PCB Photolithography and Inkjet Printers

I may be easily amused, but I’m not easily impressed.

Everything I have read about pcb prototyping using photolithography claims I needed to use a laser printer and some sort of transparent or translucent medium. Armed with this knowledge, I used acetate, or overhead projector transparencies. The film claims to be designed for laser printers and copiers, but it still distorts some when printed on. Anyway, the problem with my laser printer is toner pin-holes. For whatever reason, the output would have these little holes everywhere, and these little holes would swiss-cheese my traces and ground plane pours. This forced me to use thick traces, which would still get swiss cheesed, but generally retained enough composure to be electrically conductive. The pin hole problem seems to be getting worse … the last test I performed on my printer as an exposure test of varying width lines and different sized holes and pay layouts. In general, the performance was pretty bad and was not acceptable for more advanced designed I wanted to make, involving very small SMT components.

So, I tried printing the same exposure test, on paper, to my inkjet printer. The results were stunning. The lines were sharper, the holes clearer, the pads better defined. What a difference it makes going from a 600 dpi laser to a 4800(?) dpi inkjet. So, I felt it worth the risk, and decided to run my exposure test with the inkjet. Using some inkjet transparencies (they’re kinda coated with some type of fiber?), I printed my patterns. I exposed my board, and then developed it… the results were WOW! Of course, I messed up a few things with this first run, mostly I let the light cook for too long. So in the best un-scientific manner possible, I changed a bunch of variables at the same time, and tried another pass.

photolithography inkjet pcb

That is the result! … Please ignore the greasy thumbprint on the left side of the board – that was acquired after etching, and has no effect on anything aside from mild embarrassment on my part. My setup was fairly simple. I printed my design at best quality, monochrome mode onto a transparency. Next, in my darkroom that doubles as a furnace room, I laid down my PCB, emulsion up, then the transparency, then a sheet of 1/4″ plate glass. About 4″ above that, I have two 15 watt under cabinet lights, each loaded with a GE Daylight bulb. The lights are connected to an extension cord, so I can turn them on and off together. After making sure everything is lined up, I start my stopwatch and plug in the lights. After letting it cook for eight minutes, I turned off the lights and slid the pcb into a waiting bath of developer. The developer had been sitting for about a day, so it was a little slow. I left the board sit for about 2 min, before turning on the room lights. As the emulsion started to dissolve, I could see the results were good, real good! With the room lights on, I stirred the developer and lightly brushed the pcb using a foam brush (as recommended by the manuf.) My image grew sharper and sharper.

After I was sure all the emulsion that needed to be gone was gone, I rinsed the board and slid it into a waiting bath of ferric chloride etchant. A few min later, I pulled the pcb out, to make sure all the copper had turned pink. If the copper is not pink, it means some emulsion remains, and its time for another trip to the developer. Fortunately all the copper was pink, no problems here. I let the pcb soak for about 30 min while I had some lunch. After lunch, and without washing my hands i might add, I extracted the PCB from the etchant and rinsed it off. The results are excellent.

So, no longer will I shy away from tiny SMTs, since I can now lay down traces as thin as 8 mils without issue (determined by the earlier exposure test). Granted there were three flaws I had to fix on this board, I suspect they were caused by either dust on the transparency or those fibers that are embedded in the plastic. A quick touch-up with the sharpie solved them without a hitch.

Next stop, de-panelize with the PCB “suicide” saw.

pcb saw depanelize

25 Replies to “PCB Photolithography and Inkjet Printers”

  1. Hi,
    Nice looking job, I’ve tried using the photolithography technique….but could never master exposure times….I suppose not having a proper lightbox didn’t help!
    Although a bit pricy, you can save a bit of time by using the press-n-peel film….and still be able to do SMD devices…I now use SMD designs by choice, you can save a whole lot of drilling! 0805 or SOIP devices are fine, I’m comfortably doing 24MIL lines and dbl sided pcbs…don’t know about 8mil though!.
    It takes a bit of care though to get consistant results. After a discussion on the neonixie forum at yahoo groups I’ve come up with some guidelines that help:
    – really clean and slightly rough the surface of the copper with wet/dry sandpaper before starting
    – beware of slightly different dimensions as the film passes through the laser printer, it can stretch. Align your PCBs so most trackwork is along the width of the film, not down its length. This is particularly important in dbl sided work…something I found out the hard way.
    – print thick pcb outlines, these will help keep the artwork form slipping when ironing. Or because I usually compose up a bunch of artwork in a photo editor before printing to save using a film for just one PCB I just use normal lines and fill in the outside areas with a fill tool.
    – if doing dbl sided work at home, use biggish vias and bring connections to the bottom where possible. Onless you can plate through holes, you need to solder something to something and the bottom side is usually easier.
    – When ironing on the pattern, rotate the pcb as you go so each area gets the same pressure pattern…this’ll help transfer all lines properly.
    – heat the copper evenly and so it is just a little too hot to touch. Whilst still very hot, transfer to a running tap and cool down
    – with the water still running, lightly peel away the film, if possible let the water do it.
    – after etching drill with the toner still on, it helps in positioning the drill not to have a reflective surface.
    – again scrub down with wet/dry or use a solvent like Eucalyptus oil….only problem with the oil is it has to be removed and can leave a stain. Scrubbing with sandpaper also helps remove small burrs in the holes.
    – definately use a pcb lacquer…it’s another step, but well worth it. With SMDs it gets slightly tacky with heat and can help hold down fiddly devices.

    Would love to know a homemade method for doing solder resist layers???? It comes in handy with these SMD stuff.

  2. Just to make note that the water should not be cold. Start with warm water or you will break the bonds of the copper and board. Copper is very soft so it expands and contracts very easily. While the copper its self will Survive you may end up with lifted traces later after you etch.

    I have not tried prototyping like this yet. But from my Experience with soldering, I Found those traces to be very touchy, albiet not traces yet.

    Also Im Including a link to a site regarding this. The gent who pioneered the Gotee transfer method, Ok maybe other have done it before. he’s the fool who got his method published first.

    http://www.fullnet.com/u/tomg/gooteepc.htm

  3. I’m not sure where the concern for thermal shock comes from … the PCB is not heated at all during the photographic process. I’ve produced dozens of panels (100’s of individual pcbs) using this process, and have not had any issues with the copper delaminating. This process has nothing to do with high temperature clothes irons or waiting for paper to ‘dissolve’.

    Photolithography is accomplished using a positive mask and a light sensitive emulsion. The mask can be used over and over (until you slob developer on it).

    Most of your fast turn-around board houses use either a positive or negative photolithography method… they don’t have factories in China setup with row after row of laser printers and clothes irons.

    There are a lot of advocates for the “toner transfer” method and it has been receiving a lot of press lately because of this. My opinion is it is more difficult and slower than using photolithography, due to the large number of variables involved. With toner transfer you destroy your layout artwork and have to print another. The cost per sheet of photo paper and transparency is about the same (in my local), transparency is reusable and photo paper is not. Some argue that plain clad boards are cheaper than pre-sensitized boards; perhaps this is true in Europe, but I pay less for photo boards than I did for clad copper. Some argue toner transfer is faster somehow … I don’t see that being the case – first you manually have to use the iron for several minutes to transfer the pattern, using considerable pressure. Secondly you have to give the paper time to soften and break apart in a soap water bath – how long does this take… some claim 10 min some claim 30 min. Then you’re scrubbing the board with a mushroom brush to remove the remaining paper fibers. With photolithography, you need 8 min under the lights (turn them on and walk away), 1-2 min in a bath of developer and you’re ready to etch. Last but not least, the toner (a synthetic wax) is not compatible with the soldering process and must be removed prior to board assembly. The photo resist coating does not need to be removed prior to soldering; it is designed to quickly evaporate when exposed to soldering temperatures.

    To each their own I suppose!

  4. I have used quite a few methods to produce PCBs over the past 40 plus years. These include both positive and negative resists, silk screening, home made photo emulsions made from all kinds of weird and wonderful materials. As well as pre-coated photoresist board, you can buy the resist in a can and coat your own boards (not easy to get an even and consistent coating though)

    I recently tried the toner transfer method and found it easy and reliable. The image on the board is considerably more resistant to abrasion than the image from ANY photo board (unless you are prepared to oven bake it! In which case they are similar).

    Pre-coated photoresist board is aways going to be more expensive than bare board and it has a shelf life, which bare board does not have ! During its life, the sensitivity of the photo resist will vary, so you need to make a test before expecting to make perfect boards that way. Old board could need FOUR times the exposure required for a good image on fresh board.

    I don’t wish to argue for one method or the other. Both make good boards. Neither of these methods is good if you want more than half a dozen copies. Silk screening is good for larger numbers, but you can get boards made so cheaply by specialist PCB makers that it becomes silly to try to make any quantity at home. Just this week I ordered 1,000 double sided boards from one of my suppliers. They will be etched, drilled, tinned, silk-screened with component details, etc., and every board perfect ! All for less than I can buy the same area of bare board ! Oh yes… and they will be in my factory on Friday afternoon !

  5. Hi there guys,
    I would like to know what ink jet printer gives the best results ie
    a nice opaque finish. I have a old Lexmark 7000 and it gives poor results may pin holes etc.

    kind regards pete

  6. I use a $79 HP Photosmart 5150 … pin holes may be due to the little fibers they imbed in the transparency to make the ink stick … try a different brand of transparency, and make sure your printer is set to print on the highest quality (uses the most ink)

  7. I do this for everything down to 0402 and QFN (the cc1100 tranciever).

    My tips:
    1) Use high quality black that mixes the colors – some of the color (especially in a photo cartridge) are more UV opaque than black.
    2) for really tiny traces, use equally tiny spacing from the fill. This gets a fast etch before the traces are eaten under.
    3) for really really tiny traces (QFN) print with ink so thick you can see the globs of it. IT will take a day to dry well, but the results are really really worth it if you will use the mask several times for something that needs parts this small (QFN, 0402).

    As for soldering the small stuff – get a cheap hot air work station, they can be had for less than $50, less than $100 for a multi-function stations (I use auyoe).

  8. Like Daviant above, I have had very good experiences with toner transfers. Just laser print onto any glossy photo paper, put photo paper on copper board and iron it on for ca. 10 minutes. Let cool down for another 10 and carefully peel it off. Ready to etch…

  9. Dear sir,

    The photos show a good work and admire you for that.

    i wonder whether the ink jet will print on transperency — as, whenever i tried with my HP656C and with pigment based ink, the ink slides off the film, never got the desired result.
    Did you mean to say that i should use the photocatridge instead of pigment based ink?

    Regards
    Sarma

  10. Sarma,

    You need to use “inkjet” transparency film, not plain acetate. The difference is tiny fibers embedded into the acetate give the ink something to adhere to. also be sure to give PLENTY of time for the ink to dry before touching or even moving the transparency much. I find that 30 min. works good, but more or less might be required depending on ambient temperature and humidity.

  11. half a mil? that’s pretty damn small. most of your big time board houses usually draw the line at 3-5 mil. I don’t think it’s possible, but I don’t have any data one way or the other. of course, silicon wafers are etched using ultraviolet lithography, and have very very fine traces, so it is possible, but I’m sure they use some pretty high end hardware.

  12. hi…
    can u explain more about your darkroom…
    why do you use 1/4″ plate glass.
    what if i dont use that plate glass??

    thnaks

  13. baloma,

    the heavy glass is used as a weight mostly, to ensure the transparency lays flat against the board. it’s not as much a problem with inkjet as it was with laser transparencies – they would curl something terrible after running through the printer.

  14. I have used just about the same method to create screens to print with and have two tips that may help you…
    1) flip the original so that the printed side is touching the PCB. This prevents the light from getting under the edges of the lines. Much sharper image.
    2) print on vellum. It may raise your exposure times but it takes the ink much better than transparencies. Again, sharper image.

  15. Good explanation. I use overhead projector transparencies in a laser printer as well and experience the same problems. It is simply not black enough everywhere. I solve this by running the same film three times through the printer. It is not easy to ensure that the printer prints the second and third time at exactly the same spots but it is doable. Resolution can be about 0.2 mm and a DIL IC lay out is in this way very well possible. I wouldn’t think of doing SMD however. After three times printing it is incredibly black.

    The light source I use is an old second hand costs-almost-nothing UV facial tanner from Philips. Exposure time 2 minutes from a 30 cm distance. A thin glass plate keeps the mask flat. The printed side of the mask is faced to the copper to avoid under etching.

    It is nice to know that I can create even better masks using an inkjet printer.

  16. Most laser printers have a “print density” or “toner darkness” setting. Turn this up if you are getting holes & down to save toner. Laser printers are actually designed to print 5% coverage that is why large black areas get holes but turning up the toner density will fix this. Just put it back when you don’t need it to save toner.

  17. If a laser printer is low on toner will cause holes as will a old drum in the printer. Inkjets have the opposite problem with ink bleeding which can cause shorts between traces. Using good inkjet GLOSSY or PHOTO paper will get the best result for high density boards. For low density boards there should be more space between traces and can probably get away with normal paper.

  18. I’m looking to buy a inkjet for printing my transparency.

    You say you have an HP Photosmart 5150. I didn’t find any spec for these printer, did you have the spec for this printer, what do you recommand to buy, else than a printer that can print on transparencies ? Do you think any Photosmart Cxxxx would do the job because I think it have to be opaque trace ?

  19. @aliensim:

    I think any printer that can print at high resolution on a transparency will work fine for you. The higher your resolution, the finer pitch boards you’ll be able to make.

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