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

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.

That would be interesting to see.

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:-

Is this where I am going wrong?

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

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.

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.

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.

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).

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.

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..?!

For one-offs I don't use this method anyway, apart from (say) QFN parts, simply because I'm fine soldering 0402, TSSOP etc with solder wire and iron. But if I wanted to reflow it all and I didn't have a stencil, then the above seems a pretty reliable way of doing it.

shabaz said:

How do you keep your syringe from clogging up?

My syringe has an end just like yours.
After use, I put the paste in the fridge, and consider the amount in that tip as lost. I clean that out later, with a paper napkin and a toothpick. Before use, I let it adapt to room temperature. While that's happening, I clean that tip. It's not a lot of paste in there.

The thing with the "home use" dispensers is that they "dispense a given amount for a given temperature". If they are dialed in, and many pads have the same size, then they 'll be OK. But it needs that adaption for both pad and temp.

I see many youtube movies of people reviewing a new dispenser positively. I rarely see a movie of someone consistently using them after some time ...

shabaz said:

I used a wood splint

I do the same for the tiny components. Some wooden contraption. Often a toothpick. Sometimes a chopstick.

For QFN, I draw a line with the toothpick near the outer edges. For parts that only have contacts underneath the component, and there's no escape for excessive paste: dotting with the toothpick.

"I see many youtube movies of people reviewing a new dispenser positively. I rarely see a movie of someone consistently using them after some time ..."

Yes, this is why I'm sceptical that a simple automated dispenser will be any better. If one cannot use a manual dispenser repeatably, then it's hard to see how automating it (without actually changing the things that cause the issues) will be any better.

Yes.
For industry it's different. They have repeatable processes, better control over the parameters, and up-to-the-job tools.
And even they have to dial in for each batch run.

HI thanks for the continuing conversation it's great to hear your experiences as I am fairly new to soldering with solder paste at home. I was not sure how consistent the 'spots' of solder paste had to be and your comments have helped in that respect.
My dispenser is a throwaway system so there's no issue with drying up. A new 1mL syringe will be loaded from the paste supplier's syringe, and similarly a new tip will be used each time.
I have also used a cocktail stick to 'pick' tiny amounts of paste from a larger 'blob' on a scrap piece of pcb but It's messy and the thought of easily delivering reasonably consistent spots of paste at the press of a button seemed very appealing.
As you saw from my mayonnaise experiment, it works very well, it's just that mayonnaise doesn't reflow very well or make good electrical connections!
I've ordered some of those tapered plastic tips, as I just watched a youtube video from a paste seller comparing the flow from a needle against a taper plastic tip. Using the same pressure and 'pulse' duration the paste flowed way faster from the plastic tip, suggesting far less restriction/friction. I am heartened seeing this as it means that the back pressure that is troubling my current system may be significantly reduced and be way more controllable?
Or not...watch this space :-)
Dave

Maybe you should try this.