Aqueous and Solvent Cleaning of Assemblies

Why Cleaning Is a Decision

Cleaning an assembly removes flux residue, particulates and other contamination that can cause corrosion, leakage currents and adhesion failures. It is not automatically necessary: a no clean flux is designed to leave a residue that is safe to leave in place, provided the process has produced a full activation and the residue quantity is within the flux’s own specification. Cleaning becomes necessary where the residue is detrimental, where the product must be coated and the coating must adhere, where the assembly carries high impedance or high voltage, or where the customer requires it. The decision should be made from the requirement rather than from habit.

What Has to Be Removed

The contamination has several sources. Flux residue is the largest, and it may be ionic, which is the part that causes corrosion and leakage, or it may be a rosin or resin that is largely inert but that can trap ionic material beneath it. Particulates come from the process, from the handling and from the board itself. Oils and fingerprints come from hands. Metal salts and plating residues come from fabrication. Each has a different solubility, which is why the cleaning method has to match the contamination: an aqueous process removes ionic material well, while a solvent is needed for some organic residues, and some contaminants require a saponifier or a surfactant to be removed at all.

Aqueous Cleaning

An aqueous process uses water, usually with a surfactant and sometimes with a saponifier, at a controlled temperature and with a mechanical action such as spraying or immersion with agitation. It is effective on ionic contamination and on the water soluble fluxes, and it has the advantage of being a well understood process with a clear measurement. Its limits are the need to dry the assembly completely, since water trapped under a component or in a connector will cause problems, and the risk to components that are not sealed. The water quality matters: a rinse with high mineral content leaves its own residue, so deionised water is normally used for the final rinse.

Solvent Cleaning

Solvent cleaning dissolves the residue rather than carrying it away in water, and it is effective on the rosin based fluxes and on some organic contamination that water cannot remove. Modern solvents are chosen for a low environmental impact, and the process is often closed loop, with the solvent recovered and reused. Its advantages are rapid drying, no water to trap and compatibility with components that cannot tolerate water. Its limits are the cost, the health and safety requirements, the need for a suitable containment and the fact that a solvent may not remove ionic material that has already formed a salt. As with aqueous cleaning, the method must match the contamination.

Cleaning Under Components

The most difficult area to clean is beneath a component, particularly under an area array package where the gap is small and the flow is limited. A cleaning process that works on an open board may leave flux under a BGA, and the residue there is both invisible and difficult to measure. The countermeasures are a flux with a low solids content, a gap that is large enough for the cleaning fluid, a nozzle arrangement that directs the fluid into the gap, and a longer dwell. Where the design allows, leaving the area under the package free of vias and providing a path for the fluid improves the result. Where it cannot be cleaned, the flux choice becomes critical.

Drying

Drying is part of cleaning and is often the step that causes the problem. Water left under a component, in a connector, in a relay or in a via will corrode or cause a leakage path, and it may not show until the product is in the field. The drying should be at a temperature and for a time that removes the water from the difficult places, and it should be verified rather than assumed. Where the assembly has a component that traps water, the cleaning method should be chosen with that in mind, and a solvent process that dries quickly may be the safer choice for that product.

Verifying Cleanliness

Cleanliness is measured, not assumed. The standard method is an ionic contamination test, in which the assembly is rinsed with a known volume of a solvent and the conductivity of the extract is measured and converted to an equivalent quantity of sodium chloride per unit area. The result is compared with a limit that the product’s specification or the customer sets. Other checks include a visual inspection under ultraviolet light for flux residue, a surface insulation resistance test on a test coupon, and a direct examination for particulates. The measurement should be made on the production parts rather than on a specially prepared sample, because the process is what is being verified.

Design Choices That Help Cleaning

The ease of cleaning is decided during the layout. Keeping a gap under a package that the fluid can flow through, avoiding a component that creates a closed pocket, providing clearance around a connector so that the fluid and the air can move, and keeping the vias out of an area that will trap liquid all help. Where a component cannot tolerate the cleaning process, the design should identify that early, since the option of not cleaning may be the simpler answer. Cleaning is a process, but how well it works is often a design outcome.

FAQ

Do I have to clean after soldering? Only if the residue is detrimental. A properly processed no clean flux may be left in place.

What does aqueous cleaning remove? Ionic contamination and water soluble flux, but it requires complete drying afterwards.

When is a solvent better? For rosin based residue and for assemblies that cannot tolerate water or that trap it.

Why is cleaning under a BGA difficult? The gap is small and the flow is limited, so the fluid may not reach the residue.

How is cleanliness verified? Usually by an ionic contamination test on production parts, with a limit set by the specification.

Conclusion

Cleaning is chosen from the contamination and the requirement, and its success is measured rather than assumed. Match the method to the residue and dry the assembly completely. Process cleanliness belongs to quality management, the soldering it follows sits in SMT PCB assembly, and the coating that depends on a clean surface is described under conformal coating. Cleaning requirements for a new product are set during prototype PCB assembly in 2026.