PCB's

Introduction.

All my life making PCB's has been a pain and so it is for many others.  So recently I decided to do something about it or simply give up electronics. I think the turning point was one day when a packet of Japanese watabi mustard fell into the printer and it started outputting garbage !. I remember thinking at the time that if only I could produce a PCB with the same resolution as all those weird printed characters !.  The immediate question was why couldn't I print a PCB layout direct onto the Copper laminated board ?. I am sure that several million others have had the same idea !.

It did occur to me that I was going to have problems in feeding a PCB board through the printer .... more than once !.  What I really needed was flat bed printer capable of printing an A5 sized layout  direct onto a PCB board,  etch it and end up with a perfect PCB Everytime !.  However if you would like to have a look at the problems involved, you can do it in the following way. Take a square of aluminum kitchen foil and tape it onto a sheet of standard printer paper. Now  pass it through your printer and on it print anything with some straight lines on it. Out it comes and it looks great !. Well at least for a few minutes !.  When we print on paper it looks as if the ink dries immediately. However if you print on something non-absorbent like the aluminum foil,  it actually takes at least half an hour to start drying. During this time the ink on your nice straight lines starts to 'ball up' and your lines end up looking like dotted lines. Obviously no good for making conductive tracks !.  Of course the foil is coated with anti-oxidant and that may be part of the problem. So now get a clean piece of PCB board and paint onto it some lines using the printer ink.  The interesting thing is that you get the same surface tension  problem coupled with the long drying time  and again end up with dotted lines.  Depressing isn't it ?.  OK that is the bad news, now for the good news !. Take a permanent marker pen such as a STAEDLER Lumocolor 317 - 9 and try drawing a few lines on the same piece of PCB board.  You will notice that the ink dries very quickly and is smudge free !. There is no balling up of the ink and the end result is very durable AND ferric acid resistant!.

Printer Inks.

        Most domestic inkjet printers use a solvent ink. This is pigment in a solution of easily evaporated solvent. Well it works great on absorbent paper, but as we have seen it does not dry very quickly on non porous surfaces such as copper. Obviously the solvent used in the printer ink  is much less  volatile than the solvent in the permanent marker pen ink. This does tend to slow down the clogging of print heads and  we have a compromise situation. We need an ink that dries very quickly but does not clog print heads when the ink comes in contact with the air. 

Another type of ink uses ultra violet cured resins, instead of a solvent. Cure time can be less than 100 mS. So long as we keep the ultra violet cure light source away from the print head nozzles, there should be much less chance of clogged nozzles than there is with solvent inks. 

Thermal transfer inks I have not experimented with yet. 

 

Conclusion's.

I think it possible to print PCB layouts direct onto the PCB board, if a suitable ink can be found. This would enable the rapid printing of not only the acid resistant ink, but also flux, solder resist and layout artwork. 

A flatbed type inkjet printer would be required for the above task. I was unable to find a small flatbed printer and so have decided to modify a Cannon BC100 from conventional paper feed to flatbed,  by introducing a sliding table powered by the original paper feed stepper motor. I have a BC100 stripped down ready to be modified.

I realised that I also needed to automate the drilling of holes in the etched board.  Making an NC drilling machine seemed to be the most challenging part, so I decided to do that first. It is almost complete mechanically, and I am wiring up the stepper motors and limit switches at the moment.  As soon as I have the drilling machine working from a PC port I will move over to modifying the printer.

I suspect that the search for a suitable ink may take some time.

 

Conventional methods.

The most widely accepted method of producing PCB's is by using the photographic method. Now although I had complete memory failure I am pretty sure that I never mastered this in my previous life. The reason I think this, is because I found dozens of PCB positives laying around in my workshop, I couldn't find any good examples of boards made from them Hi!.

First we need to make the transparent 'positive' in order to create the image on the PCB board. I use Transparency sheet material that is specifically produced for use in ink jet printers. You can buy it at your local staioners for about 40 pence per A4 sheet. It has a smooth side and a rough side. We need to print on the rough side. First I mark a "T" on the bottom of a blank sheet of printing paper and the print the PCB onto this sheet. I now know exactly where the layout is going to be printed on the sheet. I then cut out a piece of inkjet transparency sheet just larger than the layout and  tape it into position over the image printed on the paper .... with the rough side out.  The next step is to put the assembled sheet back into the paper feed tray. The "T" will make sure you get it the right way round. Now using the highest quality setting on your printer ..... re - print the layout. I do this twice to get a very dense black image on the positive. It is best to wait about ten minutes between each print to allow the ink to dry properly.  You now have a positive that can be used to produce dozens of PCB's.  Since I produce a lot of PCB designs I keep them stored in photograph albums that have individual transparent pockets. This stops them becoming damaged. 

During my present 'life' I decided to ignore all of the spray on coatings and only buy pre-coated board. I used the Maplins two tube UV light box I found in the workshop and after doing a test exposure strip decided that 12 minutes exposure was about right for the board I was buying from ESR.  I think the best move I made was to stop using caustic soda to develop and instead bought "Photo resist Universal Developer 4997 Concentrate", again from ESR. I dilute this at a rate of 10 milli-liters of powder to 150 milli-liters of water ( That is about two level teaspoons). This is a sodium Meta silicate mix that has a much wider range of latitude than caustic soda and is also more human friendly. If everything is right you can expect the pattern to appear in green in about 30 seconds, and when you are happy, you then wash the developer off the board with clean water. I experimented with weaker mixtures and it works fine after several minutes. I have done dozens of PCB's with this stuff and only had two failures and this was when I chickened out and removed the board too early from the developer. You can tell when you have it right because when you dip the board into the etchant, it doesn't all turn pink .... no problem, wash the etchant off and pop it back into the developer again .... and I have never lost a board this way. 

The other interesting discovery was that I had some photo-resist board that was 20 years old. I had run out of fresh board and was desperate enough to try it !.  With an exposure time of 20 minutes and 12.5 milli-liter of developer (two and a quarter level tea spoons) it all worked !. The latitude of of this developer is such that it is difficult to get into trouble with it !. 

On the minus side mixed developer has to be used in one session. If you leave it for hours, it becomes weaker, possibly because the Isinglass in it and turns it into a jelly after a day or so. I highly recommend this developer !. If I had it years ago ... I would be ruling the world by now !. 

 

Unconventional methods.

For the sake of completion I think I ought to mention 'toner transfer'. This method requires the layout to be printed on paper using a LASER printer. Laser printers do not use ink instead they use a plastic powder that is fused by heat onto the surface of the paper. One then places the printed surface onto a clean copper clad board and apply heat using a domestic iron. The theory is that the plastic toner again melts and sticks to the copper clad board. The paper is then soaked off using warm water ... leaving the layout on the copper. A lot of people swear by this method ... but it is closer to a black art than a science. I did some experiments which were promising, but did not have the 'skill' to produce consistent results. Different people require it to be done in different ways with different methods. Ie, a special kind of paper, ten passes through a laminator etc.. If it works for you ... great!.

 

Conclusions.

I have looked at the problem of making PCB's since the early fifties and come to a few conclusions. First is that the easiest way of producing a layout is by direct printing. Second is that components are getting smaller in size every day. We do not actually need a board in the first place, as tracks can be printed using conductive ink on any sub-strata .... including flower petals. Current technology also allows components such as resistors, transistors, inductances etc to be printed on any surface. There is no reason why whole circuits should not be printed on the shell of an egg !. They tell me that times are a-changing Hi!.

Some recent PCB's.