Which flux and solder alloy should you choose for selective soldering?

In selective soldering, a great deal of attention is paid to machine parameters such as nozzle selection, soldering speed and preheating. However, the flux and solder alloy also play an important role in determining the size of the process window.

The flux needs to remove oxides and support wetting, while being applied very locally. The solder alloy must provide sufficient heat transfer to achieve good hole fill, while thermal stress on the PCB and components needs to remain under control.

The right chemistry should therefore not be selected separately from the process. PCB, component, thermal mass, flux application, soldering temperature and required cycle time all need to be considered together.

What does flux need to do in selective soldering?

Flux prepares the solderable surfaces for contact with molten solder. Its active ingredients help remove oxides, allowing the solder to properly wet the metal surfaces.

For through-hole joints, this action should not be limited to the underside of the PCB. Sufficient flux also needs to reach the space between the component lead and the plated through-hole.

How the flux spreads is therefore important.

In selective soldering, flux is typically applied locally, for example with a microjet fluxer. This makes it possible to target the required area without applying flux across the entire underside of the PCB.

However, this local application also places specific demands on the flux.

Can you use the same flux for wave and selective soldering?

A flux suitable for wave soldering can, in some cases, also be used for selective soldering. However, this does not mean that every wave soldering flux is automatically a good choice.

The process conditions are different.

In wave soldering, a larger area is typically fluxed and then passed over the solder wave. In selective soldering, flux is applied much more locally and only selected areas come into contact with the molten solder.

As a result, flux residues may remain outside the actual soldering area. Both the amount of flux and where it is applied therefore require particular attention.

Compatibility with the fluxing system also matters. A flux that performs well in a particular soldering process is not automatically ideal for microjet application.

The question is therefore not only whether a flux solders well, but also whether it is suitable for the way it is applied.

Alcohol-based or water-based flux?

The solvent used in the flux is an important consideration when selecting a product.

Alcohol-based fluxes evaporate relatively easily during preheating. Water-based fluxes require more energy to evaporate the water sufficiently.

This affects the preheating process.

If too much solvent is still present when the PCB reaches the molten solder, it can interfere with the process. Preheating should therefore be matched not only to the PCB and its thermal mass, but also to the flux being used.

A water-based flux can be perfectly suitable, but the process needs to provide sufficient energy and time for the water to evaporate.

Why is flux composition important for microjet fluxing?

A microjet fluxer uses very small openings to apply flux precisely. The flux formulation therefore needs to be suitable for this application method.

Depending on their formulation, fluxes containing certain resins or solids may be less suitable for very fine application if deposits build up in the fluxing system.

Over time, this can affect the repeatability of the flux application.

When selecting a flux for selective soldering, it is therefore important to look beyond chemical activity and consider practical properties such as:

• suitability for microjet application;
• stability of the flux pattern;
• evaporation during preheating;
• amount and nature of the residue;
• required activation level.

The best flux on paper is not necessarily the best flux for the equipment being used.

How active should the flux be?

Flux activity helps remove oxides from solderable surfaces.

When surfaces are difficult to solder, a higher activity level can broaden the process window. However, more activity is not automatically better.

Stronger activators can also affect the properties of the residues left after soldering. This is particularly relevant in selective soldering because not every fluxed area necessarily comes into contact with molten solder.

A useful principle is therefore:

Use enough flux activity to achieve reliable wetting, but no more than the application requires.

Halogen content and the reliability requirements of the final electronic assembly should also be taken into account.

When does a dedicated selective soldering flux make sense?

A dedicated selective soldering flux becomes particularly relevant when a standard flux starts to limit the process.

This may be the case when:

• the flux cannot be applied reproducibly with a microjet;
• insufficient flux reaches the through-hole;
• too much residue remains outside the soldering area;
• the required activation level does not match the solderability of the PCB and components;
• the available preheating process does not match the solvent system;
• solder balls or other process issues occur.

The starting point does not have to be that selective soldering always requires a dedicated flux.

What matters is whether the flux matches the actual process. If it does not, a flux specifically developed or selected for selective soldering can help broaden the process window.

The solder alloy also determines the process window

Flux is only one part of the chemistry. The solder alloy also has a major influence on the selective soldering process.

One important property is its melting temperature.

Many lead-free processes use SnAgCu alloys such as SAC305. These alloys require relatively high process temperatures. Sufficient energy therefore needs to be transferred to bring both the solder joint and the thermal mass of the PCB and component up to temperature.

This can become challenging, particularly with heavy connectors, thick PCBs or assemblies containing large amounts of copper.

Possible responses include:

• increasing preheating;
• providing longer contact with the molten solder;
• reducing the soldering speed;
• using a larger nozzle;
• adjusting the soldering temperature.

However, each of these changes also affects the process and the thermal load on the assembly.

What can a lower-melting-point alloy change?

A solder alloy with a lower melting point can reduce the amount of thermal energy required.

This can offer interesting process advantages. A lower soldering temperature reduces the thermal load on the PCB and components. In certain applications, it may also allow higher soldering speeds because less energy is required to bring the joint above the alloy’s melting point.

A lower-melting-point alloy can therefore influence both the process window and cycle time.

However, it is not automatically the best choice.

Mechanical properties, reliability requirements, compatibility with component finishes and the requirements of the final application remain important. Changing the alloy is therefore a process decision, not simply a machine adjustment.

Flux and alloy need to be selected together with the process

Suppose a solder joint shows insufficient hole fill. It may be tempting to immediately switch to a more active flux.

But the flux may not be the problem.

The cause could also be insufficient preheating, a nozzle that is too small, excessive soldering speed or a solder alloy that requires too much energy in combination with the thermal mass of the assembly.

The reverse is also true. Adjusting machine settings will have limited effect if the flux is not suitable for the application method or the solderable surfaces.

Chemistry and machine parameters should therefore be considered as one process.

Start with the application, not the product

There is no universal flux or solder alloy that is the best choice for every selective soldering application.

A stable process starts by asking the right questions:

• Which PCB and components need to be soldered?
• What is the thermal mass?
• How is the flux applied?
• What level of activation is required?
• Which residues are acceptable?
• Which solder alloy is being used?
• What thermal load is acceptable?
• What cycle time needs to be achieved?

Based on these factors, you can determine whether an existing flux is sufficient or whether a flux adapted to selective soldering could offer advantages. The solder alloy can likewise be evaluated in the context of the complete process.