uln2003 solder paste dispenser

This is an electric solder paste dispenser for soldering SMT boards, inspired by http://www.thingiverse.com/thing:384680. The main directory of the project is located on github at https://github.com/zapta/misc/tree/master/paste_injector which includes the hardware and firmware design of a compatible controller board that works with Arduino (it is similar to the Arduino IDE) as Arduino Pro Mini).

Schematic  :  https://github.com/zapta/misc/raw/master/paste_injector/eagle/paste_injector_schematic.pdf​

BOM :  https://github.com/zapta/misc/blob/master/paste_injector/eagle/paste_injector_bom.csv​

EEVBLOG discussion  : http://www.eevblog.com/forum/reviews/solder-pasteflux-manual-syringe-applicator/150/

I printed successfully using both PLA and ABS with standard slicing parameters (3 layers shell, 20% infill).

Tips, in order:

• When printing the motor shaft coupling, make sure the extruder temperature is high enough to achieve good interlayer adhesion.
• The stepper motors listed in the BOM are available on eBay from many sellers. They usually come with a ULN2003 driver PCB, which is not required here.
• When installing the coupler on the motor, minimize free play along the motor shaft. Before tightening the coupler screw, I used a sharp knife to pull out the motor shaft while pushing the coupler toward the motor.
• You can insert the threaded insert using any standard soldering iron that fits its hole. Preferably, you will get a tapered blade similar to what is specified in the BOM. If not, hit them with a file or Dremel tool. This is especially important for inserts that go into the plunger rod. Otherwise, the internal holes will be blocked and the M4 threaded rod will not be able to be inserted.
• When inserting the threaded insert, do not use excessive force and let the heat do most of the work. This will result in better adhesion.
• I programmed the controller board using AVRISP MKII, but you can use whatever method works for you. Github has a binary image file that includes the Arduino bootloader and scripts to program the AVR fuses.
• If you want to hack the controller's firmware, solder a 6-pin header on the pad labeled FTDI and use an adapter like FTDI Basic to plug your controller into your computer via the Arduino IDE. If you come with an interesting firmware update (e.g. red LED blinking proportionally to speed), please send me a pull request on github.
• Use a thin, sticky solder paste. For example, Chip Quik T5. If the solder paste doesn't stick well to the PCB, dilute it with IPA as needed.
• Avoid air bubbles in the syringe. Air bubbles make rate flow control more difficult.
• The potentiometer only controls the speed of the PRESS button. I got the best results by setting it on low and getting a very slow flow rate, then touching the PCB pads at a fixed rate to get a consistent drop. This takes some practice.
• The potentiometer is linear, but the firmware does a logarithmic mapping to allow very fine control at low speeds.
• This design is specific to the BD syringe listed in the BOM. Other types of syringes will most likely not work. It requires specific dimensions and the size of the rubber plunger and is designed to achieve high friction between the rubber seal and the syringe's barrel to avoid reverse rotation. If you find that the plastic plunger is over-rotating, use IPA to clean the rubber seal to increase friction.
• The controller board has a micro USB connector and can run from a standard USB charger.
• I've had good results using 22GA Luer Lock blunt needle bends. I got it from eBay. If you want to try it, buy a variety of needles and see what works for you.
• After securing the desired elbow, you can orient the needle in the preferred direction by loosening the two syringe holder screws and rotating the syringe.
• I use the dispenser with two hands. The right hand holds the syringe and controls the needle, while the left hand presses the PUSH button as needed.
• I used clear exterior mounting tape to attach the PCB to the motor mount. This is a thick, heavy duty double sided tape. You don't need to cover the entire PCB, just a few small pieces will do. Make sure there is tape underneath both buttons to provide more support.
• The bottom of the plunger model is fitted with a small conical component. Just glue it in there before inserting it into the rubber seal. It comes in two parts, allows printing without material support, and eliminates air pockets under the rubber seal (remember, air bubbles are serious).
• Solder paste lasts longer when stored in a cool place. I put the dispenser in a plastic bag and place it in the refrigerator (not the freezer). If you don't want to put the motor and electronics in the refrigerator, no problem, remove the two screws of the syringe holder and twist the syringe to unscrew it from the motor holder (do not pull the plunger out as it may air is inserted into the solder paste chamber).
• When storing solder paste it's a good idea to remove the needle and close it with a luer lock cap, you can buy them anywhere including amazon and ebay.
• If you continue to use the same needle (which I do), make sure you clean it after each use. I use a syringe filled with IPA to rinse it.
• The current PCB design on github (as of November 2015) is 100% auto-routed as part of the Autorouter Challenge and is discussed here at http://www.eevblog.com/forum/projects/ the-auto-router-challenge-will-it-work/
• The current PCB on github (as of November 2015) uses a "reverse" micro USB connector. There's no specific reason for this, just because I had something from a previous project. Make sure to get the connector listed in the BOM, or feel free to modify the PCB for your preferred micro USB connector.
• If you are looking for a PCB manufacturer for control boards, please visit http://pcbshopper.com to check prices. This is a 30x40mm two layer board.
• I superglued the two washers to the syringe holder. This way they won't fall out when the screws are removed. No big deal though.
• The 3D model was designed using Openscad. This is an open source 3D programming package. If you want to adjust any dimensions, load the provided .scad file in Openscad and regenerate the .stl file.
• Reprint the source address of this project https://cults3d.com/en/3d-model/tool/arduino-motorized-smt-solder-paste-dispenser