Laser "projector" for PCB mask creation

[Orum]

I'm an electronics hobbyist, and would love to find a better way to create one-off and prototype PCB boards. I've done toner transfer, and abhor it. UV sensitive paint transfer works much better, but has its own share of problems. But, some time ago, I found someone who used lasers with copper blanks painted black to create a etching mask (of course, I'd much rather use a pair of glavos than his 2-axis table setup). Now I finally have the time and impetus to construct such a device, I'm looking for some advice on a few topics I know little about.

Laser

• Wavelength? He uses a 100mW 405nm laser. While black paint--by definition--absorbs all visible wavelengths, is there a compelling reason to use blue/violet laser over a red laser? I've read that blue lasers are more readily absorbed than red lasers in most materials, but does this hold true with black paint? Also, supposedly red lasers are "cleaner" (rounder) when slightly out of focus (see footnote *)--is this true in your experience?
• Wattage? I was going to go with 100mW, as that's what he uses. Is there any reason to use a higher wattage laser, other than speed? Any reason to go with a lower wattage, if I can get away with it?

* The ideal distance between the laser and galvo, and galvo and PCB will be crucial. It's something I'll likely have to determine after I have all parts in hand, and even then it will depend on the radius of the circuit being printed. Regardless, I'll still obviously have a slightly varying distance where the laser need to be in focus or very close, given by sqrt(a^2+b^2)-a, where 'a' is the distance between the galvo and board and 'b' is the radius of the circuit design.

Galvos

• Where/What to buy? It seems most galvos at reasonable price are found on eBay and are all manufactured in China, like this. They claim to be closed-loop, which I will want to avoid issues with hysteresis, but are they really? What is your opinion of them? What about the drivers routinely included with them? Lastly, some include a "show card"--I assume this is for running laser shows and is completely unnecessary for my needs?
• Angular resolution & precision? All of the galvos I see are rated on PPS, which is of no use to me--10PPS should suffice! For my application, angular resolution and precision are critical. While I'm sure this depends a lot on the DACs, what can I expect with Chinese "closed loop" galvos?

DACs

While I think I can get by with 16 bit DACs for smaller boards or boards with only larger components, but I'd much rather use 24-bit DACs for more flexibility. What do you recommend?

Safety

• I'm planning on enclosing everything in something the laser can't burn though, like aluminum or steel. However, I'd still like a window to look through to verify operation. Where do you recommend getting acrylic laser-blocking flat panels? I've searched, but most places sell it in larger quantities/size than I need, for wavelengths I don't want to use use (IR), or are extremely expensive.
• Any tips on safely focusing a laser of such power?

[VDX]

... for galvo-scanning you'll need a Theta-optic to receive a horizontal focus plane, what's the most expensiv part.

I'm using diodes with 405nm@200mw, 445nm@1w or IR-diodes with 808nm@1W and 975nm@5W and @9W - here some old infos to my diodes: http://reprap.org/wiki/Laser_Cutter

The vid in your link shows a poor focussing setup - ye shorter the distance from the lens to the focus, the better/smaller the spot.

Most direct used diodes has a 'bar'-shaped beam, so simply refocussing the diode wouldn't result in a round spot - see at the attached images here: http://forums.reprap.org/read.php?27...402#msg-155402

For this I'm mostly using fibercoupled diodes - here the fiber acts as a 'homogenisator' and gives a perfect round beam, that can be refocussed to the diameter of the fiber with a 1:1 optical setup.

When refocussing diodes directly I'm using a pin-hole, but here I'm loosing a part of the power ...

Viktor

[mixedgas]

It might be cheaper for creating PCBs to just use a x-y motion stage and a microscope objective to focus the laser.
You can then use a UV diode with photo-resist coated boards, or a IR diode to burn the black paint.
No F-theta, and I know you can probably find code from PCB Gerbers to G-code.

Where the Chinese usually skip on quality is the position sensor in the galvo system. You want the 30K amp. Why? Its settling time and bearing precision will be much, much better then a 75$ pair of 10K knockoffs.

Take a look for Cambridge Technology 6450s or 6860s or similar on Ebay, you might like what you find. Too big a mirror for most laserists, and very precise. You will need position sensing amps, unless you use G120DTs or or G330DTs which have torsion bar centering.

Discuss

These are very precise but not that fast. Right up your ally, and they have the amps:

http://www.ebay.com/itm/Cambridge-Te...item1c2f7717c3

Steve

[Orum]

... for galvo-scanning you'll need a Theta-optic to receive a horizontal focus plane, what's the most expensiv part.

Yeah, I looked at these and you aren't kidding about the price. My hope is the beam will be narrow enough even when slightly off of perfect focus that it won't be a problem. Most of the boards I make are quite small anyway, and I suppose if I ever do need something large I could burn up to a certain radius, refocus, burn again, etc.

I'm using diodes with 405nm@200mw, 445nm@1w or IR-diodes with 808nm@1W and 975nm@5W and @9W - here some old infos to my diodes: http://reprap.org/wiki/Laser_Cutter

Yeah, I'd like to avoid IR for safety reasons, but if you've had good luck with blue diodes as well I'll go with those. They're easier to find in higher wattages than red anyway.

For this I'm mostly using fibercoupled diodes - here the fiber acts as a 'homogenisator' and gives a perfect round beam, that can be refocussed to the diameter of the fiber with a 1:1 optical setup.

Definitely something I didn't think about. My only worry is they're harder to find and more expensive, but it looks like they're typically well made.

It might be cheaper for creating PCBs to just use a x-y motion stage and a microscope objective to focus the laser.

Yeah, which is what he did in the article. But as you can see from this blog post, hysteresis becomes a big problem. While you can get around it to some extent with software and basically eliminate it with pair of linear encoders, it's more expensive and more work, and if I can get accurate closed-loop galvos then I could just avoid it completely.

You can then use a UV diode with photo-resist coated boards

Hrm, I actually didn't think of this. However the main reasons I want to move away from the UV-sensitive paint are the storage/aging problems I've had with it, and how long it takes to get (have to order it online) and the expense. I can get black paint locally and cheap, and I don't have to worry about storing the prepainted blanks in a dark area, etc.

Take a look for Cambridge Technology 6450s or 6860s or similar on Ebay, you might like what you find. Too big a mirror for most laserists, and very precise. You will need position sensing amps, unless you use G120DTs or or G330DTs which have torsion bar centering.

Thanks for the suggestion! I'd looked at some more scientific (as opposed to "high speed laser show") galvos and most were several grand. These on the other hand are still in my price range.

[mixedgas]

I'm not so sure your example blog used the best available cheap technology.

I work with capacitive sensors with nano-meter precision once in a while. its not hard to measure at femtofarad resolution with op-amps and a 10 Khz triangle wave excitation. What I'm saying is hobby feedback schemes never seem to match even the simple stuff that is available cheap. That would let you ditch the belt errors, which are horrid in those photos.

Example: Interpolation chips for encoders are 9-20$ each, and that knocks off the hysteresis and gives you a 16X or 32X boost in resolution, as well as storing the current position, glitch free.

Blue 1 watt laser diodes have horrid spot size, and the spot is oblong. This is true of any diode to a point. You'll need some means of correction for this. You can find single mode ones once in a while, at say 25-75 mW.

Steve

[macona]

Yeah, which is what he did in the article. But as you can see from this blog post, hysteresis becomes a big problem. While you can get around it to some extent with software and basically eliminate it with pair of linear encoders, it's more expensive and more work, and if I can get accurate closed-loop galvos then I could just avoid it completely.

That is not hysteresis, that is backlash and lost motion. Hysteresis you might see in a servo loop, not in this. Backlash is just caused, in his case, by poor mechanical design. Use good linear bearings and precision ground ball screws and you will be good for fractions of a thousandth of an inch. Heck, even rolled ball screws are typically accurate to .003" in a foot. In an light weight system like this you can even use timing belts, but they must be tight, I mean TIGHT. Play music on them tight.

I will come out and say it the mechanical design of a reprap is crap. A reprap was made to be reasonably cheap to make and uses cheap parts. Using hardware store all-thread and acrylic does not make a rigid or accurate machine. Garbage in = Garbage out.

I say ditch the idea of using a laser diode, they suck for this kind of application. Get a cheap chinese CO2 tube and power supply and build a machine around that. If you use blue you need an opaque box to put it in. Most black acrylic is actually transparent to NIR laser diodes so you would have to build the box out of metal with that or find a specific plastic that is opaque to NIR. With CO2 almost everything is opaque to it. You could build the entire box out of acrylic and the beam could not escape (I still wouldn't, though), well, not till it burns though. It takes odd-ball materials to be transparent to 10.6u like germanium, silicon, and GaAs.

A chinese laser tube is cheap! About $120 for a 40w tube and another $120 for a power supply straight from china. And they are easy to run. Dont need to worry about static killing a diode. The power supplies are designed for PWM control too.

Check out the guys over at buildlog.org. A lot of people are building their own laser cutters and he has an open source design for one. Most of use use mach 3 to control it.

I have built and retrofit many CNC machines, some pushing almost a couple tons and use 1kw+ servos on all axes. I have a couple sets of galvos that I want to use for marking so I am in the same boat as you. But I dont think you want galvos for this project. To get even a decent sized board you are going to need a lot of deflection or distance between the head and the work which means it is going to be way out of focus at the edges. You wont have this problem with a well built x-y platform. Galvos are for speed which you wont be able to use with a little 1 watt blu-ray diode.

This is my solution when I need boards: http://oshpark.com The boards are incredible. Made in the USA, double layer, gold plates, purple conformal, silkscreen for both sides. $5 a square inch and you get three boards for that price.

[VDX]

... I was building and refining/refurbishing differnt types of CNC-mills and laserplotters, have one of this chinese CO2-laserengravers sitting on my workbench, waiting for an alternative electronics/controller.

For laser-engraving/cutting I'm actually using my mill with different laserheads, but have plans to equip a really solid/rigid flatbed plotter with lasers - attached some images of the plotter and a 445nm-diode and the optical head of an IR-fiberlaser.

Attached too images showing the absorption rates for different materials and wavelengths ...

[macona]

VDX, is that a diode pumped fiber or just a regular fiber coupled diode?

Thanks for posting those charts. They are what I have been looking for a while. I can see why my q-switched 532 yag is used for marking gold and even though my 532 is more powerful than my 355nm laser the 355nm might be actually better for steel and stainless.

But in the end, even 9W CW is not much to do anything even with the extra absorption for some materials at the shorter wavelengths.

[VDX]

... the bigger 'head' is the end of a fiberlaser pumped with around 160Watts @975nm and emitting up to 50Watts @1070nm --- it's focussable to <10 microns, so good enough for cutting thin steel sheets or engraving all sorts of materials ;-)

Have fibercoupled 25Watt-diodes too, but with a 0.1mm-fiber only focussable to 0.1mm best :-/

Viktor

[macona]

... the bigger 'head' is the end of a fiberlaser pumped with around 160Watts @975nm and emitting up to 50Watts @1070nm --- it's focussable to <10 microns, so good enough for cutting thin steel sheets or engraving all sorts of materials ;-)

Have fibercoupled 25Watt-diodes too, but with a 0.1mm-fiber only focussable to 0.1mm best :-/

Viktor

Now that sounds like fun! That must have cost a small fortune.

I have a bunch of the 50w FAP modules with 800um fibers. I am hoping I can use them to drive a Spectra Physics T70 head when I can find the right cables.