mikeselectricstuff wrote:I'm just about to start looking into converting an old X-Y plotter into a paste dispenser for low-tech PCBs. I was thinking of just using a standard air dispensing system with controlled air times - I assume the auger head has a screw type element inside - do you have any info on the design of your head?
mikeselectricstuff wrote:I assume the main issue with air based systems is uncertainty due to changes in viscosity versus temperature/paste age.
From the point of view of simple engineering, I wonder if there would be any mileage in something like a very small peristaltic pump, using small -bore silicone tube. A nice advantage of this is that when it clogs you just replace the tube.
This was the very first thing I tried and it worked to some degree. Didn't last long though because the flux was attacking the screw... Here's one of the the first prototypes that was pumping grease successfully. Looks very ugly because I hacked it together in under one hour. See that metal screw on the bottom?As regards auger type dispensers, I wonder if you could use something as simple as a woodscrew as the drive element?
scsi wrote:mikeselectricstuff wrote:I'm just about to start looking into converting an old X-Y plotter into a paste dispenser for low-tech PCBs. I was thinking of just using a standard air dispensing system with controlled air times - I assume the auger head has a screw type element inside - do you have any info on the design of your head?
That's right, it's got a screw inside driven by a small stepper motor. Weights around 250g and has a very small footprint, smaller than a regular auger dispenser. I think a plotter should be able to handle it no problem. Shoot me an email if you want to beta test one.
For the time-pressure dispensing, change in viscosity is an evil but not the biggest. The amount of air necessary to build up the pressure in the barrel varies as the syringe empties.
This was the very first thing I tried and it worked to some degree. Didn't last long though because the flux was attacking the screw...[/quote]As regards auger type dispensers, I wonder if you could use something as simple as a woodscrew as the drive element?
mikeselectricstuff wrote:Not sure my plotter would manage 250g! - I'd be interested to see the shape of the screw etc. and the general mechanical arrangement, e.g. how do you segregate the paste from the motor? A magnetic coupling might be nice if you could make it work.
Regarding auger type drives, I was thinking more along the lines of a small gearmotor to reduce size and weight compared to a stepper - maybe use the motor and box from an RC servo as a nice small light gearmotor, probably with an encoder for feedback.
Yes, simple linear motors are being used for coarse dispensing. For example http://www.fishmancorp.com/download/LDS_Technology.pdf. These cannot do microdispensing though. The resolution is not there.mikeselectricstuff wrote:I also wonder if there may be scope for something really crude - a simple (high-res) leadscrew into a syringe, like an infusion pump - this would eliminate the air volume issue. Maybe use a fairly small-bore syringe to reduce the 'gearing' between piston and paste movement. I also wonder if hydraulics may have advantages over pneumatics.
Closed loop pressure control is quite difficult to acheive in very short intervals (milliseconds). Some advanced time-pressure do exist. Martin Clever Dispens is one example: http://www.martin-smt.de/en_p_d_cd.phpmikeselectricstuff wrote:I wonder to what extent you could reduce this effect by upping the pressure and having precise timing (although the latter may get difficult). Or maybe have closed-loop control of pressure to effectively compensate for volume changes.
Screw corrosion was not the only problem. Precise positioning and centering of the screw was challenging. For solder paste dispensing the auger screw cannot touch the walls and some stable gap has to be maintained. Otherwise, it will squish solder balls and weld them together which will eventually clog the nozzle.mikeselectricstuff wrote:Stainless screws are readily available, and even if corrosion is a problem, if you could figure out a way to use an unmodified screw, you could just treat it as a consumable.A hex socket, torx or hexagon bolt head comes to mind, or maybe something like this
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