Aqueous Cleaning: Bath Concentration and Rinse Control

Cleaning is the step that decides whether the flux residue on a board is a harmless cosmetic film or a future leakage path. It is also the step that is most often run on a schedule rather than on a measurement, which is why boards that pass a visual inspection can still fail an ionic test. This article covers the bath, the rinse, the dry and the measurement that ties them together.

What Aqueous Cleaning Has to Remove

Aqueous cleaning removes flux residue, handling salts and particles, using a chemistry chosen for the flux rather than for the board. Water alone removes ionic material well and organic residues poorly, which is why a saponifier or a detergent is added for rosin based fluxes.

The result is specified as an allowable ionic contamination, commonly between 1.5 and 2.0 micrograms of sodium chloride equivalent per square centimetre for high reliability work. That figure is measured by extraction rather than judged from the appearance of the board.

Bath Concentration and Chemistry

Detergent concentration is measured and controlled, because a bath that is too weak stops emulsifying the residue while one that is too strong leaves a film of its own. Conductivity or titration is used for the measurement, and the value is held inside a band rather than kept above a minimum.

The bath also accumulates what it removes. Dissolved flux, salts and saponified rosin raise the total dissolved solids, and beyond a certain level the boards come out of the wash with a film that is worse than the residue it replaced.

Concentration and total dissolved solids move in opposite directions as a bath ages, which is why a bath is judged by both rather than by the detergent addition alone.

Temperature, Time and Mechanical Energy

Temperature improves the kinetics of dissolution, and most chemistries are operated between 45 and 65 degrees Celsius. The upper limit is set by the chemistry and by the parts, because hot water at a high pH attacks some finishes and removes some component markings.

Mechanical action reaches what chemistry alone cannot: spray pressure, immersion agitation and ultrasonics all bring fresh solution to the surface. Ultrasonics are used with care on fine pitch assemblies, because cavitation energy that is harmless to a bare board can damage a wire bond.

Rinse Stages and Water Quality

The rinse removes detergent and the material it holds, and most cleaning failures are actually created there. A rinse that carries detergent forward from the wash leaves a film that becomes visible only after the board dries.

Rinse water is monitored by resistivity, typically between 1 and 8 megohm centimetres depending on the specification, and the final rinse should be fed from a deionised supply. Cascading stages with counter flow use less water and hold a higher resistivity in the last tank.

Drying and the Residue It Leaves

Water left on the board concentrates whatever is dissolved in it as it evaporates, so a poor dry leaves a ring of residue around every via and joint. Drying uses filtered air knives and a heated zone, and the air itself has to be clean.

A useful check is a clean glass slide that has been through the whole process and then examined under a microscope, because the slide shows what the process leaves behind without the complication of the assembly.

Ionic Contamination Measurement

Ionic contamination is measured by extracting residue from the board into a known volume of water and alcohol and measuring the conductivity change against a blank. The result is expressed as an equivalent mass of sodium chloride per unit area. The method is described in our cleanliness guide.

The measurement is taken from the board rather than from the rinse water, because the water describes the bath and the board describes the product. Both are useful and they are not interchangeable.

Effects of Incomplete Cleaning

Residue left under a component traps moisture, and with voltage present in service it forms an electrochemical cell that corrodes the joint. The failure takes months to appear and it is usually found as a leakage path rather than an open circuit.

Conformal coating over an unclean surface is worse than no coating, because the coating seals the residue in place and stops the humidity from drying out. Cleaning therefore precedes coating in the flow, and the interval between them is controlled. The coating side of this is covered in our conformal coating notes.

Bath Life and Maintenance

Bath life follows the measurements rather than the calendar: concentration, pH, total dissolved solids and the cleanliness result together show when the bath has to be dumped. A bath changed on a schedule alone is either dumped too early or left far too long.

Filters, nozzles and the rinse cascade are part of the maintenance plan. A blocked spray nozzle lowers the mechanical energy locally and produces a board that is clean in the centre and dirty at one edge, which is a pattern worth recognising.

Integrating Cleaning with the Line

Cleaning sits between soldering and coating, so its output is judged by what follows it. Where the coating dewets, the cause is often upstream in the wash rather than in the coating process itself.

The records that make this diagnosable are bath concentration and pH at the time of the run, rinse resistivity, the dry settings and the ionic result from a sample board. With those four numbers, a change in the coating can be traced back to a change in the wash.

Saponifier concentration is titrated at the start of each shift rather than once a week, because drag-out and evaporation change a bath measurably within a single day of production.

Spray pressure is checked at the nozzles rather than at the pump, since a partly blocked nozzle reads normal on the gauge while delivering half its flow to one side of the board.

Assemblies with a large thermal mass take longer to reach the bath temperature, so the wash time is set from the heaviest board on the line rather than from the average.

Rinse water resistivity is recorded before and after each run, and a fall of more than a factor of two across a shift points to carry-over from the wash stage.

Drying is confirmed with a humidity indicator or by weighing a sample before and after, because a board that looks dry can still hold water inside a connector body.

Where the same line cleans bare boards as well, the detergent and the rinse quality have to satisfy both, and the ionic result from a plated-through coupon is the common reference.

FAQ

Can a board be cleaned with water alone? For ionic residues, yes, but organic flux residues need a saponifier or a detergent, and the choice has to follow the flux chemistry rather than the board.

Why does the final rinse matter more than the wash? Because the last water on the board is the water that dries there, so anything dissolved in it becomes the residue that remains.

How often should a cleaning bath be changed? When the measurements say so. Concentration, pH and dissolved solids together define the point at which the bath starts leaving residue rather than removing it.