PCB manufacture

[skippy]

I’ve been trying to learn about pcb manufacture and back in May 2004 I asked “Does anyone know of any good articles on PCB manufacturing? Specifically what I'm looking for are articles that describe the processes involved, give an explanation of the terms used, both on a hobby level and also on a professional level (high volume) relating to single sided boards……….” Since then I’ve read Atan Basir’s excellent article on pcb manufacture using laser toner, however I still have a lot of questions relating more to professional pcb manufacture. Maybe I’ll state what I know and ask a few questions and hopefully someone can fill in the blanks.
The artwork is printed on a transparency (a) is it called a bromide? with a special printer used by graphic houses (b) what are these printers called? (similar to laser but the black has 100% light block out and no pin holes). A screen (for screen printing) is then coated with a light sensitive coating (c) is it generally sprayed on? and the bromide is placed on top of the screen and is subjected to a very strong light for a certain amount of time to expose the screen. The light sensitive coating hardens where exposed by light. Non-hardened areas of the screen coating are removed by pressure washing. PCBs are then screen printed with a masking product which later hardens. PCB’s are then dropped into an aerated acid bath and afterwards are washed leaving just the copper tracks as required. (d) I’ve read about inline pcb printer machines. Are these just an inline automatic screen printer or are they an inline gantry style inkjet using an acid resistant stencilling ink? Next, the procedure can go one of two ways.
Wave Solder Method: the boards are “stuffed” or “propagated” with their components via a pick and place machine, legs of components are trimmed off, and then placed in a wave soldering machine which solders all components to board (e) Does this method require tinning of the tracks and how is this done? (f) after soldering is there another process to protect the solder from the atmosphere in the long term?
Infared soldering process: another screen printing procedure is used to print a solder paste (I think? Or was printing of a tinning layer or solder mask?) around the holes where component legs stick through. Boards are stuffed, then passed via conveyor belt through an infared heating oven which briefly heats and fuses (solders) the components to the boards as they pass through the zone. Right, that’s what I know, now please, please, someone help me out or at least point me in the direction of someone that does know.
I'm not planning on going into production of pcbs but I do have a commercial reason for wanting to know about the process based on an idea I've had kicking around in my head for the last year.
Thanks in advance for your help
Skippy

[vladdy]

>The artwork is printed on a transparency (a) is it called a bromide? with a >special printer used by graphic houses (b) what are these printers called?

The transparancy is nothing [in most cases] more complcated than a sheet of negative photo emulasion, with very fine grain structure..If you want to do your own, you can use standard negative stock from a photo shop.
The printer is a bit more complicated, actual older units used a light source with specific size apertures that determined track width, the 'standard' system was produced by Gerber, hence 'gerber files'. For home use you can laser print to transparancy, or opaque paper, and develop your own film transparancies..

> A screen (for screen printing) is then coated with a light sensitive coating
>(c) is it generally sprayed on? and the bromide is placed on top of the >screen and is subjected to a very strong light for a certain amount of time >to expose the screen. The light sensitive coating hardens where exposed by >light.

hobby use, purchase 'pre sensitized board, light / medium, use a emulsion boner that 'glues' the resist to bare coper, large scale can use 'spray and spin' applications for even coats.. When exposed to UV light, the resist changes strength, not all the same though, check postive and negative resist types, the exposure / developing methods are slightly different..

>Non-hardened areas of the screen coating are removed by pressure washing.

The developer washes away the 'etch area' resist, water rinse after

>PCBs are then screen printed with a masking product which later hardens.

After etching, for solder mask areas, or silkscreen legends, etc..

>PCB’s are then dropped into an aerated acid bath and afterwards are >washed leaving just the copper tracks as required.

basically right, production will use a 'spray wash', and specialized etchants depending on copper type and end use..

>I’ve read about inline pcb printer machines. Are these just an inline >automatic screen printer or are they an inline gantry style inkjet using an >acid resistant stencilling ink?

Production is 'a lot' of seperate stages...you can make a stencil [similar process to transparancy with pre sensitized stencil material], and silkscreen the resist on before etching, and silkscreen placement layer after the same way.

>Wave Solder Method: the boards are “stuffed” or “propagated” with their >components via a pick and place machine, legs of components are trimmed >off, and then placed in a wave soldering machine which solders all >components to board (e) Does this method require tinning of the tracks and >how is this done?

In production Tinning is not usually done for completed PCB's, the wave solder tank will coat all cover not protected by solder mask at the same time it solders the components. Bare pcb's are sometimes coated if they may sit for a while to prevent corrosion and subsequent difficult soldering operations.

>(f) after soldering is there another process to protect the solder from the atmosphere in the long term?

Usually in better boards / miltary / industrial, not cheap consumer stuff, one of the standards is "Conformal' coating, you can get spray cans for hobby use.

>Infared soldering process: another screen printing procedure is used to print >a solder paste (I think? Or was printing of a tinning layer or solder mask?) >around the holes where component legs stick through. Boards are stuffed, >then passed via conveyor belt through an infared heating oven which briefly >heats and fuses (solders) the components to the boards as they pass >through the zone.

Kind of, usually used on SMD components, probably could be used on thru hole, more money though, and a short shelf life on the paste [sometimes only 30 days even in the fridge]


Right, that’s what I know, now please, please, someone help me out or at least point me in the direction of someone that does know.

Check out some the the places that will do short runs PCB prototypes, they often have a desciption of what machines they use. Maybe try a search for PCB fabrication equipment, there's always lots on ebay, and it's usually pretty cheap, not a real long life cycle..

enjoy

[skippy]

Vladdy,
thanks a million for helping to fill in some of the gaps as I really appreciate that you took the time to answer. Anyone else?
Regards
Phil (Skippy)

[abasir]

Originally Posted by skippy Vladdy, thanks a million for helping to fill in some of the gaps as I really appreciate that you took the time to answer. Anyone else? Regards Phil (Skippy)

I'm assuming you've been here http://www.thinktink.com/
Plenty of info regarding the whole PCB process.

[miljnor]

Has anyone thought of or tried taking a laser printer and taking it apart and using the low powered laser you heat a pcb board directly and just have the toner adhear to it. I know thats how it adhears to the paper.

The paper passess thru the laser zone being heated in the pattern that the computer tells it (ie your text or whatever) and then passes in front of the toner cartridge where toner basically melts onto the paper.

I have a few working older laser printers that I have been diing to modife to maybe a gantry still laser printer to print on flat surfaces. And while I know that copper is a serious ugly on the laser cutting front I wounder if the low powered one from the printer could heat it up enough to apply toner. The temp cant be higher than 451 degrees for obvious reasons.

Anybody have any thoughts on this?

[vladdy]

>>Has anyone thought of or tried taking a laser printer and taking it apart and using the low powered laser you heat a pcb board directly and just have the toner adhear to it. I know thats how it adhears to the paper.


Actually the laser in a printer will develop virtually no heat at all, even on paper..All it's there for is to remove the charge from the drum..

The toner is fused to the paper by the fuser unit at the exit... hot rollers..
If you stop one half way through a print, you will find the the image will wipe [even blow] off the paper until it hits the fuser asssembly..

[miljnor]

That actually could be good news as the copper and the laser arn't the best of mixes.

could the heated drum be set up to roll across the copper plate for tonner transfer?

[sbrpollock]

Yes, I have given thought to modifying/building a laser printer to "print" directly to a copper clad-board by applying the toner directly to the board. This would increase the resolution and improve efficiency by eliminating the "transfer" step of the "toner transfer" process.

However, I haven't seen a way to apply the laser printing process directly to copper-clad boards.

Read how the process works at the following link and then let me know what you think.

http://computer.howstuffworks.com/laser-printer.htm

[vladdy]

It's not readily possible to transfer the technology of drum transfer to PCB foil directly. The drum is composed of millions of seperate particles per square inch that can be charged, a PCB foil is ONE particle electrically, until it's cut / milled / or etched..
One possible way would be to discharge the drum to PCB rather than paper, and then fuse the result, but you would need a printer capable of a virtually perfectly straight path, as PCB doen't bend very well, even phenolic.
They do make flexible substrata circuit boards, primarly toys, games ,and automotive dash circuit panels, but these are mostly a silkscreen transfer of conductive resins, as opposed to an etched or milled style of PCB.
One easily obtainable type of conductive resin is rear window defrost repair kits which contain high percentages of copper or aluminum fines, but these are fairly low current capacity.

not sure if this reply is what you were thinking about, but??

[miljnor]

Its still possible just more complicated. The biggest stradle would be the fact that copper clad boards are probably not perfectly flat, nor are they flexible. so getting a roller to transfer the toner might be a problem.

But then again maybe not. The charge put to the copper board might be strong enough to pull the toner off the roller and the small gaps (if any) would be a non issue.

Then there is the fuser issue. that would probably have to change but maybe not.

I will have to get some time to tear a printer apart a do some tests. Even if the image isn't good, if any toner transfers then all the hurdles can be dealt with.

Another Idea this gave me is maybe applying the toner electrostatically to the copper board and using a CD-R laser to melt the toner onto the copper. This way the laser never actually touches the copper. The only problem here would be teh mess. becuase the layer of toner you get would cover the whole board and would come off except for the heated area.

Do any of these ideas seem feasable?

we should probably make a new thread for this.
or if we could get Paul to chop these last few posts and put them in a new thread thats labeled something like
"PCB laser printer: fact or fiction?"

hint hint hint

[sbrpollock]

Vladdy:

I agree, there is no way to use the laser to charge the copper directly and have it pick up the toner. The drum has a very special (and delicate) surface that responds to the laser. *If* this could be done, the drum would still have to be part of the process.

Here is where my thoughts have taken me:

Once the image is written to the drum by the laser, it is transfered to the paper by use of "a difference in electrical charge". The one thing that I've never been able to acertain is, does the paper actually make physical contact with the drum as is passes the drum and picks up the toner? Or, is the paper held a few microns away from the drum as it passes, and the toner has to jump this small gap from the drum to the paper?

The reason I pose this question first is that if there is actually a small gap (microns) between the paper and the drum, then maybe we have a chance to transfer toner from the drum to the copper-clad board. However, if the transfer actually requires physical contact, there is no way the delicate surface of the drum is going to survive making direct physical contact with the copper-clad board. Plus, with the copper-clad board being as conductive as it is, the moment the physical contact would take place, the differing electrical charges necessary for the transfer to take place would be dissipated.

If the tranfer from drum to board were to be possible, then it is certainly conceivable that a machine could be built/modified to have mechanics that would transport the board past the drum instead of paper. It might be difficult but it is certainly not impossible.

I suspect however that transfering from drum to board is not possible because direct physical contact between the two would be necessary. This would complicate the idea immensly.

[skippy]

Wow, now that's amazing what you guys are discussing. I'm attempting to make oversize circuit boards for an led application in a specialist market and was therefore wondering if there was another way or another way could be developed to facilitate making larger boards. Some of the ideas I've had (which may or may not work) are:
(A) If you laminate a pcb with a light sensitive mask (this is standard procedure in pcb making), the boards could then be passed under a low powered gantry style laser (like Paul's first unit) and the laser light hardens the necessary areas. Remember the laser light hits the mask not the copper. Wash your boards to remove unwanted mask then acid etch. I think bare pcb boards (fr4 etc.) can also be bought precoated with a light sensitive mask but I think it's expensive because it's aimed at the hobby market. You would set up registers like in screen printing so that each board registers the same as the previous one. This would in order facilitate the correct registration/alignment in the drilling process that follows.
(B) Modern vinyl signs of the more expensive type are made using a flat bed printer. One of the manufacturers is Zund (Switzerland??) and I recently saw one of their machines that was about 2.5m x 1.2m working area. Anyway a vinyl substrate (roll of vinyl, generally white) sits on top of the bed and an x/y gantry with printer head basically paints the signs. When that area is done the machine automatically advances the vinyl to the next point so that a continuous sign can be made that is say 50m long. On the ones I have seen, they were using an individual reservoir for each of the primary colours plus black and the reservoirs are put under low pressure (air) to assist flow. If I understand correctly the actual printer head technology is just a big version of your everyday inkjet style printer head except that it can use solvent based inks/paints. The inks/paints are interchangeable with other types. Now, if we could find an acid resistant ink/paint we just paint our mask and again using the registration system for positioning product.
(C) A flat bed vinyl cutter (i.e. the expensive type such as Zund or Wilde TA10????, not the cheap "see-saw" roller type) as used to make every day vinyl stick on lettering, could also possibly be used. Not all acids attack all vinyls and adhesives. If you could find a vinyl that was resistant to the acid used in the pcb process, you simply laminate the bare pcb, lay in on the flat bed cutter with the registration stops (they have a vac hold down system), machine cuts out the pattern, you then weed (remove) all vinyl except that which is covering the tracks, roll to assure good adhesion, acid dip.
(C2) As above but remove vinyl material covering tracks instead of that which is not covering tracks, this will act as a mask. Again, roll for good adhesion, spray with enamel or whatever is resistant to acid, remove vinyl, acid etch OR if vinyl can be attacked by the acid, leave the vinyl on and let the acid remove it as well (one less job).
I've done plenty of internet searches over the last year looking to see if anyone was already doing this but didn't find anything. If one of these processes worked it would be particularly beneficial in the 1 - 25 boards market. Obviously after that number a screen print situation would be more economical.
Skippy