No-Clean vs. Water-Soluble Flux

The PCB industry is facing volatile market conditions. Manufacturing the highest quality products is critical to staying competitive. Solder flux plays a crucial role in the performance of your PCB assembly. Flux helps create a stronger solder joint. It cleans the metal surfaces and removes metal oxides that can interfere with the soldering process.

How can you decide between no-clean vs. water-soluble flux?

These two flux types have advantages and disadvantages for the PCB assembly process. Learn more here so you can choose the right flux for your application.

Advantages of Water-Soluble Flux

Water-soluble flux was the industry standard for decades. It remains a popular choice, with analysts expecting demand to grow almost 5% over the next five years. It continues to hold many advantages for the PCB assembly process.

Water-soluble fluxes are usually solvent-based. Three water-soluble flux types are:

• Halide activated (pH neutral)
• Halide activated (low pH)
• Halide free

Water-soluble fluxes have a high solid content. They’re highly active with long life in the process.

The high activity of water-soluble fluxes gives them excellent soldering performance. These fluxes rarely burn off during soldering, and they clean the metal to be effectively soldered.

Challenges of Using Water-Soluble Flux

Despite the advantages, a water-soluble flux has some challenges; mainly, the chemistry is aggressive and corrosive. Water-soluble flux is long-lasting on circuit assemblies and will continue to react after soldering.

Water-soluble fluxes require cleaning, and the machine wash process must be thorough to ensure clean circuit assemblies. Using a washing system makes the PCB assembly process longer and may be more expensive.

Advantages of No-Clean Flux

No-clean flux leaves a negligible amount of residue. It leaves little to no active chemistry on the PCB after soldering. This process eliminates the need to clean the board in most cases.

No-clean fluxes have low solids. They can be alcohol-based or water-based. Keep in mind that a water-based no-clean flux isn’t necessarily water-soluble or washable.

Alcohol-based no-clean flux can contain rosin or not. Alcohol-based fluxes evaporate quickly, and this reduces flux spattering during soldering.

No-clean flux streamlines the PCB assembly process. Cycle time is shorter, and the process may be less expensive.

Challenges of Using No-Clean Flux

No-clean flux has some potential downsides to keep in mind. For example, it has very low solids and low activity. Getting enough active chemistry to be effective can be a challenge. The flux may not last long.

This process decreases soldering performance. Adding active chemistry improves performance and increases the amount of residue left on the board.

No-clean flux is designed to leave little residue. However, modern circuit assemblies are getting smaller and smaller, and the components are closer together these days.

PCB assemblies are less tolerant of residue now than in the past, and the risk of residue-related failure is higher.

No-Clean vs. Water-Soluble Flux

Choosing which type of flux to use for your PCB assembly process depends on several factors. The same flux may be safe in one application but poorly suited for another one.

Your contract should specify the type of flux and the cleaning process to use. This detail helps ensure you get the results you need for your components.

Through-Hole or Surface Mount

Through-hole mounting was the original technology for PCBs. Water-soluble flux is often a better choice for this mounting method.

Surface mounting is much more common today. No-clean flux can work well with surface-mounted PCB assemblies.

Component Density

A PCB assembly with enough space between components is a good candidate for no-clean flux. Assemblies with a high component density may need a water-soluble flux.

One consideration is the spacing of components on the board. The spacing of conductors in a single component is the second factor when determining density.

Too much residue in a small area increases the chance of failure from electrochemical migration (ECM). ECM causes a short from decreased electrical resistance.

Lowest Component Standoff Height

Component standoff height (CSH) has a large effect on the structural performance of the assembly. CSH depends on the bond pad and the soldering reflow profile. A lower CSH improves the shear strength of components.

Components with a very low standoff height may trap flux activators underneath. This increases the potential for shorts. It can also cause solder voiding.

Before using a no-clean flux, you should verify that the CSH is high enough. Otherwise, a water-soluble flux may be a better choice.

Service Environment

Moisture is a critical factor in causing a component to be short. If a PCB assembly has a relatively low component density and little residue, moisture from a standard operating environment probably won’t trigger failure. However, an assembly with high component density and high residue may fail from everyday moisture.

Determining what a severe environment depends on the PCB assembly.

A board that will be in a harsh environment will likely benefit from a water-soluble flux. No-clean flux is likely safe for an assembly with a standard service environment.

Cost of Failure

The cost of device failure is a final consideration when choosing between flux types. If a smart lightbulb stops working, the consequences are probably minor. If a medical device fails, the consequences could be serious for the user and your company.

In applications with a high cost of failure, the additional security of water-soluble flux may be worth the extra time and expense.

Choosing the Right Flux Type for Your PCB Assembly

The decision of no-clean vs. water-soluble flux is important for the performance of your PCBs. Many factors go into choosing the right flux. The experienced professionals at Geospace Technologies can help you get the best flux for your application.

Geospace Technologies is a PCB assembly manufacturer with the top-tier capabilities you need. Our verification and testing processes help ensure your product will function as designed in its service environment. Contact us today to start designing your PCB assembly solution.