe bulbs on the sides managed to shine under the mask where it was lifted from the board very slightly.Does anyone have any better idea why this might've happened?

author: sleemanj
author_rank: Super Contributor
timestamp: December 31, 2020, 09:51:28 am
content: In the boxes I've made I've either used glass out of old multi-function printer scanners, or glass out of cheap dollar store picture frames. Glass is flat.You need to press it tightly and evenly against the glass, I tape the artwork to the glass, then I place the board on top, and then I have a layer of squishy but firm foam, and a stiff board on top, and then that all gets weighted down (or cinched down with velcro straps). I use the foam layer to help spread the load and make sure it's evenly pressed down.

author: Fflint
author_rank: Frequent Contributor
timestamp: December 31, 2020, 12:34:32 pm
content: Quote from: sleemanj on December 31, 2020, 09:51:28 amYes, I have to make something similar.BTW, my exposure time went from 40 minutes to 10 seconds!Yes, that's right. I got an "80W" led nail curing lamp. I put 80w in quotes, because that's what the box says while the power brick maxes out at 56W... Anyway, even after I taped over all, but overhead leds I managed to expose the film pretty well in 10s,then after developing I give it extra 30s just in case. This is with pretty small testing boards though. I hope it works similar with actual pcbs.

author: jmelson
author_rank: Super Contributor
timestamp: December 31, 2020, 04:20:28 pm
content: Quote from: Fflint on December 31, 2020, 08:58:03 amI have two pieces of Plexiglas, and made an "O ring" out of some micro vinyl tubing (sold on eBay to protect fishing hooks!) to make an air seal.I have a hole in the corner where I connected a hose to a vacuum pump. This sucks the Plexi down against the master artwork so that there will be no air gap between film and sensitized board. I just flip it over to expose the other side. Keeping the vacuum on the whole time prevents the board and artwork from slipping out of alignment.Jon

author: Fflint
author_rank: Frequent Contributor
timestamp: January 01, 2021, 01:51:47 am
content: Quote from: jmelson on December 31, 2020, 04:20:28 pmThat's cool. I might need to try this when making larger boards. So far weighting 3mm glass with 4 550 grain lead fishing weights worked for my small board.One additional thing I found out which I haven't seen mentioned anywhere is how to develop both sides of the board without over-developing one side.Basically after exposing both sides I remove the protective film from one side only. I put it in shallow dish filled with developer (potassium carbonate 3%) and use a small brush while developing to help unexposed film removal in masked areas. Once done (in 60s usually). I wash in cold water and I stick it under the lamp again for longer (just developed side up) to harden it fully.Then I remove protective foil from the other side and develop the same way.I did two boards like this today and both worked fine.Also I found best resolution can be achieved if the film is exposed only slightly. It doesn't have to be fully exposed before developing as long as it is exposed again before etching as described.

author: nctnico
author_rank: Super Contributor
timestamp: January 01, 2021, 02:40:57 am
content: Nowadays I just buy boards because it is easier and super cheap.But back in the days I used tracing paper and a laser printer. Tracing paper is cheap and it lets UV through just fine. Set the laser printer to print a thick layer. Developing time depends highly on the board and I suggest to buy pre-sensitised boards from a quality brand. There is a lot of cheap crap out there (even sold by Farnell) which has very inconsistent results.

author: jmelson
author_rank: Super Contributor
timestamp: January 01, 2021, 06:19:19 pm
content: Quote from: Fflint on January 01, 2021, 01:51:47 amI peel both sides, pre-heat the devloper in a pyrex dish to about 40C and then put the board in. I start wiping the board immediately with my fingertips, and flip the board over every 30 seconds. After one minute the unexposed resist is already dissolving, and after 2 minutes or so it looks completely devloped. I continue the wiping and flipping for another minute to make sure all the unexposed resist has dissolved before washing.Washing completely is important to make sure none of the dissolved resist remains to create a film over the board. I usually wash/rinse for about 2 minutes under freely running water.I have never done that post-develop exposure, the resist, when dried, is PLENTY solid. And, the one minute exposure with my blacklight bulbs seems to not overexpose it. A long time ago, I did use a longer exposure, and did get some broadening of the traces.Jon