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

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.

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!

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Originally Posted by Eclipze

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

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

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

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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

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.

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Originally Posted by Eclipze

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.

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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.

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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.

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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

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.

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

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.

Pics of the new (aka far more accurate) Y-Axis coupler...

BTW... there is a reason it isn't round. I need to machine the slot in the bottom, so I first mill tight cavity in wood to press the coupler into upside down... and being an odd shape, won't turn/move while being machined.

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Originally Posted by scsi

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

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My guess is that in it's day it was a cheaper and more reliable solution than steppers. Neither you or I know what the requirement were for the 'Engineers'

Cheers,


Peter.

The linear stepper I have has 15 degree step. By experimentation with a 30g paste syringe (2/3 full), I determined you need at least 6N force before the paste would move, and around 9 to 11N for confident dispension. I can achieve 10N with this linear stepper via half-step drive, however with the 10-bit driver it drops to near 2.5N.

Using half step, the linear stepper has a step resolution of 0.0127mm. I have calculated a half sphere dome of paste, 1mm in diameter to require 0.28mL. In order to get the minimum resolution, 1 half step = one 1mm pad, the tube needs to be 5.3mm in diameter.

Given in application this linear stepper would give me 12mm of travel, the total volume of the reservoir is a mere 0.26cc. That will get me 945 dots, and given each board will probably use in the order of 100+ dots, this volume wouldn't last long. I was previously thinking around the 2 to 3cc size.

Would be good if someone could double check if my calculations are in the right ballpark please.

Options.
- Not keen to put a more powerful/heavier linear stepper in the system.
- Pre-loading the screw with weight might be advantagous, as it could reduce the torque requirement and I might be able to use the 10 bit driver. That is assuming this force will translate through the thread to linear movement, not just increased friction. Weight is a design constraint though, so it may need to be implemented as a spring under compression.
- I could find a longer/compatible thread to triple the plunge distance.


Somewhat thinking it might be best to make a bracket to hold a 5mL syringe and the linear stepper rigid, so I can see what happens experimentally.

Great thread!

I can't imagine ever needing a pick and place, but I'm learning a lot. I'd be more likely to build one of your dispensers for other purposes like reprap-style additive machining.

Cheers,

BW

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Originally Posted by Eclipze

The linear stepper I have has 15 degree step. By experimentation with a 30g paste syringe (2/3 full), I determined you need at least 6N force before the paste would move, and around 9 to 11N for confident dispension.

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Was that measured with dispensing tip or without? What ID of the tip are you looking to use?

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Using half step, the linear stepper has a step resolution of 0.0127mm. I have calculated a half sphere dome of paste, 1mm in diameter to require 0.28mL. In order to get the minimum resolution, 1 half step = one 1mm pad, the tube needs to be 5.3mm in diameter.

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My calculations come down to 0.26uL for the half-sphere dome 1mm dia. Keep in mind that these can stack steppers don't like microstepping abuse.

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Given in application this linear stepper would give me 12mm of travel, the total volume of the reservoir is a mere 0.26cc. That will get me 945 dots, and given each board will probably use in the order of 100+ dots, this volume wouldn't last long. I was previously thinking around the 2 to 3cc size.

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Why only 12mm of travel? Is this captive shaft motor? I have some Haydon motors with 4x better resolution and long travel. Can mail you one if you like. Here's the spec. Mine's non-captive variety.

There is a smaller stepper with the same resolution as yours (0.001"/step) selling for a nice price: http://www.mpja.com/prodinfo.asp?number=17284+MS

Actually, the prices directly from Haydon are not that scary either. For most can stack motors it's well under a hundred. For hybrids they charge a lot more.

-scsi

What do you think about peristaltic pumps? They are capable of precision pumping and have simple construction.

However, I think solder paste may need some additional pressure pushing it instead of just tank.

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Originally Posted by Xerxes

What do you think about peristaltic pumps? They are capable of precision pumping and have simple construction.

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Don't get me started on this! Peristaltic pumps are not meant to be used for precision pumping and cannot produce small amounts with any kind of accuracy. Small amounts being anything smaller than several peristaltic pulses. And yes, they all produce pulsing output. Also the tubing ages and wears out with usage and the throughput drops over time in rather unpredictable manner. And last, but not least, peristaltic pumps use flexible tubing and we are trying to eliminate any flex in the dispenser to prevent oozing/drooling of the paste. In my experiments with auger valves I was surprised to find that even plastic dispensing nozzle flexes enough under pressure to cause oozing. Before augers I was attributing all oozing to the flex in syringe/piston/air bubbles but with all these factors eliminated in auger, some oozing is still there and it's due to the slightest flex in the nozzle.

-scsi

I think a syringe-type dispensation would be more accurate if the paste came out horizontally rather than vertically onto the pad. An L shaped nozzle with a square opening would give you a good shaped paste bead and maybe a rotating slotted wheel to cut off the paste at the end and avoid a dilly-bar like swirl.

Finally have a datasheet for these :-) You da man scsi!

The one from MPJA is the same model as I have. My linear stepper is a Haydon 20841-05-006, which is a 15 degree with a linear step of 0.001". I believe it's 40 TPI thread.

I haven't considered actual tip size, only the volumetric reduction of the stopper within the syringe.

Your steppers are definitely nice, more powerful and more precise. I really wished they were smaller, as it won't fit well with this design. Appreciate the offer though :-)

These size 8 hybrid models from Haydon are verrry nice! I contacted the Australia distributor to get a price... ~$244 + tax and freight. Don't look so nice now :(

If I remove the requirement to hold the top of the screw from rotating, then the travel is going to be ~24mm. As you say, the syringe stopper is likely to hold it from rotating anyway. In which case, the volume increases to 0.5cc of paste - more respectable.

I'm going to setup the stepper with a small syringe to see how it actually performs. Will have to find a 1cc and 3cc syringe first. Choice pending step-stroke vs volume.

I have been thinking that this stepper may need a more complicated driving strategy to avoid the heat, and may use go one step further with over driving for dispension. The heat rise on these at 100% power is apparently 75'C. Doesn't take too long to get too hot to handle, which isn't good for the conducted heat to the paste. I may try driving 200% power for dispension movement, with 20% holding current (if needed). Both to increase step movement force and also to minimise total power exerted in heat.

Told you not to look at hybrids. :)

Also, I don't want to discourage you or something. Just get ready to face a situation when you'll have to perform dozens of steps both ways to make a successful and repeatable dispense. There is backlash and compliance and flex and friction in the system. Don't assume that one step forward gets you the material at the end of the tip. It never will.

I was driving the 200 series motors (like yours) with 800mA peak current from a 12VDC rail with no adverse effects. Just make sure you have at least 5:1 duty cycle and they should handle 1A.

-scsi

I couldn't help myself looking at the hybrids :rolleyes:

I was thinking of monitoring the current used by the servo to determine the weight change and use an automatic volume/weight dispension control loop. So I can meter the drops.

Only kidding hehe... I'm starting to realise the difficultly associated with flex/oozing etc... Very helpful understanding/avoiding or mitigating the potential traps.

I can see it's not going to be straight forward, not x steps = 1 dot. I'd certainly prefer a high torque, high resolution linear drive mechanism to make the control easier to deal with. I can only hope that limiting reservoir diameter and over driving the stepper for plunging will compensate enough that I can implement a suitable step control strategy. One that is consistent and also quick.

I rigged up a 5mL syringe with a very crude setup, just to get some experience on what the stepper could do and how the flex in the system responded. I initially filled the syringe with ~2mL via another solder paste syringe (with needle), so managed to avoid air pockets. The stepper didn't have the power to suck paste up from the jar (admitted cold from the fridge). However with a bit of assistance it did.

There was "relatively" a lot of flex in the system to produce a small spot. However I found I needed to introduce a step down, delay and step up. Because of the delayed response inherent with the flex, it was really important to get the right amount ejected. Just to avoid ejecting too much on the first squirt, and leave the second one short. I needed to bump the voltage up to ~9.5V to get consistent and prompt movement from this stepper (about 1 second worth of traverse movement). However since I'm dealing with half step movements, I powered off at the end of a squirt to avoid over heating. With a better mechanism it should operate quicker, as there is a lot of compensation for flex.

After playing with the timing a fair bit, I managed to achieve reasonably consistent drops with no persistent drooling. I suspect I was dropping some steps here and there, however difficult to measure without a proper setup. At least stepping resolution wasn't much of an issue with so much flex in the mechanism hahaa...

Going forward... I definitely want to go to a narrower vessel, and one specifically designed to minimise flex at all points. But otherwise I'm a fair bit more confident the the stepper will cope with the load (over driven) and the resolution is within the usable range (half stepped).