Flux Acid Value Control: 7 Rules for Consistent Soldering

Acid value is a laboratory number that describes how much acidic material is present in a flux, expressed as the amount of alkali needed to neutralise a known weight of it. It is one of the few measurements that predicts how a flux will behave on the line before any boards are soldered.

Flux does not stay the same in service. Volatile components evaporate, activity is consumed as boards are processed, contamination accumulates and top ups change the balance. Acid value is the number that catches those changes while they are still small enough to correct.

What Acid Value Measures

The test titrates a sample of flux against a standard alkali solution to an indicator end point, and reports the result in milligrams of alkali per gram of flux. The value reflects the total acid content, which includes both the activator that does the work and any acidic material that has been added for other reasons.

Because it measures the total, it does not identify which acid is present or whether the activator is still effective. It is a trend tool: the nominal value for a fresh batch, and the way the value moves with use, describe the state of the flux far better than a single reading does.

Titration Method and Repeatability

The method needs a defined sample weight, a defined solvent, a defined alkali concentration and a defined end point, and the same person or the same training should be used each time. A test that is performed with a different indicator or a different normality gives a number that cannot be compared with the previous one, and the trend becomes useless.

Repeat the titration at least twice on each sample and record both results. Where they differ by more than the repeatability of the method, the sample is not homogeneous, which usually means it was taken from a surface film rather than from the bulk of the liquid.

Where the Value Drifts in Service

Evaporation is the commonest cause of change in a bath or a reservoir. The volatile fraction leaves, the solids content rises and the acid value rises with it, so a flux that is recirculated through a wave machine becomes stronger and thicker over a shift.

Top ups dilute the bath and pull the value back down, which means the number oscillates between the addition of fresh material and the loss of volatiles. Contamination from the process, from the board and from the solder adds a slow drift that is not corrected by a top up, and that drift is the one worth watching most closely.

Acid Value, Activity and Solids

The activator in a flux is provided by the acidic species, so the acid value is related to the activity that will be available at the joint. The relationship is not proportional, because some acids volatilise before they reach the joint and some are more effective than others per unit of acidity.

Solids content is the other half of the picture. A flux with a high acid value and a low solids content behaves differently from one with the same acidity carried in a heavier vehicle, because the vehicle decides how much material is left on the board after soldering. Both parameters should be measured together, as they are in the flux solids work, and the thermal behaviour of the activator is described in the activation temperature notes.

Applying the Number to Foam and Spray Fluxers

Foam fluxers depend on the flux forming a stable foam, which in turn depends on the density and the surface chemistry of the liquid. Acid value alone does not predict foam behaviour, but a change in acid value almost always comes with a change in the other properties, so the foam height is a useful cross check at the machine.

Spray fluxers are less sensitive to foaming and more sensitive to viscosity and solids, because the pattern depends on how the liquid atomises. Specific gravity and solids are the numbers usually watched on a spray line, with acid value used to confirm that the chemistry has not moved in a way the other measurements do not reveal.

Limits, Corrective Action and Records

The specification should state a nominal value and an acceptable range, and it should state what happens when the value falls outside it. In some cases the answer is a top up, in others it is a partial replacement of the bath, and in others the flux should be discarded. Deciding this in advance removes the tendency to adjust with the first convenient liquid.

The record should show the sample point, the measured value, the action taken and the date, and it should be linked to the boards produced in that period. That link is what makes it possible to establish whether a soldering defect occurred before or after a change in the flux condition, and it supports the same evidence chain used for solderability decisions and for the residue risk described in the no clean residue review. Where the flux is a cleaning type, the balance between activity and removability is described in the water soluble flux notes.

Sampling Points and Laboratory Practice

The sample point decides what the number describes. A sample taken from the surface of a reservoir describes the film that has formed there, while a sample taken from the bulk describes the liquid that the boards actually see. Where the flux is recirculated, the sample should be taken from the delivery line or from a point close to the board, and the point should be the same every time so that the results can be compared.

Laboratory practice matters as much as the sampling. The burette should be clean and the alkali fresh, the balance should be calibrated for the sample weight, and the end point should be judged by the same person or against the same reference. Where the result is near a limit, a repeat measurement with a fresh sample settles the question, and the two values should both be recorded. Over time, a chart of the measurements against production hours is a far better tool than the acceptance limit alone, because it shows the rate at which the chemistry is moving and allows an intervention before the limit is reached.

The same reasoning applies to the flux in a cored solder wire and to the liquid flux used at a hand soldering station. A small pot that is topped up and never tested will drift in activity just as a wave bath does, and the change shows up as joints that suddenly need more heat. Extending the measurement to every flux container in use, rather than only the largest one, closes that gap.

FAQ

Does a higher acid value mean a better flux? No. It means more acidic material is present, which may be more activity than the process needs and may leave more residue. The correct value is the one the process was qualified with.

How often should flux acid value be checked? On receipt of a batch, and at an interval set by how quickly the flux changes in service. A wave machine running continuously will need a check every shift, while a small hand soldering pot may need one every week.

Why did the value rise overnight? Because volatiles evaporated while the bath was idle and warm, leaving the solids and the acidic material behind. A reading taken first thing in the morning should be compared with one taken at the same point in the previous cycle.