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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 $parentForumReply.Author.DisplayName

    Wow thanks MP2100 that is a very helpful article.
    Whilst the text considers the shearing of the paste in a 'squeegee' scenario, I wonder what is really happening in my application where I am forcing the paste down a microbore tube. Is this going to shear and liquify the paste or cause it to compress and become thicker?
    I have seen an alternative solder paste applicator whereby the paste is supplied down a tapering plastic cone where the final 22G restriction is right at the point of application, like this:-

    image
    Is this where I am going wrong?

    image

    I did manage to deliver some solder spots yesterday (shown above) but they were inconsistent in size and very dry as they exited the needle.
    As you can see they appeared to have relaxed and become 'wetter' in the time I took to get them under the microscope!
    How do my observations correlate with the shearing description in the aimsolder article?
    I'm not sure that squeezing is the same as this 'shearing' action?

    Thank you everyone for all your helpful comments so far........

    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.