Thursday, January 12, 2006


PCB production

We had an idea for making PCBs in a RepRap machine more or less directly, as opposed to the usual route of masks, photoresist, UV lamps, and ferric chloride solution.

Instead use electrochemical machining. I put a blank PCB in a bath of concentrated salt (NaCl) solution and connected it as an anode to a 12v supply. I used a soft-iron lead from a resistor as the cathode and held it a very short (0.5mm? I didn't measure it) distance from the copper of the PCB. The cathode had plastic sleeving round it in an attempt to cut out extraneous "sideways" currents. I set up a little fish-tank pump to circulate the solution and squirt it directly at the cathode to stop local build-up in solution of reaction products.

When I turned on the current (about 200 mA according to the meter) it punched a hole through the copper pretty quickly (about a minute). But the hole was about 2mm in diameter, which was a bit big.

I'll do some more experiments and blog some pictures soon.

If it can be made to work you could have an array of cathodes, each switched on and off as the array is scanned over the copper surface in X and Y to remove the copper in the pixel pattern of the PCB you want. If the scan happens from one edge of the board to the other, there is always going to be continuous copper on the unscanned side to ensure that islands (that cannot be removed as no current gets to them) don't occur.

Dremel tools put stress on the workpiece, and our designs won't cope with that very well - excess movement, sensitivity to swarf etc. All the reasons we went to FDM, in fact.

Using the RepRap as a plotter loaded with etch resist is one thing we've talked about. Should work, but needs nasty chemicals.

Adrian, could we shield the PCB with Polymorph, then electrically etch?

Electroplating, sputtering etc. are all possible, but need a substrate that can subsequently be soldered.

Vik :v)
Just a thought (applying the reversal heuristic proposed by R J Weber, if anyone's interested) - could one reverse the electrolytic erosion process? Use copper sulphate solution or something similar and move an electrode to deposit copper in lines, instead of eroding it? Probably impractical, but then again, you never really know until you try to build one. Of course, I'd be being hypocritical after my recent posts if I didn't note that copper sulphate isn't that easy to get hold of, but it's still an interesting concept.
COme to think of it, could you use copper sulphate recirculating inside a nozzle with specially shaped electrodes to extrude copper filaments, without having to mess around melting it? Get the rate of deposition on the rear of the extruded work to equal the rate at which it was forced out of the nozzle, maybe? You might get lousy quality, though, if it could be persuaded to work at all.
i'm no expert in building electronic circuits, but what is the feasibility of simply extruding solder? its easily available, simple to melt, and would mesh very well with a working reprap.
Solder needs to stick to something, or to be contained in a fireproof trench of some kind. In order to stick it need flux.

That said, we are looking at a low-melting point alloy and Ed says he's going to pass that through the Mk2 nozzle at some point.

Vik :v)
We're designing the project using Open formats throughout, so in theory converting a PCB layout to an STL and vice-versa should be quite practical.

Vik :v)
How about laying down the circuit paths in an adhseive and covering with graphite powder. Then the copper could be plated onto that. A method like that has been used successfully in jewelry making for decades (at least)
Individual paths could simply be connected by insulated jumper wires.
I have to admit the electro-erosive method your working on is a novel approach I look forward to tinkering with myself. You could think about tailoring the amps and volts and also the on/off frequency. Higher amps from a smaller tip like a needle for shorter durations might give you the hole size you want.
Graphite powder - ingenious! Come to think of it, I think I read somewhere that that's how they used to make the master metal stamper from the wax impression for old 78 rpm records - the recording in the wax would be dusted with a powder, and then electroplated to form a rugged metal negative form used to stamp out production records.

I was envisioning a slightly more Heath Robinson (US: Rube Goldberg) approach: Have a polymorph canal in the shape of the tracks you want, then place the source electrode in the centre and the sink electrodes at the end, and fill the canal with the electrolyte - the copper deposition would then start at the ends of the track and gradually creep towards the centre, forming into the shape of the liquid cross section in the trough. I didn't do a very good job of describing that, but I'm in a hurry.
Okay, I think we've got it there. Instead of using an adhesive and dusting it with graphine, however, lay a trace of our ordinary CAPA 6800 polymer with carbon black used as a filler. Once you have your traces done, dump the board in your polysulfate bath and pop an electrode onto the traces one at a time till they're plated. Looks like a special bath attachment and a board rack to hold the boards in place in the bath will be necessary. A special head on the reprap will hold the electrode.
Slight problems there, according to the vikscope. First off, a carbon/Polymorph mix is unlikely to be any more conductive than, say, copier toner. The reason you dust items to be electroplated with graphite is to get a continuous conductive surface. If the graphite is encased in plastic, it doesn't conduct well enough until you reach something very much like coal...

Second, the Polymorph trapped under the graphite is going to vapourise when soldered. Bubble, bubble, toil and trouble.

Vik :v)
Um... Vik? How do they solder on flexible polymer PC boards then? My technicians were making flexible PC boards in my lab in Hong Kong 10 years ago. The base material was a polyester which is not exactly a high temperature matherial with a really thin coat of polyamide over it sometimes but not always.

Mind, I can't get into the details of how soldering was done, but I know they managed to do it with a fairly short learning curve. Those tiny sequin shaped resistors and capacitors were a pest to hang on to when you were soldering, though. I learned a few really choice Putonghua epithets while my technician was building boards. :-)
Polyamide (Kapton) and Polyester (PET) may melt slightly while being soldered, but their high molecular weight stops them vapourising. Just prod a piece of polycaprolactone with a soldering iron and you'll appreciate the problem.

I noticed this while I was pushing bolts into a block of Polymorph with a soldering iron, and Wups! >Ftzzzzz!<

Vik :v)
I think we have a winner here, then...

CAPA® 6800 is a high molecular weight linear polyester derived from
caprolactone monomer. It has a molecular weight of 80,000 and is used in a variety of applications.

It's a polyester with a very high molecular weight! :-D
This comment has been removed by a blog administrator.
Like I say. Poke it with a soldering iron and tell me what you think.

Vik :v)
Now I just have to come up with a soldering iron. :-(
Old-fool historical reminicence: I can remember as a child grinding up 6B pencil 'lead' for graphite, spraying fallen dried leaves with glue, dusting on the graphite, then electroplating with copper from CuSO4 solution to make a copper leaf. I like the idea of a glue trail and graphite dust. Tom says copper sulphate is hard to get hold of; this seems strange as it's in every child's chemistry set; also I can buy it by the kg over the counter at my local chemist as a fungicide.

However, the advantage of electrochemical removal as opposed to electroplating addition is - as pointed out above - that you never get disconnected islands. Or you can run your cathode round each track to delineate it in a sea of ground plane (which is good design practice anyway).

As above as well, we have thought of just putting an etch-resist pen in the RepRap X-Y head, drawing the circuit onto a PCB, then etching in the usual way. Again many years ago when I was an undergraduate I did exactly this in the big Kingmatic XY plotter that we had at Imperial College. The staff were very dubious when I turned up with the pen holder that I'd made in a lathe, but fortunately the head of the computer centre walked past, saying, "Good. That's exactly the sort of thing we got this for. Not for endlessly drawing geological maps for the School of Mines..."
//But the hole was about 2mm in diameter, which was a bit big.//

Would it help any to use two leads (anode and cathode) instead of making the entire PCB an anode? It might be easier to control the spread that way.
Cool, lots of new thoughts. I was thinking of using a syringe as your anode that way the pumped solution could act as part of the anode and you wouldn't need a seperate entry point for the wash liquid.
Another advantage the X-Y plot etcher would be the ability to make truly complex shapes and even shades of thinner copper for things like board mounted antenna, coils and capacitors.
In the long run, to manufacture a larger subset of items it may be necessary to have both additive and subtractive methods which is why all this alternative thinking is good.
On the binder/graphite issue: there are probably glues/binders that would handle solder level heat. How about a sodium silicate solution?
Ooops, darn my americanisms. By "syringe" I meant a hypodermic needle. Not the little thing you squeeze eyedrops with or a turkey baster. I've seen needles easily as narrow as a resistor lead. The fluid could be under considerable pressure and that would help erode the copper. As a matter of fact, isn't there a device called a water-jet saw? You could apply a similar principle but using salt solution and electro-corosion.
Copper is remarkably resistant to erosion by flowing fluids, hence its use in water pipes and fountain nozzles. The electrochemical effect is going to be the major player here, I feel, and Coulomb's Law limits the rate at which we'll remove the copper.

Besides, if we really crank the pressure up, we'll likely delaminate the board!

I saw one of our guys at the fire station blast a hole in the tarmac out in the yard with a fire hose. Quite spectacular when it all blew back at him out of the crater :)

Proper waterjet cutting nozzles need to be made of sapphire to resist the forces involved.

Vik :v)
You don't have a choice over anode and cathode. The workpiece, if it's metal to be got rid of, has to be the anode. It's the current flowing through it and taking metal ions off the surface into the electrolyte that results in material being removed.
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