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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

  • Solder paste will vary in consistency according to age, temperature and stirring.

    The stuff in small syringes is often of very poor quality - you should make sure you buy a recognised brand (not from Amazon or Ebay).

    Commercial dispensing machines can suck back to prevent the dribbling problem.

    I have had no joy at all dispensing solder paste using a very cheap and fairly grotty pneumatic dispenser.

    I use stencils (which are cheap at £5 - £10 if you buy them with the boards)) although the frame for holding them will cost a bit.

    https://www.aliexpress.com/item/1005004545082599.html?

    You might be able to find/make a cheaper one.

    Talking of ageing of solder paste - I buy it in pots that cost about £70 per pot and I have never used all the paste from one. I decided to replace the current one a few weeks ago and was amazed to find it was three years old (official shelf life < 6 months). I do keep it in a fridge. However, the new pot was dramatically easier to print and use - you don't notice it going off because it happens slowly and progressively.

    So my advice would be to try warming the paste you have first - (to maybe 30C, 35C at most) - this will reduce its shelf life. And if you can give it a good stir before using.

    MK

  • in reply to michaelkellett

    HI Michael,

    Yes it's a quandary for as you can see my boards are tiny and this (shown below) is the 11th iteration

    image

     so the expense of stencils would be considerable, also holding the boards steady would require quite a decent stencil/board alignment tool.
    Equally hardly any solder paste is required meaning large pots of paste are not economically viable :-(
    So you are correct that small syringes of paste might be poor quality but I have had good results with my 'cheap' paste on many larger boards.
    However for this application hand 'spotting' solder paste for 0402's is very difficult, hence my having a go at this semi automatic idea.
    The annoying thing is it works with runnier medium (yoghurt/mayonnaise/toothpaste) so the idea is good.
    My software does 'pull' the piston back to remove unwanted pressure after each droplet as you commented, so it's just down to the correct viscosity solder paste.

    I have had recommended solder paste from ChipQuik SMD4300AX10 Farnell 1850224 but I don't understand how the viscosity(Malcom?) relates to toothpaste etc..lol
    Hence my seeking expert guidance.

    Thanks

    Dave

  • in reply to michaelkellett

    I second the need of pull-back that  mentioned, it will effectively prevent your creeping problem. Also use reputable paste.

  • in reply to davebullockmbe

    It needs to actually pull the piston not just release the pressure.

  • in reply to davebullockmbe

    How many of these are you making ?

    With a suitable iron, flux and 32swg solder wire its perfectly hand solderable.

    I don't like hand soldering 0402 parts but it can be done.

    I'd rather hand solder with wire than hand blob the paste and reflow.

    MK

  • in reply to anniel747

    HI yes the 'pull back' is adjustable as are all the other parameters though I haven' t tried increasing this actual parameter. The problem being (i guess) getting just the right pull back to not cancel out the next tiny advance. A balancing act !
    Well i've made 11 of these in different stages of development and I can hand solder 0402's but they don't look as neat as flow soldering as it's so tricky to get them 'square-on' to the pads etc. Maybe I could put them in the reflow oven after hand soldering?
    And, as the 'worm' of solder paste I was dispensing wasn't properly 'wetting' the pcb pads I was unhappy with it's viscosity without the added problem we are discussing now.

    Does anyone have experience of which manufacturer's paste is the most 'fluid'  that may suit my quest?
    Has anyone use the  ChipQuik SMD4300AX10 Farnell 1850224 and is it runny?
    Has anyone tried diluting SMD paste and with what? (I notice chip quik sell a syringe of  flux but is is compatible with their solder paste)?

    Thanks keep your helpful thoughts coming it's greatly appreciated
    Dave

  • If more aggressive pull-back (as indicated by  and  )doesn't solve your problem, you could have a look at the materials Voltera uses (and sells) for their V-One printer, where they seem to have solved the dispensing issues. I think you can even buy their dispensing nozzle modules.

  • in reply to davebullockmbe

    Even with professional dispenser parameters need to be exactly set. IPA can thin the paste to some extent. 

  • in reply to dougw

    Any idea what is different from others?