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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 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

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  • in reply to davebullockmbe

    I think it's expecting a bit much for such a simple method (DC motor, gearbox and syringes that are not designed for such tiny application) to work consistently, but you can always manually correct things (for instance lifting off any excessive paste occasionally applied using folded paper or a wood splint etc). Maybe to do it repeatably, it needs glass/steel syringe components and feedback (like calipers). That would also possibly require a stepper motor.

    Each time you replace the solder with a new batch, or each time the syringe clogs up a little bit, or the ambient temperature changes, the conditions have changed and the paste will either come out intermittently or at varying rates as what's stuck on the inner walls of the syringe and syringe tip change slightly. You'll need to clean and swap out syringe tips and try to maintain temperature each time you use it.

    It doesn't need to be neat, it just needs to be in the right ballpark of the correct amount of solder. The rest will ball up and can be brushed away during cleaning.