Flux Specific Gravity and Acid Value Control
Liquid flux is supplied as a concentrate or as a ready-to-use mixture, and its activity depends on the concentration of the solids it contains. As flux is used, the volatile solvent evaporates and the solids concentration rises, so the flux becomes progressively more active and leaves more residue. Flux control is the discipline of measuring that change and correcting it before it reaches the board.
Why Flux Has to Be Controlled
Flux that is too weak does not remove the oxide, and the result is poor wetting, incomplete hole fill and dull joints. Flux that is too strong leaves aggressive residue that attacks the board in service if it is not cleaned, and produces a heavy deposit that can interfere with the operation of test probes.
The change with use is gradual. A flux bath in a foam or spray fluxer loses solvent to the air continuously, so the specific gravity rises through a shift. Without measurement, the change is discovered when the joints begin to look different or when the residue becomes difficult to clean.
Specific Gravity and Its Limits
Flux specific gravity is the simplest measurement and the one used most often on the line. A hydrometer or a digital density meter gives the figure in a few seconds, and the value is compared with the target from the flux supplier. The measurement must be taken at a defined temperature, because density changes with temperature by enough to matter.
Typical ready-to-use fluxes are specified around 0.85 to 0.90 specific gravity, with a tolerance of a few thousandths. The figure is a proxy for solids content rather than a measure of activity, and for a simple flux the relationship is good. For a flux containing several solvents it is weaker, because two different mixtures can have the same density and different compositions.
Acid Value and Titration
The acid value is the amount of acid in the flux, expressed as the quantity of potassium hydroxide needed to neutralise a given mass of flux. It is measured by titrating a weighed sample against a standard alkali solution to an indicator endpoint, and it gives a direct measure of the chemical capacity of the flux to remove oxide.
Measurement by titration is more informative than specific gravity but slower, so it is used less often. A typical schedule is a daily or per-shift titration, with specific gravity checks more frequently. The two measurements together show whether a change in density reflects a real change in activity or simply a change in solvent.
Solids Content and Flux Type
Solids content is the fraction of the flux that remains after the solvent has evaporated, and it is quoted on the data sheet for the concentrate. The solids contain the activators, the resin or resin-free carrier and the additives, and their concentration is what the specific gravity is trying to track.
The flux type determines how much solids is acceptable. A no-clean flux is designed with a low solids content so that the residue is minimal, while a water-soluble flux contains more active material because it will be washed off. Comparing a specific gravity figure between two flux types is therefore meaningless unless the solids content is considered at the same time.
Top-Up, Dilution and Contamination
Top-up is done with fresh flux or with solvent depending on the measurement. When the density is above target, the correction is to add the solvent specified by the supplier; adding an arbitrary solvent changes the evaporation behaviour of the mixture and may leave residue that does not dry properly.
Contamination arrives from the boards and from the process. Flux that has been foamed or sprayed and has drained back carries particles, and a fluxer with a pump recirculating the liquid will concentrate debris over time. Where the flux is filtered, the filter has to be compatible with the chemistry; where it is not, the flux should be replaced at a defined interval rather than topped up indefinitely.
Alcohol Loss and Closed Systems
Alcohol-based fluxes lose solvent quickly, which is why the density rises fastest in a spray fluxer with a large exposed surface. A closed or covered flux tank reduces the loss, and a fluxer that recirculates through a closed loop changes less than one that exposes the liquid in a foam tower.
Water-based fluxes behave differently. Water evaporates more slowly than alcohol at room temperature but much faster when the flux is heated, so the loss pattern depends on where the flux is used. The measurement frequency has to be set from the rate of change in that particular machine rather than from a general rule.
Effect on Joint Quality
The effect of a drifting flux on the joint is a slow change rather than a defect. Wetting time lengthens, hole fill falls, and the residue becomes tacky and darker. Where the flux is applied by spray, a denser flux also produces more spitting, and the controls for flux spitting include the flux density as well as the nozzle settings.
If the flux is not cleaned, the residue left on the board depends directly on the solids that were applied, and the flux residue standard is therefore a flux control issue as much as a chemistry one. Where the boards are cleaned, the load on the cleaning system changes with the flux density, and the cleaning after wave soldering parameters may need adjustment when a new flux batch is introduced.
Measurement Frequency and Records
The specific gravity should be checked at the start of each shift and whenever the tank is topped up, with the temperature recorded alongside the value. Acid value can be measured less often, and solids content is best confirmed on receipt of a new batch rather than in production.
The record should carry the flux type and batch, the measured density with its temperature, the titration result, the quantity and type of material added and the date of the last full replacement. That record is what makes a change in joint appearance traceable to the flux rather than to the wave or the preheat setting.
Comparing Batches and Suppliers
A new batch of the same flux should match the previous one on density, acid value and solids content. Where it does not, the difference will appear on the line as a change in residue or in wetting, and it is cheaper to find it at goods-in than on the wave.
Comparing suppliers is harder, because the formulations differ. The relevant evidence is the joint quality and the residue left at the same settings, together with the solderability result on a standard test board. Changing flux supplier without that comparison is one of the more common causes of a wave soldering line that stops behaving as it did.
FAQ
What specific gravity should wave soldering flux have? Many ready-to-use fluxes are specified between 0.85 and 0.90, with a tolerance of a few thousandths, measured at a stated temperature. The figure should come from the flux supplier’s data sheet.
Is specific gravity enough to control flux? For a simple flux it is a good proxy, but it does not measure activity directly. Periodic titration of the acid value catches the cases where the density is right and the chemistry is not.
Can any solvent be used to dilute flux? No. The solvent specified by the supplier should be used, because the evaporation and residue behaviour of the mixture depends on it. An arbitrary solvent changes how the flux dries and how the residue behaves.
