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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
    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.

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  • in reply to Jan Cumps

    How do you keep your syringe from clogging up? Of course, if the syringe tip is cleared to a consistent level each time (or a new tip is used), then consistent results can be expected, and the skill simply boils down to applying minute pressure manually to the syringe plunger (no need for the blue thing in the photo below - it's not very good anyway).

    image

    To try to reduce drying, I use a silicone cap (as shown in the photo) but I think it compresses some air into the end, and then that still causes it to dry up. It needs cleaning each time. I use decent brand solder, no AliExpress stuff.

    In contrast, here's what I did in about 3 minutes using a pot (not stored in the 'fridge, although I do keep a separate pot in a small fridge in the shed, I keep this pot for quick minor bits of work). I used a wood splint, and I think this would be totally fine with reflow, since the solder collects together, and it doesn't need to be precisely applied.

    image

    So I'm just curious what the actual goal was (i.e. any specific components that the user has problems with?), not the objective of improving the syringe method. Not saying it's not a good thing to solve, but surely the goal is what's important..?!