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Solder paste viscosity - making the right choice - help please?

davebullockmbe

Calling all hand builders.

I am building a homebrew semi-automatic solder paste dispenser to build one-off SMD boards.
I attach a picture of where I am up to below.
The system uses a 1mL syringe with a 22G needle being driven by a threaded rod connected to a miniature motor and gearbox.
The motor is controlled by an Arduino that can activate the motor for a variety of milliseconds to deliver the correct solder paste 'droplet' size.

My problem is selecting a solder paste with the correct viscosity.
Whilst testing I used various household 'pastes' to save wasting expensive solder paste. I tried toothpaste and Dijon mustard and the system delivered acceptable sized droplets (see toothpaste attached below):-

image

However when I tried my solder paste, the system delivered the paste but due to it's thicker consistency it continued to 'creep' out the needle after the motor advance.
Clearly the paste is too thick and unduly pressurises the system, slowly releasing over time rather than being delivered all in one droplet.
The effect is that I get a slowly emerging 'worm' of paste rather than a nice droplet.

image

So my plea for help is this.
Without going to the expense of buying a variety of solder pastes in the hope one is suitable, does someone in the community have experience of hand delivering droplets
of 'leaded' solder paste in a similar manner. And can you kindly save me a lot of experimenting, by suggesting a suitable paste please?

Thanks in anticipation......
Dave

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  • in reply to davebullockmbe

    Even with quite large diameter tubes things are inconsistent unless conditions are perfect (freshly filled syringe). I can't remember the last time a syringe of solder paste worked smoothly first time. I always have to use a piece of wire as a pipe-cleaner to try to get it going. 

    For normal PCB use, I have a small tub of paste (not syringe) and use a metal squeegee and metal stencil, which I guess is what solder paste consistency (in good condition) is best geared towards. 

    If these are one-off prototypes (for which the metal stencil is not worth the expense perhaps), are there specific components for which you wish to use the syringe method, or for all components? There may be alternate techniques for some of the footprints of interest.

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  • in reply to shabaz
    shabaz said:
    f these are one-off prototypes (for which the metal stencil is not worth the expense perhaps), are there specific components for which you wish to use the syringe method, or for all components? There may be alternate techniques for some of the footprints of interest.

    For off-the cuff-application, where a stencil isn't affordable: a lot of practice with an out-of-box solder paste syringe works (for me). Without dispensers, without other tools. 
    If you practice, you 'll learn how hard to press to get an amount that can be used to tap a few footprints. And at what temperature it works the best. And to adapt the push based on the circumstances.
    Like soldering, it's a skill you can learn by doing it and getting the hang of it.

  • in reply to shabaz

    HI Shabaz
    well the current board I am developing is so small that the stencil cost would enormously outweigh the cost of the boards (which are £3.55 for 5 pcb's manufactured and shipped to my door) Also managing to make a fixture to align and hold the board pretty time consuming and tricky too, just not worth the time and effort.

    image

    It's mainly 0603's and 0402's that I CAN hand solder, but the boards would be much better looking, neater and quicker to assemble (with my 72 year old eyes!) if I can get the solder paste dispenser to work.
    And ..... surely you must know the feeling of not letting a challenge beat you?
    Dave