Eclipze's SMD Pick'n'Place Build....

[Eclipze]

It wouldn't be to difficult to add a dolley wheel setup for vertical stability, or to use a rod and bearing for full support. I have specifically made provision to support this, as shown in the picture. But until it can see the necessity, I want to avoid over engineering the design and making it more complexity. It will mostly depend on the acceleration used, carriage mass, flex in the Y-Axis rail and the stability of the supporting X-Axis.


I'm interested in the Auger system, however it seems a rather messy mechanism to deal with. I'II post a drawing of the system I'm thinking about for paste dispension... as soon as I draw it hahaa...

I re-machined one of the brackets, as I didn't provide adjustment for the C-Axis stepper and it's mating with the gear. The carriage holes were also mis-aligned - blunt drill bit wandered.

The biggest issue I've had so far has been the stepper coupler. I had a massive 0.3mm offset error, which wobbled. That is a massive oscillating error introduced into the accuracy, so I had to machine another paying particular attention to getting the hole pockets perfect. It really is a part that should be done on a lathe. The new one was within 0.1mm before I used a dremel to carefully grind the pockets with constant checking. Managed to get the offset down to 0.01mm Took me a few hours though The X-Axis coupler is much larger/deeper... I'm going to have to get that done on a lathe.

[phomann]

Hi,

I also have a positive displacement paste dispenser. A paste syringe is
screwed into the unit and has a small +ve air feed into the end of the
syringe to help feed the paste. The unit has a small DC motor that runs
contineously.

The paste is fed by an auger (looks like a 3mm ballscrew). The auger is
driven by the DC motor via an electronic clutch. To dispence the paste,
activate the clutch, to stop, deactivate. This head is off a
commercial machine and is quite heavy. It's about 10" tall 1.5"
diameter. Also has a mic adjustment on a dovetail slide to set the
nozzle height.

Haven't played around with it much but it does seem promising.

[scsi]

The auger is driven by the DC motor via an electronic clutch.

It always surprises me how some engineers would do anything to avoid using stepper motor. Also, when I see an instrument of almost any kind with a stepper or two in it, it typically costs over a thousand. Why it's so hard to use a stepper?

-scsi

[Eclipze]

Hahaaa... I know what you mean.

This is just a rough concept of what I've been thinking of implementing for the solder paste dispension. The yellow rod through the stepper is a threaded, fine pitch.

I've only been thinking of having a small solder paste reservoir (for example, 2mL), for a few reasons. I'm concerned about long term vibration separating the paste components for a large amount. I want to use a stepper that isn't too heavy/large to plunge the paste (low torque). More accurate control over a smaller amount (actuation length vs total volume). Finally, it's something that I'd use for small jobs. Anything that would use a lot of paste (multiple board) I'd just make a stencil.

What I'm looking to achieve is having a paste block I can easily un-dock and put back in the fridge. The approach shown has two bolts to remove the paste holder, and the rod can be driven straight out of the stepper so I can leave the rod with stopper in the top, which means less mess and avoids the unnecessary introduction of air from removal of the stopper.

Having a thin/narrow reservoir will give me a higher precision with dispension, as well as helping to support using a stepper with lower power.

What I actually need to do with this design is make a slightly squarer profiled stopper, which will importantly prevent the rod from free rotation. The shaft isn't keyed, and if the rod is allowed to rotate it won't traverse up/down. This approach avoids having to provide a more complicated method to stop the shaft rotating (less weight, less complexity).

Using a linear stepper will hopefully give a nice consistent control, as well as supporting a suck back function.

Thoughts, ideas?

[frazz]

Instead of a syringe-type dispensation of the paste, why not build a mini-stencil with a simple solenoid to move a little squeegee? You'd need a syringe to dispense more solder paste onto the mini-stencil, of course, but it wouldn't need to be nearly as accurate as if you were dispensing the paste onto the pad with the syringe directly as the mini-stencil would keep the paste in a uniform shape and thickness.

I was just browsing threads, I don't have a lot of experience with PnP but I'm familiar.

[Eclipze]

Component pads on the board are much to close to use a small stencil, one pad or one component footprint at a time. Apart from mechanically complex to have a mini stencil, stenciling is messy work. You really need to stencil a whole PCB in one go.

Using a dispenser like this has limited application, whereby some fine pitch rows of pins you cannot do individually because you can't deposit very small thin amounts cleanly. But there are techniques to draw lines of paste across a row, although can involve re-work. The dispenser however will be fantastic for doing a limited amount of work, where you wouldn't want to go to the effort of making a full stencil. Much better than manually paste dotting by hand!

[scsi]

The yellow rod through the stepper is a threaded, fine pitch.

When you first said "screw based dispenser" I thought you meant auger valve.

Using a linear stepper will hopefully give a nice consistent control, as well as supporting a suck back function.

So, this type of dispenser is called a "positive displacement pump" or syringe pump. I played with it a lot and can say that auger is ~100x times better for micro-dispensing for many reasons. First being repeatability and close second is ease of use. With some of the best linear actuators of the kind you have in your 3D model (can stack) you can hopefully get 0.0005"/step resolution but there is a huge backlash in the nut/rod combo that would cause serious problems when you want to perform pullback at the end of each displacement cycle. This backlash can be as big as 10-20 steps and returning back to the original position will be challenging to say the least. This kind of pump is very good for continuous dispensing but a nightmare for dots. I have three motors like this laying around with a very fine pitch shafts - let me know if you need some.

Another issue to be aware of with syringe pumps is how are you going to re-fill the syringes. Getting rid of bubbles is one of the challenges.

One more issue that comes to mind is elasticity of parts in a typical syringe. The thing has to be as rigid as possible to prevent oozing. Yet there is a rubber stopper, air pockets and plastic plunger that tend to flex/compress with every dispense cycle. Definitely requires some generous pull-back routine to get rid of the oozing.

-scsi

[Eclipze]

I wasn't too accurate when I mentioned screw type... darn terminology :-) Wasn't thinking of an auger at the time as a method of implementation.

Understand the problem with the syringe flex areas, as well as how air bubbles (apart from misfires) will introduce more flex into the system. It's one of the reasons why I'm not keen to use the actual syringe parts, or to use air pressure dispensing.

The reservoir would be a block of acrylic, which being that thick won't flex with significance and provide clear view on contents The grommet in the design is an aluminium part with a rubber O-Ring.

I'II have to look at the precision of the stepper with respect to volumetric dispensing resolution. I can see where the auger has benefit mechanically, however it's a physically larger and more complex system. Also requires additional air supply to the head. Difficult to support such a mechanism with this feather weight design. I'm still hoping there is some middle ground where it's practical given a small diameter tube of paste.

Filling isn't something I have considered yet. Perhaps what I need to do is to keep the original dispensing tip intact/removable. Then I can remove the tip and inject paste using a syringe (via larger diameter hole channel). Shouldn't take long for only a few cc's worth and would avoid introducing air bubbles if done with good connection.

[scsi]

The reservoir would be a block of acrylic, which being that thick won't flex with significance and provide clear view on contents The grommet in the design is an aluminium part with a rubber O-Ring.

That rubber o-ring is concerning. There will be a lot of friction of rubber on acrylic no matter how well it's polished. The movement will be jerky and will cause a non-linearity. I'd stay away from rubber or use some Teflon encapsulated o-rings. They are using FEP or PFA for encapsulation - don't remember. This will reduce friction but won't eliminate it completely. In regular air syringes they use PE pistons on PP barrels or even PE barrels at times. This pair has much better friction properties and the movement is more or less smooth.

Don't worry about the non-captivated shaft on that linear motor. If you secure it to the piston well enough there will be enough resistance between the piston and the barrel to prevent the shaft from rotating.

Filling isn't something I have considered yet. Perhaps what I need to do is to keep the original dispensing tip intact/removable. Then I can remove the tip and inject paste using a syringe (via larger diameter hole channel). Shouldn't take long for only a few cc's worth and would avoid introducing air bubbles if done with good connection.

Yes, they fill syringes from the nose, but they have to evacuate the air from that nose before they introduce any solder paste (or other material) into the syringe. Otherwise there will be enough air trapped just below the piston to cause trouble later.

-scsi

[Eclipze]

Hmm... very good point about the friction! Perhaps an alternative approach would be to use the tube of a small syringe as the sleeve.

I could still possibly use the plunger rubber (instead of O-ring), however when you mentioned about evacuating the air I had the thought of have a small air bleeder screw in the plunger head.

[scsi]

Okay, since you're not giving up the idea of using syringe pump yet, here's the site you need to study: http://www.fishmancorp.com/
Lot's of good information in there on barrels, pistons and such. They can also sell a box of barrels with pistons for $30-$50. Their pistons are gugged enough and designed for motor actuator. The piston has a standard male luer thread in it, so it's easy to make a coupler to the motor shaft. You'll notice that they use can stack motors as well. I believe from Haydon.

-scsi

[Eclipze]

Nice find!

They have a white paper that shows diagram on how they have implemented their dispensing system.
http://www.fishmancorp.com/download/LDS_Technology.pdf

Same approach as I'd like to take. Still needs some thought on the implementation, however I do like this approach.