In this section I’ll explain how to deposit repeatable drops of fluid, specifically solder paste in “Dot mode”. In this mode pressing the Start button once moves the motor a predetermined amount or distance. It’s not too dissimilar to “Timer mode” except here the volume extruded is defined by piston distance in mm -not time- which makes it much easier to compare with the “Retraction” parameter (also in mm).
1. The first thing you’ll need, is to make sure that there’s no air inside your syringe at all, be extra careful when filling the syringe to avoid pockets of air. This is because air is compressible and will drastically reduce responsiveness and slow down the dispensing time for each pad.
2. Second, fill your syringe to a low level. This means filling it with just around 3cc of paste, up to this line:
When you insert the piston, the level will rise slightly and it should look like this:
The reason for this is somewhat related to point #1, some solder pastes have tiny air bubbles “dissolved” inside which is actually the gap between the solder balls not filled by flux.
Prior to filling, you can test the paste in its original syringe: push the plunger firmly with the bottom cap still on and see if it’s compressible. If that’s the case then follow the advice.
3. Third, go to menu item #13 and set “Backlash distance” to 0.00mm. Backlash helps in Manual dispensing where the movements of the piston are not identical each time, it compensates for play in the leadscrew when the piston goes backwards and forwards. But since we’ll be controlling these movements much more accurately in Dot mode, it’s not necessary to implement it.
As always, higher viscosity fluids tend to make it harder to find the correct settings quickly. Solder paste is very thick and requires a lot of force to be pushed through the small hole in the tip.
Because of this, there’s a spring effect where the paste, piston and syringe “store” energy during operation if the internal pressure isn’t released on each cycle. Failure to extrude the correct amount will produce oozing or dripping over time, leading to inconsistent deposits.
For this reason it’s important to know exactly how much volume is supposed to be extruded for each pad. For instance, a 0805 component pad needs just 0.001mm of piston displacement. Here’s a test with 0.001mm displacement of water:
The test was done with water because it’s the easiest liquid to dispense: it comes out instantly when the piston moves forward and there’s no need for a “pull back” movement since there’s no oozing.
So we now know the distance the piston has to move for each deposit, however these same settings on solder paste would be impractical, because it would take a long time (minutes) for this volume of paste to ooze out.
To make it faster we need to temporarily increase the forward distance by a large margin, then wait for the desired amount to come out and finally retract the piston to stop the flow. For example:
First the piston is moved 0.141mm which applies a large pressure quickly extruding the paste.
Then we wait a fraction of a second “wait before retract: 0.2s” now the paste is flowing.
And finally a retraction movement of 0.14mm is applied, this instantly eliminates the pressure and the paste stops.
The net distance is 0.001mm and the equivalent in volume was dispensed correctly. This means that there will be no oozing and therefore every drop will be identical to the previous one. The dispense tip in this case is the pink conical 20ga, and the syringe fill level was 3cc as shown previously.
(These specific values were found by trial and error, you can use them as a starting point to save setup time)
What happens if we need to dispense smaller or larger pads?
You can increase the extruded volume by changing the net forward distance, for example by using the settings: displacement=0.143, retraction=0.14 and also increasing the “wait before retract” value to give the paste more time to flow.
For smaller volumes you can decrease distances overall, for instance 0603 pads can be dispensed with the settings shown above.
Remember this is a one time setup, for each pad size in your board you can store the settings to a specific profile (menu item #5) and quickly recall them each time.
“Dot mode” can also be used with 3D printers and CNCs or in other open-loop systems, simply send a pulse and wait until the dispense cycle has finished to proceed to the next pad.
Note: 0603 pads are in the lower limit of what’s possible in Dot mode without oozing. For smaller deposits I recommend using manual mode with a dispensing speed of 3 or lower.