Actions
Forum Thread Details
  • Replies 30 replies
  • Subscribers 147 subscribers
  • Views 5811 views
  • Users 0 members are here
Related

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

Parents

  • Temperature makes a significant difference, in this experiment. I used quite a fine nozzle, and bunged it with solder and gently heated with hot air tool (trying not to soften the plastic!). 

    image

    Ordinarily I would have needed loads of pressure and it might not even have been possible with the small 3ml syringe without buckling the slim plastic plunger.

    I was able to follow the line and stop the solder by bringing the tip vertical to break off the extruded column of solder. I might need an even thinner diameter nozzle for TSSOP, but I think that's feasible (or even draw the bead at 90 degrees, in a long line over TSSOP pads, since the solder will separate between pads when reflowed anyway).

    image

    And this was possible even with air pressure (I didn't put any paste in the syringe, I merely packed a small amount in the plastic end of the nozzle for this simple test. 

    The results should be even better if there were no air in between. 

    image

    The effects of heat, and perhaps a mechanism for heating a syringe might be worth investigating.

Reply

  • Temperature makes a significant difference, in this experiment. I used quite a fine nozzle, and bunged it with solder and gently heated with hot air tool (trying not to soften the plastic!). 

    image

    Ordinarily I would have needed loads of pressure and it might not even have been possible with the small 3ml syringe without buckling the slim plastic plunger.

    I was able to follow the line and stop the solder by bringing the tip vertical to break off the extruded column of solder. I might need an even thinner diameter nozzle for TSSOP, but I think that's feasible (or even draw the bead at 90 degrees, in a long line over TSSOP pads, since the solder will separate between pads when reflowed anyway).

    image

    And this was possible even with air pressure (I didn't put any paste in the syringe, I merely packed a small amount in the plastic end of the nozzle for this simple test. 

    The results should be even better if there were no air in between. 

    image

    The effects of heat, and perhaps a mechanism for heating a syringe might be worth investigating.

Children
  • in reply to shabaz

    Switching down to an even finer nozzle, and this time Type 4 solder paste. Still warmed with hot air tool. It still flows well. 

    This time I had to treat the PCB with a thin film of flux paste beforehand (a blob of it, spread over the board with a dry PCB wipe), because the column of extruded solder is so thin, it doesn't have enough tack to stay on the board without it.

    Again, I had air in the syringe in this quick test, otherwise control would be better.

    image

    Poor quality images from mobile phone, but hopefully it's enough to demonstrate.

    image

  • in reply to shabaz

    This time with dots. Again using a thin film of flux paste on the board, to make the solder stick, allowing me to lift off the nozzle without the solder lifting off too.

    image

  • in reply to shabaz

    Brilliant experiment and encouraging results.
    I like the 'thin film of flux' idea as this is one of the issues with my device, I often got small enough spots of paste but they wouldn't "stick" / stay in place on the board, preferring to stay on the end of the needle.
    Heating the needle may also be an option, just a small amount of local heat makes sense.
    Well,  clearly I have several more options to try......
    Thank you for your involvement in my quest, you are all showing the true spirit of engineering isn't dead :-)
    Marvellous!
    Dave