Russel, thanks for the suggestion. I have admittedly never tried Press-n-Peel, but as far as I can see it goes most of the way back to chemical etching. It still needs ferric chloride (which has ruined more pairs of jeans than I care to remember) and still needs the intermediate stage of plotting/printing artwork. The only process it seems to avoid is UV exposure, which is actually the least messy part of what I do now, and not a problem anyway.
And I'm not sure that double-sided image location would not be tricky with Press-n-Peel. Double-sided is admittedly tricky with any system, but at least with both UV and milling it's possible to get precision - with care!
Milling appeals to me on two counts (well, three if you include having a new toy to play with) - (1) Not having to worry about slippage in a laser printer, or less-than-black inkjet printing, and of course (2) No messy bubble tanks. Not to mention the accumulated guilt of years pouring spent ferric chloride down the drain ("What me, gov? Nah, it's ketchup.").
Hank, you're right on all counts. Fresh developer each day is a must (prepared the night before to stabilise), though I've found I can usually go on using ferric chloride until it's spent. But temperature control is very critical, and it took me a lot of experimentation to get it right. I was making boards for production then, not just prototyping, and scrap costs money.
Another nice thing about milling has just ocured to me: there's a big price difference between plain copper boards and pre-sprayed photo-sensitive boards (NB to anybody thinking of doing it: use Fotoboard 2. Don't even think about spraying your own photo-sensitive stuff on. You've got to spray it evenly with no missed bits, bake it dry at the right temperature, and never let it see daylight until you use it. You really don't want to go there).
A pick-and-place m/c for SMD is next on my list after PCB-making. Sounds as though you and I are travelling the same road. Are you using an "Archimedes Screw" device to dispense solder?
....for 100pcs it works out about 36p inc vat & delivery for a 6" x 4 board" (cheapest I know of but I'm happy to be corrected!) ....the cheapest fotoboard of the same size I was ever able to source worked out at about £2.00 delivered per board, so yes a lot cheaper! (and the board I'm milling with are so cheap that it doesn't ruin your day if you screw up!)
re your comment...
2. Don't even think about spraying your own photo-sensitive stuff on. You've got to spray it evenly with no missed bits, bake it dry at the right temperature, and never let it see daylight until you use it. You really don't want to go there).
Still got the can - doub't I'll ever use it....it's twatful stuff.
Last edited by HankMcSpank; 11-03-2012 at 07:36 PM.
I'd be interested in details of your solder paste dispenser (apologies if you have already posted them somewhere - I've been away from this site for quite a while). What air pressure do you use? Is the amount dispensed repeatably accurate? I admit to a prejudice against pneumatic control of such small amounts of thick and gooey (and expensive if lead-free) stuff, but I'd be very happy to be proved wrong.
Grateful thanks to Mike for steering me towards a Proxxon mill. Their new KT150 compound table is the perfect size for milling my PCBs, and looks sturdy enough to hold the work properly. I have settled on:- a KT150 table, BFB 2000 Mill/Drill Stand, and BFW 40/E Mill/Drill Motor and Controller, all at http://www.proxxon-direct.com/index.html .
And to Hank for advice about leveling, calibration, cutters, and warping. All greatly appreciated.
I'll do the CNC conversion myself, and I should be able to machine the stepper motor mounting plates on the mill. Then I'll make a cabinet with extractor to keep the dust under control. So I now have an enjoyable summer ahead, and I'll post the results here when I have something to show.
Thanks again to all for your very helpful advice.
I also use that cheap PCB from rapid, but tend to need the bigger sheets.
Have you considered toner transfer, where you print the mirrored artwork off using a laser printer onto the correct type of paper, place it over the bare board, iron it on then etch as normal? I've used it quite a lot and you can do very accurate boards with a bit care. The best paper is the type that sticky labels peel off as the toner doesn't stick very well to it. The finest I did was an 80-pin TQFP as a test. PCB was fine but I couldn't solder it!
Anyway, clearly milling PCBs is superior. Whilst selecting the track outlines and offsetting them to create the toolpaths works, if you're trying to make fine board it can be difficult to ensure pieces don't overlap. So instead I convert the tracks to the minimum required lines to isolate each connection.
(drawn with funny pads/track widths to make conversion easier)
Unless you're doing fancy RF stuff the odd shape tracks shouldn't be a problem. It's also less cutting, so clearly tools last longer and it doesn't take as long to cut. The latter isn't really an advantage unless you're making several of the same as it takes a while to redraw it.
As long as you don't skimp on the stepper motor and drivers there's no reason to expect long term problems with missed steps. This is particularly important since you seem to have chosen a relatively heavy machine bed with dovetail slides, not rolling balls, so the total force required by the stepper motors (F=uC+ma) will be high compared to a moving gantry router if you want to get decent acceleration and speed, which you do for PCBs since they have a lot of short moves and changes of direction.
Re what you've done to minimize thecut lines...there's a free app out there called visolate - it's basically a midline voronoi region generator ...it takes a gerber as an input file & creates an NC file. It's poorly supported & the documentation sucks, but it does work ok (you just need java loaded on your PC with JAva's 3D libraries loaded too, it's explained a little better here...
Terrible stray capacitance though so forget it for anything over 1khz or so!
(I'm also in email contact with a chap who after a bit of communication to/fro, is soon about to lauunch a web portal where you upload a dxf & it'll generate the voronoi midlines & give you the file back with the midlines done for you)
Re worring about offsets.....in cambam you can import a gerber, fine tune the offset & see the cut width onscreen...the trick is to get the offset so that the midlines of ajacent pins of your smallest IC pitch so they almost share the same path. Difficult to explain, but look here...
the IC pads are black, the tool cut width is turquoise & the midline for the toolpath is the dark blue thin line in the middle ....it's important to set your offset to get these midlines to almost kiss, but not go over, else you'll be unintentionally cutting into the neigbouring pad.
Last edited by HankMcSpank; 12-03-2012 at 07:16 PM.
Thank you Jonathan. Yes, I'll make sure the steppers and drivers are beefy enough. I haven't done any CNC yet, but I used to plot PCB artwork on a Roland plotter and I know what you mean about all the stops and starts and changes of direction. Dovetails have more friction than ball bearings, but unless there's a similarly solid XY table out there with ball bearings, my guess is that it's easier to compensate for high friction by beefing up the steppers, than it is to compensate for a less-than-rigid (aluminium) ball bearing table of the kind I have seen being offered by the oriental suppliers, some of which look a bit flimsy to me. But I confess to a total lack of experience.
I hadn't even got as far as worrying about minimising toolpaths, and I'm going to have to print off your two examples and lay them side-by-side to ponder them. Obviously there are things to learn about this CNC business! Is there a good book out there which deals with subjects like toolpath optimisation?
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