Cleaning Chemistry Selection for PCB Assembly Lines

Cleaning is often treated as a piece of equipment rather than a chemistry, and the choice of cleaning chemistry is made after the machine has been ordered. That order of work puts the process in a corner, because the chemistry determines the temperature, the cycle time, the rinsing requirement and the materials the machine has to tolerate. This article explains what has to be removed, how the main chemistry families differ, and how to select a process that removes it without damaging the assembly.

Why Cleaning Is a Process Choice

Cleaning is required when the residue left by the soldering process is harmful to the product. That is a reliability statement rather than a cosmetic one: a residue that is harmless on a consumer board can cause leakage currents, electrochemical migration or adhesion failure on a high impedance or high reliability assembly.

The decision should come from the residue and the environment, not from a general preference for clean boards. A no clean process with a benign residue is a legitimate answer for many products, while a cleaning process that is added without a defined acceptance criterion is an expense rather than a control. The residue that matters is the one that changes the electrical or the mechanical behaviour of the finished product.

What Has to Be Removed

The residue from a soldering process contains flux activators, rosins or resins, reaction products from the soldering chemistry, and the ionic species that the activators leave behind. It also contains particulate contamination from the process and anything the board picked up during handling.

The ionic part is the most damaging, because ions in the presence of moisture and a voltage produce electrochemical migration and leakage. The organic part matters for coating adhesion and for appearance, and the particulate part matters for both. A cleaning chemistry is chosen to attack one or more of those categories, so the first question is always which one is present. Our solder defect notes describe how residue-related failures present on an assembly.

Water Based, Semi Aqueous and Solvent Chemistry

Water based cleaning uses a detergent or a saponifier in water, with the water doing most of the work. It is the most common choice because it is effective on ionic residue, it is easy to control and it does not carry the environmental burden of a chlorinated solvent.

Semi aqueous cleaning uses a solvent or a terpene in a first stage and a water rinse afterwards, which gives good performance on rosin based residue at the cost of an extra step. Solvent cleaning uses a non aqueous chemistry, which is effective on organic residue and can be gentler on components that cannot see water, but it needs tighter control of the solvent and of the vapour.

Saponifiers and Their Limits

A saponifier is an alkaline additive that reacts with rosin flux to form a water soluble soap, which the rinse then removes. It raises the pH and the temperature of the wash stage, so it works quickly on rosin residue that plain water would leave behind.

The limit is compatibility. A high pH chemistry attacks aluminium, some platings, some anodised finishes and certain connector housings. The chemistry to be used should be checked against every material on the assembly, and the check should include the parts that are only present sometimes, because a material that is incompatible usually fails gradually rather than immediately, and the failure often appears as a field return rather than as a line defect.

Compatibility With Materials and Components

Compatibility covers more than metals. Labels and marking inks, adhesives, conformal coating, connector lubricants, plastic housings and the components themselves all have to survive the wash, the rinse and the dry. A chemistry that is excellent at removing residue but attacks a membrane or a label is the wrong chemistry for that product.

The surface finish on the board matters too, because some finishes are more sensitive to an alkaline wash than others. Our surface finish guide describes the different coatings and how they behave in a cleaning process.

Rinse Quality and Water Purity

Rinsing removes the chemistry and the dissolved residue together, and a poor rinse leaves a film that is worse than the residue it replaced. Deionised water is used because the ions in tap water would themselves be a contaminant, and the resistivity of the rinse water is the parameter that shows whether the stage is working.

The rinse also has to reach the same places the wash reached. A dense assembly with low standoffs, connectors and shielded areas traps liquid and traps residue, so the mechanical energy of the rinse matters as much as the chemistry. A spray that cannot reach a gap cannot rinse it. Immersion stages help in those areas, and the fixture design that holds the assembly often decides whether the rinse reaches the underside of a large component.

Drying and Its Effect on Residue

Drying removes the water and leaves behind whatever was dissolved in it. That is why the final rinse quality determines the final residue: a rinse with a high ionic content leaves a conductive film even though the board is dry. Hot air, infrared and vacuum drying all work, and the role of the drying stage is to complete the removal rather than to substitute for a good rinse. A board that comes out of the dryer with a visible film has already told the operator that the rinse stage is not working.

Trapped water is the other risk. Liquid left inside a connector, under a shield or in a via can be released later and cause corrosion or a coating defect. The drying stage has to be long enough and hot enough to drive that liquid out, and the assembly design has to allow it to escape.

Monitoring and Verification

Verification uses the ionic contamination test, which measures the conductivity of a solvent extract from a cleaned assembly, and surface insulation resistance testing, which measures the effect of the residue under bias and humidity. The two answer different questions: the first measures what is left, the second measures whether what is left matters.

Our short circuit inspection notes describe how a conductive residue can produce a bridge that only appears when the board is wet, and our quality guide describes how a cleanliness defect is classified at gopcb.

Choosing a Chemistry for a Product

The selection follows a defined sequence: identify the residue, identify the materials that must survive, define the cleanliness acceptance criterion, and then choose the chemistry and the equipment that meets both. The cheapest chemistry that meets the criterion is the right answer, and a more aggressive chemistry is only justified by a residue it can remove and a material that tolerates it.

The decision should be documented with the concentration, the temperature, the cycle time and the rinse quality, because a cleaning process that is not recorded cannot be repeated.

FAQ

Is a no clean process always acceptable? It is acceptable when the residue it leaves is benign for the product and the environment it will see. For high impedance or high reliability assemblies, the residue usually has to be qualified rather than assumed.

Does a higher wash temperature always clean better? It usually cleans faster, but it also increases the attack on incompatible materials and the risk of damage to components. The temperature should be set from the residue and the material compatibility rather than pushed to the maximum.

How is cleanliness verified on a production line? By an ionic contamination measurement on a sample and by surface insulation resistance testing for the qualification. Both should use a written method so that the results are comparable over time.