Flux Solids Content: What the Datasheet Tells You
Flux solids content is the mass fraction of a liquid flux that is not solvent: the resin, the activators, the inhibitors and the additives that do the work at the joint. A flux listed at 35 percent solids carries 35 grams of active material in every 100 grams of liquid, and the remainder leaves in the preheat.
That figure decides how much chemistry lands on the copper for a given deposition weight, and it can move between batches without any change to the part number. It also sits behind most spray fluxing arguments, because it converts a volume setting on the machine into a mass of active material. This note covers how solids are measured, what shifts them and how to react.
What Flux Solids Content Means
Solids are what remains after the solvent has been driven off under a defined drying cycle. For a water based flux the solvent is mostly water, for a solvent based flux it is an alcohol or a glycol ether blend, and for a VOC free flux it may be a low volatility carrier that partly stays on the board.
Because the definition depends on the drying method, the number is only comparable between suppliers if the method matches. The standard approach is described in the IPC flux specifications, which define the test temperature, the time and the sample size. Quoting solids from two different methods is a common source of confusion when a process is transferred to a second supplier.
Why the Figure Matters in Wave Soldering
Activity at the joint is proportional to the mass of solids delivered, not to the volume of liquid sprayed. Raise solids from 30 to 40 percent and the same spray volume puts a third more chemistry on the board. That can fix a dewetting problem or create a residue problem, depending on where the process was sitting.
The effect is sharper on a spray fluxer than on a foam or wave fluxer because the deposition weight is low and tightly controlled. On a machine where the flux is applied as a thin film, small changes in solids change the film thickness needed to deliver the same activation, and the fluid dynamics change with it.
Measuring Solids in the Lab
The gravimetric method is simple and needs care rather than equipment. Weigh a clean dish, add a known mass of flux, dry it under the specified conditions, cool it in a desiccator and weigh again. The mass that remains, divided by the mass of flux added, is the solids content.
Two errors ruin the result. The first is uneven drying, which leaves solvent trapped in a skin and reads high. The second is overheating, which can drive off volatile activators and read low. Run the test in duplicate, dry to a constant mass, and keep the dish covered during cooling so that the sample does not pick up moisture from the air.
Solids, Density and Specific Gravity
Density and solids content are related but not the same, and confusing them leads to wrong corrections. Density is the mass per unit volume of the liquid as supplied. Solids are the fraction of that mass which is not solvent. A supplier can keep the density constant while changing the solids by swapping solvent blends.
In practice both are monitored. Density is the quick shop floor check, and it is convenient to track with a hydrometer or a cup. Solids are the confirmatory test that explains why a density reading moved, and the two should be logged together. Our notes on specific gravity control describe the daily check that most wave lines already run.
Spray Fluxing and the Deposit on the Board
A spray fluxer delivers a thin film, and the mass of solids in that film is what the joint sees. Spray fluxing therefore has three variables that interact: the flux solids content, the flow rate at the nozzle and the coverage of the spray pattern across the board width.
Coverage is usually the weakest link, because the pattern is not uniform and the edges of a wide panel receive less than the centre. Measuring the deposition weight with a paper coupon at several positions across the conveyor shows the true distribution, and the method is set out in our notes on spray fluxing density and coverage.
Solids and Flux Residue After Soldering
Everything that does not evaporate remains on the board as flux residue. Higher solids therefore mean more residue for the same film, and a no-clean process has a budget for that. The budget is set by the ionic cleanliness requirement, by the cosmetic standard and by any coating that has to adhere over the residue.
Residue that is still tacky after the wave is the classic sign that solids are high or that the preheat is low. Residue that has charred into a dark crust means the opposite: too much heat for the chemistry that is present. Both are read from the board, and both are corrected by matching the solids to the thermal profile. The acceptance limits are covered in our notes on flux residue cleanliness.
No-Clean Limits and the Residue Budget
No-clean fluxes are formulated so that the residue is benign, not absent. That means the solids level and the activator system were chosen together, and a change to one invalidates the assumption behind the other. Substituting a higher solids flux into a no-clean process can push the residue past the point where conformal coating will adhere.
A flux density check and a solids test run together will separate a genuine formulation change from a measurement artefact. Where a batch is found outside its window, the safe response is to quarantine it and ask the supplier for the certificate value, rather than to compensate with a machine setting that alters every joint on the board. Flux chemistry behaviour in the wave is also affected by contamination, which is covered in our guide to flux and dross control.
Reacting to a Shift in Solids
When solids move, the first question is whether the change is in the material or in the machine. Check the batch certificate, then repeat the spray deposition weight on a paper coupon. If the deposit weight has moved with the solids, the flux is the cause. If the deposit weight is unchanged, the spray settings are the cause.
Corrections follow the cause. A material change is handled with the supplier, or with a deliberate formulation switch that is qualified on the line. A machine change is handled by cleaning the nozzle, checking the flow meter and re-establishing the pattern. Making one change and re-measuring is the discipline that keeps the wave process under control.
FAQ
Is higher flux solids content always better? No. More solids means more activity and more residue, and the trade only pays where the boards show insufficient wetting or where joints are heavily oxidised. For a clean board with good solderability, extra solids add cost and cleaning risk without improving the joint.
Can solids be checked on the shop floor? A full gravimetric test belongs in the lab, but a refractometer or a density cup gives a usable trend. Set limits around the value that a known good batch produces, and send any sample that falls outside them for the full test.
Does flux solids content change in the tank? In a foam or wave fluxer the solvent evaporates during the shift, so the solids in the bath rise slowly. Top up to the marked level with the correct thinner rather than with more flux, and check the density at the same point in every shift so the trend is comparable.
