Calculations on flux consumption with a spray fluxer

The flux needed for soldering can be applied by several fluxing methods.
Only with spray fluxing one can apply just the amount of flux necessary for the job. The correct flux amount becomes more important with the use of Low Solid-No Clean fluxes. Flux suppliers will give advised flux layer thicknesses, mostly expressed in grams per meter square, in their relevant data sheets.
Mostly they mention the weight to be applied for the wet flux.
A calculation of the layer thickness of the predried flux can be made by the multiplication of this wet layer with the solids percentage.
As an example we calculate the dry layer thickness of a flux with 2% solids, that has been applied in a wet layer of 50 micron, which approximates 50 gram per meter square. The 50 micron wet flux will leave 50 x 2% = 1 micron dry flux on the board before soldering.
For a real calculation of the volume of flux needed to apply this 50 micron, one has to calculate with the "real" density of the flux. In general one can use 0.8 g/ml for this calculation. This means that in theory 50 grams: 0.8 g/ml = 63 ml flux per meter square is needed. The practical amount will be higher, due to some evaporation and over-spray.
Depending on the setting of the system and the type of flux, the real flux consumption can be 30 - 50% higher.
The flux consumption is further depending on the board area to be soldered.
If one has to solder 200 boards with a 200 x 250 mm board size and one has to apply a 75 ml wet flux layer per meter square, than the theoretical consumption will be:
200 x 0.2 x 0.25 x 75ml = 750ml
With relatively small boards in relation to the width of drum spray fluxer the evaporation loss may be larger than the common loss of 30 - 50%.
This is due to the cover plates over the "not used" area of the spray drum.
The air which activates the fluxer will however also blow the flux from the drum over this covered area. This flux will drip back in the spray flux unit, but has lost some solvent due to evaporation. This extra loss will give an increase in flux density which can be compensated with solvent.
The amount is depending on the "covered" area of the spray fluxer cover.
With a nozzle spray fluxer, there is never a need to adjust the flux, since the flux in this case is really used in a "one way" system.
When the drum spray fluxer is used over its full width, one often can also use this system as a "one way" system, although one will always have some evaporation.
When the solvent loss due to evaporation is not too high, the addition of the fresh flux which will be added due to the consumed flux, will often establish an equilibrium, were the flux consistency is still within the data given by the flux supplier. In that case one does not have to add solvent either.
With a drum spray fluxer, there is a linear relationship between the applied flux layer thickness and the drum rotation speed. On the other hand the applied amount is reciprocal to the transport speed.
The final amount is further depending on the flux type and viscosity.
If the flux factor is called "C", the drum rotation "N" in RPM and the transport speed "V" in m/min, than the amount of wet flux layer "F" in microns can be expressed as:
F = ( C x N ) : V