Wave soldering has been an established process in electronics manufacturing for decades. At the same time, the requirements placed on the fluxes used in this process are changing.
In addition to wetting performance, process stability, and residue behavior, other factors are becoming increasingly important: VOC emissions, occupational safety, fire protection, storage requirements, and sustainability.
As a result, water-based, VOC-free fluxes are attracting increasing attention.
But can they really replace conventional alcohol-based flux systems?
As is often the case in electronics manufacturing, the answer depends on the entire process.
1. What Is the Function of Flux in Wave Soldering?
Before the PCB comes into contact with molten solder, the flux must perform several important functions.
It helps to:
- Remove oxide layers from the surfaces to be soldered
- Limit renewed oxidation during the soldering process
- Promote wetting of the surfaces by molten solder
- Provide a sufficiently robust process window for reliable solder joint formation
The performance of a flux is therefore not determined by its activator system alone.
Carrier system, solids content, wetting characteristics, application quantity, and preheating conditions must all be considered together.
2. Why Are Alcohol-Based Fluxes Commonly Used in Wave Soldering?
Alcohol-based fluxes have been successfully used in wave soldering processes for many years.
Alcohols such as isopropyl alcohol evaporate relatively quickly, allowing the applied flux to dry rapidly.
This characteristic simplifies process control and is one reason why alcohol-based systems have become established in many production lines.
However, there is another side to this advantage.
Organic solvents used in these systems can be released into the environment as volatile organic compounds, or VOCs. Depending on the solvent system, additional requirements may also apply regarding flammability, storage, transportation, and workplace safety.
This raises an increasingly relevant question:
Does every wave soldering application really require an alcohol-based flux?
3. What Does VOC-Free Mean?
VOC stands for Volatile Organic Compounds.
In conventional flux systems, volatile organic compounds may serve as the carrier for the active ingredients of the flux.
In water-based systems, water primarily performs this function.
This can offer several potential advantages:
- Reduced solvent-related VOC emissions
- Lower fire load
- Easier handling of non-flammable systems
- Reduced use of organic solvents
- Potential for a more sustainable manufacturing process
However, replacing alcohol with water is not simply a matter of changing the carrier liquid.
Water has significantly different physical properties, and these differences affect the soldering process.
4. Preheating Becomes Particularly Important
One of the key differences between alcohol-based and water-based fluxes is their evaporation behavior.
Water requires considerably more energy to evaporate than typical alcohol-based solvents.
For this reason, preheating becomes particularly important when using water-based fluxes.
Before the PCB reaches the solder wave, the necessary evaporation should have taken place in a controlled manner and the flux system should have reached the intended activation condition.
Important parameters include:
- Preheat temperature
- Preheat time
- Conveyor speed
- Thermal mass of the PCB assembly
- Component density
- Amount of flux applied
- Performance of the preheating system
A water-based flux should therefore not simply be introduced into an existing process without reviewing the relevant process parameters.
5. Flux Application Must Be Controlled
Preheating is not the only important consideration. The quantity and uniformity of the applied flux also influence the soldering result.
Too little flux may result in insufficient activation. Applying unnecessarily large quantities, on the other hand, increases the amount of liquid that must be removed during preheating.
Controlled and uniform application is therefore an essential element of a stable wave soldering process.
Spray fluxing systems can provide good control, provided that parameters such as nozzle condition, pressure, conveyor speed, and spray pattern are regularly checked.
When changing to a different flux system, the existing application equipment should therefore also be included in the process validation.
6. Wetting Performance Remains the Decisive Criterion
Regardless of whether water or alcohol is used as the carrier medium, the primary function of a flux remains unchanged:
It must enable a reliable solder joint.
To achieve this, oxide layers must be sufficiently removed and the surfaces to be soldered must be protected during the relevant stages of the process.
The performance of a modern water-based flux therefore depends heavily on the formulation and balance of its activators and other components.
A VOC-free product represents a genuine alternative only if it also provides the required technical performance.
Sustainability should not come at the expense of process stability.
7. What Does the Change Mean for Existing Equipment?
A common question is:
“Can we simply replace our existing flux with a VOC-free product?”
From a technical perspective, such a change should be treated as a process modification.
Before release for series production, several factors should be evaluated, including:
- Flux application and spray pattern
- Application quantity
- Preheating capacity
- PCB temperature
- Conveyor speed
- Wetting performance
- Through-hole fill
- Solder bridging
- Solder balls
- Post-soldering residues
- Interaction with different PCB surface finishes
It is particularly important not to change several process parameters simultaneously.
A structured comparison with the established process makes it considerably easier to evaluate the new flux and identify the cause of any differences.
8. Consider Process Costs – Not Just the Price per Liter
The price per liter should not be the only consideration when evaluating a flux.
Other factors can have a significant impact on the actual process cost:
- Storage of flammable liquids
- Internal handling requirements
- Transportation
- Exhaust and ventilation requirements
- Flux consumption
- Equipment cleaning
- Scrap and rework
- Energy requirements for preheating
A water-based system may offer advantages in some areas, while potentially requiring additional energy for evaporation during preheating.
A meaningful economic assessment should therefore consider the entire manufacturing process rather than the purchase price of the flux alone.
9. PRO W30 – VOC-Free Wave Soldering Flux from SOLDER CHEMISTRY
With PRO W30, SOLDER CHEMISTRY offers a water-based, VOC-free flux for lead-free wave soldering processes.
The flux has been developed for industrial applications where reliable soldering performance is to be combined with the advantages of a water-based, non-flammable system.
However, the same principle applies here:
Successful implementation depends not only on the flux itself.
Application quantity, preheating, soldering parameters, equipment configuration, and the specific PCB assembly must work together.
For this reason, when evaluating a conversion to a new flux, we consider not only the material but the complete wave soldering process.
Conclusion
Water-based, VOC-free fluxes can provide an effective alternative to traditional alcohol-based systems in suitable wave soldering applications.
The potential advantages go beyond reducing volatile organic solvents. Lower fire load, storage considerations, workplace safety, and sustainability can all contribute to the decision to evaluate a water-based system.
At the same time, water behaves differently from alcohol. Flux application and, in particular, preheating conditions must therefore be carefully considered.
The decisive question is not:
“Is water-based better than alcohol-based?”
It is:
“Which flux system provides the most stable, safe, and economical process for our assembly and our equipment?”
That is the question that should be at the beginning of every successful process conversion.
