Computing stuff tied to the physical world

CD drive as X/Y table?

In Hardware on Aug 20, 2009 at 00:01

(Warning: this post is a digression into robotic tools…)

I’m still struggling with the assembly of JeeLinks: after 4 attempts, none of my boards work and each with different results – not good :(

The answer is probably to have SMD stencils made to apply the solder paste evenly and in precisely the right spot. Manual parts placement with a tweezer is tricky but it seems to work ok for me.

But I’m hesitant. Stencils add a considerable cost overhead to each pcb run, and I’d like to be able to do lots of prototype runs in the future.

The other way to do this is to automate. Unfortunately, SMD machines are expensive – even the manual-assist or “semi-automated” ones are in the multi-$1000 range. And I don’t really need a CNC router with extreme precision or a 50x50cm work area plus a full 3D Z-axis. I just need to deal with printed circuit boards of say 3x12cm and position them within 0.2mm or so. Plus a reasonably accurate automated solder paste dispenser.

Hm. Wait a minute. That’s essentially the range of a CD/DVD drive: the tray comes out about 12 cm, and the laser inside moves about 3 cm. Why not take apart an old CD drive and mount the laser motion system on top of the tray and perpendicular to it?

Here is an old Philips drive I still had, disassembled:

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Here is the drive, “reorganized” to perform X/Y positioning, with both actuators at the opposite ends of their respective travel:

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Here’s another one from AOpen:

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A close-up with the laser platform turned and on top of the tray:

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The motors in both drives create a smooth and not overly fast motion when driven by 1.5 .. 3V, but the tray motor from AOpen draws much more power (about 250 mA).

The main idea is that such a setup can move the laser platform over an area of 3x12cm. But instead of turning on the laser, some sort of pcb holder could be attached to it so that the board is positioned. Then, all I need is a syringe mounted in a fixed spot over this X/Y table and some move-down-and-dispense mechanism.

Voilá – a dirt cheap automated solder paste applicator for small PCBs!

Given that there is no sideways strain, I hope that the tray/X-axis positioner (which has some slack) can be made accurate enough for solder paste by spring-loading it. The laser/Y-axis accuracy should be ample without much effort.

One problem with both drives is that they use linear motors instead of stepping motors, so there is no easy way of figuring out the current X/Y position.

Hm again. Could we somehow accurately measure the position of the X/Y table? Adding quadrature encoders to both rotating shafts would solve it, but that means hacking the mechanism – which I’d rather not alter. Not to mention getting suitably small encoders.

So here’s a second idea: the X/Y table positions the pcb, but perhaps it could also move around a bit of graph paper, attached alongside the board. And then… maybe an optical mouse could be hacked to detect the changes? Though I don’t know how hackable optical mice are. Another option might be to use IR proximity sensors, but that’s pushing it and would require precise calibration.

The third issue is the solder paste dispenser. Some sort of syringe, with the plunger tied to a servo, perhaps?

There’s a second, simpler, use for such a system: a PCB inspection system. Just place a loupe or microscope on top and meander the board underneath it.

I’ve even got an old Ikea drawer to fit the entire thing into:

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I know, I know, it’s crazy. But still… ideas are cheap, and so far my materials cost is zero!

  1. If this doesn’t work out you might wish to try Lego Technic, or fischertechnik.

  2. I have tried similar “crazy” ideas :P before I finally bought a Taig 4 axis bench top CNC machine.

    I have combined an old scanner and an even older Epson dot matrix printer for the X-Y table. http://acholic.turningaway.net/pics/XYPlot.jpg

    The main problem I see is controlling the solder paste. The kind that I have is really thick and usually requires a substantial effort to manually dispensing it out of a syringe even after adding a fair amount of thinning paste flux.

  3. Maybe this could be used / adapted… http://www.thingiverse.com/image:4316

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