Specific Gravity Control for Liquid Flux: 5 Checks Before the Wave

Specific gravity is the oldest measurement in a wave soldering shop and still the one most often taken without a record. A liquid flux changes as solvent evaporates from the tank and as thinners are added, and the change moves the solids that reach the board. Controlling the reading is what keeps the flux chemistry where the profile was set up.

Why Specific Gravity Is Used to Track Liquid Flux

The flux that arrives in a drum contains resin or organic solids dissolved in a solvent carrier. As the flux circulates in an open tank, solvent evaporates, the solution becomes more concentrated and the specific gravity rises, so the reading is a convenient proxy for the solids reaching the board. The reading is also compared with the last value recorded for the same tank, so the trend matters as much as the absolute figure.

It is a proxy rather than a measurement of activity. Two fluxes can show the same specific gravity with different solids content, which is why the figure is only meaningful against the specification for the exact product in use and against a temperature at which it was taken.

Measuring Specific Gravity: Hydrometer, Cup and Temperature

The classic method uses a hydrometer floated in a tall cylinder of flux, read at the eye line of the meniscus. The cylinder has to be tall enough that the hydrometer does not touch the bottom, and it must be filled to a level that lets the float settle freely without wetting the stem above the scale.

A digital density meter removes the reading error but still needs the same discipline: a clean sample, the correct sample volume and a cell that has been rinsed with the previous sample. Where a shop uses both methods, the two should be compared on the same sample so that the limit on the record means the same thing to every operator. The reference method and the flux classification used to set the limit come from documents such as J-STD-004, which also defines how the solids are expressed.

Temperature Correction and Why It Changes the Reading

Liquid density falls as temperature rises, so a sample taken at thirty degrees Celsius reads lower than the same flux at twenty. Most specifications state the reference temperature, usually twenty or twenty five degrees Celsius, and a correction factor per degree is applied when the sample is warmer or cooler.

The correction is not a formality. A shop that measures at the tank, on a warm day, without correcting, will read a flux that looks dilute and add solids or reduce thinner until the flux is genuinely too concentrated. Take the temperature with the sample and write both figures on the record.

Solids Content, Thinners and What the Reading Does Not Show

Thinner added to a tank drops the specific gravity immediately and dilutes the active solids. Where the tank has lost solvent and gained contamination, adding thinner restores the reading without restoring the chemistry, and the deposit that reaches the board is diluted flux rather than fresh flux.

The part of the picture the reading cannot show is contamination: metal ions, flux residue carried back from the wave, and water absorbed from the air. Those are the reasons a tank has a life limit and a periodic change, and the reason a perfect specific gravity is not evidence that the flux is fit to use.

Specific Gravity Limits for Spray Fluxing

Spray fluxing is less forgiving than foam or wave fluxing because the deposit is thin and the solids that reach the board depend on the density of the spray and the percentage of solids in the liquid. Most spray specifications hold the specific gravity in a narrow band, often within about 0.005 of the nominal figure.

The band has to come from the flux supplier’s data sheet and the site’s own deposit measurements, because the machine setting and the density interact. Confirm the band whenever the flux product changes, and read it with the spray fluxing coverage data for the same period so that a density change is not mistaken for a machine fault.

Automatic Density Control on the Fluxer

A density control loop measures the flux continuously and doses thinner when the reading rises above the set point. The loop is only as good as its sensor, so the refractometer or the density sensor needs its own calibration and a periodic comparison against a hydrometer sample taken by hand.

The rate at which thinner is consumed also carries information. A tank that suddenly needs much more thinner is losing solvent faster, which points to a higher tank temperature, a leaking seal or a change in how the flux is returned from the board. Read the dosing record rather than only the density display.

What a Rising or Falling Reading Tells You

A reading that rises steadily over a shift is normal evaporation. A reading that jumps in one step is usually a top up with concentrated flux or a thinner line that has been connected to the wrong drum, and both are found by checking the delivery rather than the tank.

A reading that falls without thinner being added points at water or contamination entering the flux: a chilled water line that has been drained into the tank, a leaking coil, or residue washed back from the board. In each case the tank has to be sampled and often changed rather than adjusted back into band. Take the sample from the tank outlet rather than from the surface, because a surface sample picks up the film that floats on the flux.

Records, Intervals and Operator Checks

The record holds the date, the time, the tank identification, the flux lot, the sample temperature, the reading and the corrected value. A reading without its temperature is not a measurement, and a corrected value without the raw figure cannot be audited later.

Frequency depends on the duty: a shift start and a mid shift check is common on a busy line, with an extra check after every top up. Keep the record beside the tank so the trend is visible to the operator, and read it with the solder defect data for the same product so that a defect trend can be tested against the flux trend.

When to Change the Tank

A tank is changed on a scheduled interval, on a contamination finding, or when the flux cannot be brought back into band without excessive dosing. A wave solder pallet that traps flux and returns it dirty, or a board that carries a heavy residue load into the wave, both shorten the practical life of the charge.

Change the tank completely rather than diluting a tired charge, and record the change as an event in the same file. The specific gravity after the change should be written down as the new baseline, so that the next operator can see the starting point rather than inferring it. A tank life that is set from experience should be written into the procedure, because an unwritten rule is one that a new operator cannot follow.

FAQ

What specific gravity should liquid flux hold? It is set by the flux supplier for the exact product, and a common band is within about 0.005 of the nominal figure. The number is only valid at the reference temperature in the data sheet, so the sample temperature has to be recorded with the reading.

Can specific gravity be adjusted by adding thinner? Thinner restores the reading but dilutes the active solids, so it corrects the number and not the chemistry. Use it to compensate for normal solvent loss, and change the tank when the flux is contaminated or the reading cannot be held within band.

Why does the reading drift during a shift? Solvent evaporates from the open tank whenever the flux is circulating, so the concentration rises and the reading climbs. A step change instead of a drift usually means a top up or a dosing fault rather than normal evaporation.