Dispensing Solder Paste Automatically

Through-hole chips are slowly falling by the wayside, and if you want to build something with new parts you will be using surface mount components. This means spreading paste and throwing it in the toaster oven. Of course, if you don’t want to take the time to get a stencil for your solder paste, you can always lay it down by hand. For that, [owhite] has created a tiny, handheld, robotic solder paste dispenser. It’s a robotic pen that dispenses just the right amount of solder paste on your pads.

The design of this solder paste dispenser is basically a syringe filled with paste and a stepper motor to push the plunger down. Devices like this already exist, and the i-extruder can be had for somewhere around two hundred bucks. Why buy when you can build, so [owhite] set out to create his own.

The key to a successful solder paste pen, it seems, is driving the plunger with a small NEMA 8 stepper motor, using a very fine pitch on the threads of the gears pushing the plunger down, and surprisingly finding a small-diameter syringe. [owhite] found the last bit in the form of a gas-tight syringe with a nylon gasket. The electronics consist of just a Teensy 3.2, DRV8825 stepper driver, footswitch, and an OLED for a UI.

With just a few parts, [owhite] managed to create a solder paste pen that’s better than the commercial i-extruder, and with a bit of practice can be used to place paste on some SMD pads.

46 thoughts on “Dispensing Solder Paste Automatically

  1. While we are on the subject we need something that will keep this solder paste usable when stood for months. My soldering iron and roll of solder will work just fine if I don’t use it for a year, when I turn it on it will work perfectly but this new fangled solder paste is a real pain, I buy a syringe full at lots of $ then next time I need to use it I have to buy another at lots of $ again and the dried stuff is something I haven’t found a use for.

    What do other people do to keep it usable or what alternative is there?

    I tried mixing it with flux again but it doesn’t work the same as it did originally.

    1. FWIW:
      After I use solder paste syringe, I dip the end in few drops
      of liquid flux and withdraw the plunger ~ 2mm to suck up a
      small qty of flux wetting the end and lasts several months.

      If you don’t have liquid flux handy can do the same with a
      little iso-propyl alcohol (IPA) & if I have some used IPA/flux
      cleaner waste left over from last few hundred boards I use
      that, just needs a little filtering.
      Cheers

    2. Surprisingly (and after a *long* search) it seems we Europeans are for the first time at an advantage here. I’m using Vieweg BLF 03 paste in a syringe and it officially has a shelf life of 6 months at room temperature. I’m at my third syringe now and so far I can totally confirm that claim.

    3. We buy the little containers (50g jars) of paste and not the syringes. From my experience the paste in the jars holds up better (longer), and you can stir it up good in there before you put that small amount you’re going to use into a syringe. After the job is done, we clean out the syringe (push the rest of the paste back into the jar, then clean the syringe and needle with isopropyl alc).
      There are even 15g kits from Chip Quik that are delivered with flux and tin separated, so you could mix the two as needed (SMD291SN15T4 for example).
      Buying the paste in syringes gave us a lot of headaches too, as the stuff would always “dry” out on us if not used for a few months.

      On top of that, we have a little fridge (set at around 6°C) where we store all our solder pastes and other adhesives.

    4. I buy the syringes and store them in the fridge between uses. After each use, I take the dispense needle off and put the screw on cap back on. Then I clean out the needle with isopropyl alcohol and a piece of wire wrap wire threaded through the needle a few times. I get up to a year of use out of the paste. After the paste is getting too old, it does not coalesce properly around the pads when you reflow the board.

      1. I simply use medical needles, they come with a proper cap and the area that comes into contact with air (and dry out) can be simply pushed out of the needle before use.

  2. Good work, but I would like it more if it wasn’t electronic. Some sort of purely mechanical apparatus where you can set the amount dispensed (the travel of the piston) in one click of it seems like a better fit for a hand tool, unless operator has arthritic fingers.

    1. In my former lab we had a syringe hooked up to a pressurized container – hit a foot paddle and compressed air was applied to the syringe – worked like a charm and with very good control over the applied amounts (until someone hit the silicone air supply tube with a soldering iron)

  3. What’s the smallest needle this could work with? Or how much pressure can the motor generate? I’ve tryed to use different syringe/needle combos for paste dispensing (manually pushing the syringe) but i’ve found that for small pad sizes smaller than 0603 passives and 1mm pitch connectors either the blob of paste gets too big, or if i take a smaller needle gauge i can’t push the paste thru the needle anymore. The paste is very fine and worked extremely well with a 70µm stencil for 0.4mm pitch QFN, so i’d say the tin-ball size should not be the problem with the needle gauge i tryed to use. (I even bought the Metcal 900-NK Needle Kit to try and find the smallest working needle, but now i’m much happier with using the stencil…)

  4. I buy my paste in a jar. I keep it in the fridge, sealed again in a ziplock bag with as much air removed as possible. It has lasted for a few years so far, and doesn’t look like expiring any time soon.

  5. Btw, for hand soldering, I use very fine solder and pre-flux the pcb (and QFP pins too) with a flux pen. My eyes are no longer what they used to be, so I use jewellers glasses over my normal computer bifocals. Use a fine tip temperature controlled iron.
    Again, no problems soldering 0603 an 0.5mm pitch parts. I find 0402 parts too small though.

    Having said the above, I now use a commercial reflow oven and a stencil where possible.

  6. Throwing this in as a wild idea, as I know nothing about reflow soldering or solder paste: insulin “pens” are nice mechanical devices designed to deliver as little as 10 microliters (100 units per mL, has 1 unit click-stops). Diabetics like my mother in law go through four or more pens per month.

  7. “Through-hole chips are slowly falling by the wayside, and if you want to build something with new parts you will be using surface mount components. This means spreading paste and throwing it in the toaster oven.”

    Surprised no one’s doing their homemade wave-soldering machine.

    1. Was visiting our board assembly fab last week. It was very interesting to see the new selective solder wave line (Ersa Versaflow) they installed, with selective pre flux and all.
      If someone comes up with a machine like that, affordable and usable for the low volume/ DIY market, i bow my nose to the floor. Normal wave soldering for DIY already is quite a strech. It sounds pretty simple from the outside, just melt a lot of solder and pull your board underside thru. But in reality, stuff like preheating, temperatures, speeds, soak times and shielding of parts that should not get in contact with the solder wave make it quite difficult to get a reliable and good result.

  8. “Through-hole chips are slowly falling by the wayside, and if you want to build something with new parts you will be using surface mount components. This means spreading paste and throwing it in the toaster oven.”

    I reject the 2nd sentence. Yes, some parts require reflow, but most surface mount components are easily soldered by hand. Obviously it makes no economic sense to mass produce this way, but small batch one-offs and hand soldering go well together.

    1. Even for a batch of five boards (OK, with thermal pads on some chips) it saved much time. Although I had no time to wait for a commercial stencil. So we cut it out of overhead transparency film on a laser cutter. Without access to the laser cutter it would really have been difficult.
      The vacuum tweezers were self-built and the temperature control of the oven was done by estimation and close watching of the melting of the solder, which of course was lead-containing. In my opinion lead-free solder is only good for industrial manufacturing.

  9. The 5V non-captive NEMA-8 linear stepper (20x20x30mm, 43g wt., linear 2mm per revolution, 1.8 deg./step, 4-wire) linked from the build notes is $98.50 USD in unit quantity. Ouch! I seem to remember seeing another attempt at a paste dispenser build similar to this that used a syringe (or was it a pipette) and a lead-screw and runner from a stepper salvaged from an old floppy disc drive. Of-course the link eludes me (I’m lousy at archiving old Web stuff – sorry). I think the linear displacement using a salvaged FDD stepper might be a bit big though. There are disposable laboratory syringe pipettes that fit holders for automated dispensing. Maybe worth a search. Remember, typical FDD linear stepper have captive lead screws which can be cut and spliced with another of higher pitch (but it’s still captive). To get the heavy stepper away from the hand held tool to lighten it, maybe hack a long flexible coaxial manual camera shutter-release cable?

  10. Maybe I’m just getting old, but if we really get to the point where everything is SMD I think it might be the end of the hobby for me. With through hole components and a halfway decent iron you can knock something together on protoboard in a few minutes. But if I’ve got to get a stencil cut and then put the board in the oven…ugh.

    1. Not sure what you mean. I neither have an oven for reflowing nor have I ever ordered or created a stencil and yet I’m pretty much exclusively working with SMT nowadays and quite prefer it over THT. Even a hot air gun, while extremely useful (and not just for SMT!) is not really a hard requirement.

  11. OP here. I hear ya about smd. I started electronics in the sixties so I grew up on through hole. Once custom PCBs got so incredibly cheap to make it was hard to resist going to smd, but if I’m making point to contact I still use through hole components. But dont get hung up on stencils and ovens. i’ve made a lot of boards and you can just dab paste on by hand if you want with a toothpick, or get a commercial dispenser. To melt the solder, hotplates were very well. Dont bother with an oven.

    1. Nah, don’t use toothpicks. Simply get a syringe with a blunt needle and manual plunger. Works quite well up to 200 applications in a go or so, after that it starts becoming a bit annoying.

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