Introduction
The applied flux amount per unit time, gflux (mg/ms), of a dropjet fluxer depends on the type of flux and the fluxer settings. In order to measure the behavior of the fluxer; a practical test is developed to determine this dropjet flux capacity figure in relation with the nozzle aperture.
Testing the flux amount gflux
The flux container should be filled with the flux to be used and then set at its working pressure of 0.5 to 1 bar.
Note: For this test often 1 bar is used for comparison, but if a lower pressure is used in production this production setting might be used also.
After all supply hoses are completely filled with flux and all the air in the flux supply is removed the test can start. For the test we need a timer (watch), a cup (coffee beaker) and an accurate balance (0.01 g accuracy).
During the test the flux coming from the dropjet should be collected in the cup.
The cup is weighed before and after the supply of the flux in the cup. For testing the flux amount G the following fluxer settings should be used:
• Open-time setting 4 ms
• Fluxer frequency 100 Hz
• Test duration 120 s
Note: These are just proposed test settings and might be changed to actual user settings if required. However test duration of at least 120 s is recommended to provide sufficient accuracy.
With the given settings the actual fluxing time ta during these 120 s is:
ta = t × f × to
Here is: t = test duration (120 s), f = frequency (100 Hz) and to = open-time per cycle (4 ms), giving for these settings a ta of 48000 ms.
After this test the supplied amount of flux G in the cup should be determined and expressed in milligrams, using a balance with 0.01 g accuracy or better.
Next the flux amount gflux in mg/ms should be calculated using the formula:gflux = G / ta mg/ms
Note: This figure depends also on the applied pressure on the flux vessel, so it is important to mention this pressure with the figures when they are used for comparison.
General figures
Just to give an indication what can be expected under different situations next figures can be used as a reference. Note that these figures should be used as a directive and not as absolute values, since the real value depends on flux properties, which might be different depending on the brand type.
Using a vessel pressure of 1 bar common figure values are:
Nozzle 130 micron IPA based flux gflux = 0.08, water based flux gflux = 0.12
Nozzle 270 micron IPA based flux gflux = 0.5, water based flux gflux = 0.64
Effects to expect when using a dropjet fluxer
When the nozzle is opened, a thin liquid column will be formed. After a short time this column will separate into single droplets. During its way to the PCB this column will contract to single droplets as a result of the surface tension and cohesive forces acting on that liquid column.
What effect this mechanism will have before the liquid comes to the PCB, depends on the travelling time and on the amount of liquid.
With low vessel pressure the travelling time for the liquid column increases, so that more single droplets can be formed from the original column. At high-pressure the liquid travels faster, so that the column might be still intact, but also collides with more energy to the PCB surface. This again might then create more splashes around the fluxed target.
Both effects are more or less unavoidable. But with a good tuning of the settings these side effects can often be reduced to an acceptable level.
The best balance, between the formation of single droplets around the fluxed target and the formation of splashes, is found with a 130 micron nozzle at a pressure of about 1 bar, using an open time of 1 - 10 ms .
For a 270 micron nozzle, the formation of splashes with water based fluxes is almost unavoidable, however this nozzle might give a practical solution for a specific job.
In general a low flux vessel pressure of about 0.5 bar might create satellite droplets around the target area, while a pressure of more than 1 bar might create splashes. The 130 micron nozzle is the most universal since it works well in a relative wide range of settings. The 270 micron nozzle is difficult to tune without the mentioned side effects, but has is advantages when more flux must be applied.
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