The use of low solid content fluxes is advised when SMD's are to be soldered on a wave soldering machine.
These fluxes are very difficult to control for the right composition. Since they have a solid content of less than 5%, the third digit behind the decimal pointer is already important in the density control.
This means that automatic density control with a hydrometer has reached its limit.
Due to the fact that the densities of rosin (= approx. 1.07) and alcohol (= 0.79)are so close to each other, it is almost impossible to control the flux consistency within ± 1% with the use of a hydrometer. The solid content variation of ± 1%can give a great change in flux behaviour, as the total solid content is less than 5%.
Therefore the flux suppliers, knowing this problem and having no satisfactory technical solution yet for the solid content control, advise to replenish the consumed flux with a mixture of 50% new flux and 50% diluent. From time to time you can control the flux density with a laboratory hydrometer. If necessary you can change the mix-percentages of the blend used for replenishing.
EXAMPLE: If the density is 0.795, the solid content is 1.8%. When the density changes to 0.800, the solid content is 3.6%. At 0.805 the solid content is 5.4%.
An other important factor is water absorption e.g. by condensation of moisture from the air. This can change the flux behaviour considerably. To control this effect one should regularly check the acid value of the flux e.g. with the Alpha Metals Densograph.
One way to overcome all these difficulties is to use just so much flux that it can hardly change its composition due to its constant consumption. This can be done by using a nozzle spray fluxer.
ITW EAE Support Center