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

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

HI Michael,

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

 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

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

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 michaelkellett and anniel747 )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.

A bit of a science-experiment level thought rather than a serious suggestion, but I've always wondered if there wasn't another technique feasible, which would be to glue down every part precisely where it's needed! and then, using very runny solder paste (warmed in the syringe), run it along the pads and let capillary action suck it up, so that it doesn't spread all over the board. And then reflow. Kind of like a cold upside-down version of wave soldering. Could even have a cooled PCB so that the flux quickly thickens.

OK
I might try IPA then to see if it makes a difference to my 'cheap' paste delivery and the fiercer 'pull back' is good advice too.
However, because the paste I have isn't 'wetting' the solder pads I think the first priority is the viscosity?
My experience with toothpaste and mayonnaise wetted really well (but it doesn't solder the parts at all...lol!)
The only issue I have with thinning such small amounts of solder paste is getting consistency between batches. And my big syringe of solder paste has it's plunger integral so not easy to remove and dribble IPA to thin the whole lot. 
I am soooo tantalizingly close yet so far.
Thanks......
Dave

Warmth makes solder paste extremely runny, so it might just be a case of slightly warming the syringe contents (maybe in hands). In summer time I find it difficult to quickly work with a pasted board (using a stencil) compared to colder weather, when I have more time before it slumps.

0402 parts look nicely filleted with 0.274mm solder wire (which has almost no flux in it due to the thinness, so it does require the board to have a smear of flux beforehand (I use a gel-like flux, but it could be coated with liquid flux too). And a very small soldering tip (1mm).

It could also be worth experimenting with spoon tips for these small parts, even though those tips are intended for SMD ICs.

The fact that you used mayonnaise to experiment with jogs my memory.  Mayonnaise is non-Newtonian, it is thixotropic

https://en.wikipedia.org/wiki/Thixotropy

Which means when you are pushing your syringe, the viscosity thins our.  Stop pushing, it's viscosity increases, so much less likely to overflow, no "worm" on your circuit board.  Now I have to figure out if solder paste is thixotropic. 

Oh, ok, never mind.  One quick search, it is thixotropic.  So mayonnaise was a perfect choice.

aimsolder.com/.../go-with-the-flow

Good advice on that web page, and examples of materials' viscosity

How about peanut butter?