What requirements must a flux meet? Chemical activity, thermal stability, and wettability

Flux is an indispensable part of the welding process; it serves multiple functions that help ensure high-quality welds. In this article, we will take an in-depth look at the basics of flux, explore its role in welding, and examine key factors related to the welding process.

Requirements for Surface-Mount Soldering Flux

1. Chemical Activity

Flux must possess a certain degree of chemical reactivity to function effectively during the soldering process. It reacts chemically with the solder and the substrate surface, helping to remove oxides and promote solder wetting.

2. Thermal Stability

Good thermal stability is one of the key properties of a flux. During high-temperature soldering, the flux must maintain its performance without decomposing or losing effectiveness.

3. Wettability

Flux must have good wettability, which means it can quickly cover the soldering surface and reduce the surface tension of the solder, allowing it to spread evenly and wet the soldering area.

4. Expanded Role

The flux should promote the spread of the solder so that it fully covers the soldering area, ensuring the quality of the solder joint.

5. Cleanability

Flux residues must be easy to clean off to ensure that no residue remains on the surface after soldering, which could affect the product's performance.

6. Low chlorine content

The chlorine content should generally be kept below 0.2% (W/W) to avoid adverse effects on the welding materials.

The Role of Flux

In the welding process, flux plays a critical role, primarily in the following areas:

Auxiliary Heat Transfer

Flux helps improve heat transfer and ensures that the solder melts evenly and wets the surfaces to be joined.

Removing Oxides

The active ingredients in the flux react with the oxides on the substrate's surface, removing the oxide layer and making the solder joint more reliable.

Reduce surface tension

By reducing the surface tension of the solder, the flux helps the solder spread evenly across the soldering area, thereby improving the quality of the solder joint.

Prevent Reoxidation

At high temperatures, the flux forms a protective layer over the surface of the solder, preventing re-oxidation and ensuring the quality of the solder joint.

Physical Properties of Flux

The physical properties of flux include melting point, boiling point, softening point, vitrification temperature, vapor pressure, surface tension, viscosity, and miscibility. These properties are closely related to soldering performance and must be taken into account when selecting and using flux.

Defects Caused by Flux Residue and Countermeasures

Flux residue can cause certain problems, so appropriate measures must be taken to address them:

Question

It is somewhat corrosive to the substrate.
Reduced electrical conductivity may cause migration or short circuits.
Non-conductive solid residues may affect component contact.
Excessive resin residue can affect product reliability.

Countermeasures

Select a suitable flux with a moderate level of activator activity.
Use a flux that forms a protective film after welding
Use a flux that leaves no resin residue after welding
Use a no-clean flux with low solid content
Clean the area thoroughly after welding.

Flux Classification Codes Specified in QQ-S-571E

According to the QQ-S-571E standard, fluxes can be classified using different codes, which reflect their type and properties. Some common codes include:

S: Moderately solid (no flux)
R: Rosin flux
RMA: Low-activity rosin flux
RA: Active rosin or resin flux
AC: Flux containing no rosin or resin

Synthetic resin fluxes in the United States are also classified using specific codes, such as SR, SMAR, SAR, and SSAR, which reflect the different types and activity levels of synthetic resin fluxes.

Flux Spraying Methods and Process Factors

During the welding process, flux can be applied to the weld surface using various spraying methods. The following are some common spraying methods:

1. Ultrasonic Spraying

Ultrasonic spraying uses oscillating electrical energy to atomize the solder paste, which is then sprayed onto the PCB through a pressure nozzle. This method ensures uniform application.

2. Screen Sealing Method

The screen-coating method uses a fine mesh screen and an air knife to spray the flux, creating a mist that covers the PCB.

3. Spray Coating with a Pressure Nozzle

Pressure-nozzle spraying involves using pressure and air to eject flux directly from the nozzle; it is typically used for welding large areas.

Key factors to consider in the spray coating process include pore size, nozzle shape, nozzle spacing, ultrasonic atomizer voltage, nozzle movement speed, PCB conveyor belt speed, and the solid content of the flux.

Key Characteristics of No-Clean Flux

No-clean flux is a special type of flux with the following main characteristics:

It has good solderability, producing full solder joints without defects such as solder balls or bridging.
Non-toxic, environmentally friendly, and safe to use.
After welding, the plate surface is dry, non-corrosive, and non-stick.
Online testing capabilities are available after welding.
It is compatible with SMD and PCB boards.
After welding, the surface insulation resistance (SIR) meets the specified requirements.

In summary, flux plays an indispensable role in the welding process; it helps improve weld quality and ensure the reliability of the weld. Selecting the appropriate flux, along with the correct application method and process parameters, is crucial for successful welding.

If you have any questions about flux or other welding-related issues, please feel free to contact us. We will be happy to provide you with assistance and advice.

Frequently Asked Questions

1. What adverse effects can flux residue have on a circuit board?

Flux residue can cause corrosion of the circuit board, reduced electrical conductivity, poor component contact, and compromised product reliability.

2. What is no-clean flux?

No-clean flux is a soldering material that does not require cleaning after soldering and offers a range of advantages, such as being non-toxic, pollution-free, and safe to use.

3. Why is it important to choose the right flux?

Choosing the right flux can improve soldering quality, ensure the reliability of solder joints, and minimize adverse effects on the circuit board.

4. How can you ensure uniform flux application?

To ensure uniform flux application, it is necessary to consider the nozzle orifice size, shape, and spacing, as well as other process factors, such as the ultrasonic atomizer voltage and conveyor belt speed.

5. Why is a flux with good wettability important?

A flux with good wetting properties ensures that the solder is evenly distributed across the soldering area, improving the quality of the solder joints and ensuring the reliability of the solder joint.