Comprehensive Guide to Fluxes in Soldering Materials

For any electronics enthusiast, the art of soldering holds a certain mystique. Behind the satisfying sizzle and smooth flow of molten solder lies a crucial, often overlooked component: flux. In solder pastes, flux plays a vital role in ensuring strong, reliable joints, making it the unsung hero of the soldering world.

So, what exactly is flux, and how does it work its magic in solder pastes?

What are the functions of flux in the soldering process?

Before soldering, metal surfaces might have thin oxide layers and impurities, such as dust. These imperfections may prevent the surfaces from being soldered to form a strong bond.  Flux acts as a cleaning agent, dissolving oxides and promoting wetting, i.e., the ability of the solder to spread evenly. To put it simply, the flux prepares the stage for a perfect solder joint.

What are the types of fluxes in solder formulations?

Solder pastes consist of solder powder suspended within a flux paste. This premix offers convenience, saving time and ensuring consistent flux application, which is important for efficient mass production in electronics manufacturing. As you would have imagined, different applications require different flux formulations.  Here are the three primary types of fluxes:

• Rosin-based: Rosin-based fluxes are soldering fluxes primarily composed of natural or synthetic rosin obtained from pine trees. They excel in promoting wetting, ensuring a strong bond between solder and metal surfaces. However, a main disadvantage is their tendency to leave a corrosive residue if not thoroughly cleaned after soldering, which can affect electronic components over time.
• Water-soluble: Water-soluble flux contains resins, activators and solvents that are soluble in water. While this type offers excellent cleaning and soldering performance, the residue needs to be thoroughly washed away with water after soldering to avoid corrosion.
• No-clean: No-clean fluxes can be either made with natural rosin or contain synthetic resins and activators. This type of flux cleans surfaces and promotes good solder connections while leaving minimal residue, eliminating the need for post-soldering cleaning. However, it is important to apply no-clean fluxes correctly to prevent corrosion from any leftover residue.

What are the factors to consider when choosing the right flux type?

Selecting the right flux for solder pastes depends on several factors, including:

• Flux–metal compatibility: Different flux formulations are designed to work with specific types of metals or alloys. For example, rosin fluxes are effective with pads made of copper and its alloys,  like brass and bronze, because rosin can dissolve copper oxides. Rosin fluxes with low halide content may also work with silver-plated pads that tend to tarnish easily. If cleaning can be performed, water-soluble fluxes may also be used for these pads to ensure long-term durability despite their tarnishing potential.
• Level of Surface Oxidation: Surface oxidation plays a crucial role in selecting the right flux for solder pastes.  The appearance of the surface can provide insight into the thickness of the oxide layer. A dull or discolored surface typically indicates a thicker oxide layer, which requires a more aggressive acidic flux to remove effectively. In contrast, a shiny or bright surface suggests a thinner oxide layer, which may only need a milder flux for oxide removal.
• Cleaning Requirements: Cleaning requirements refer to the extent to which the flux residues need to be removed after soldering. Some fluxes leave behind residues that are more difficult to clean, while others produce residues that are easier to remove.
• Environmental Regulations: Selecting the right flux for solder pastes involves considering environmental regulations, such as RoHS that mandate lead-free and halogen-free fluxes in electronics manufacturing. Lead and halogens, such as Cl and Br, are toxic and pose environmental and health risks. While removing lead and halogens can be challenging, advancements have led to effective lead-free and halogen-free fluxes that perform well, ensuring both efficiency and environmental safety.

Classifying and Characterizing Flux using the IPC Classification: J-STD-004C Standard

The guidelines outlined above in choosing the right type of flux is an oversimplification. To help users properly classify, choose, and characterize flux, a standard published by the Institute for Interconnecting and Packaging Electronic Circuits (IPC) outlines the classification and characterization of fluxes for high-quality solder interconnections. It acts as a common language for manufacturers, suppliers, and users, ensuring consistency and quality in the electronics industry.

The IPC J-STD-004C covers:

• Flux classification: Categorizes fluxes based on their chemical composition, activity level, residue characteristics, and cleaning requirements. This helps users quickly identify suitable fluxes for their specific needs.
• Performance testing: Defines various tests to assess the flux’s cleaning ability, wetting performance, corrosion resistance, and other critical properties. This ensures consistent quality and performance across different flux brands.
• Application guidance: Provides recommendations for selecting the right flux based on factors like materials being soldered, cleaning requirements, and environmental considerations.

The table below breaks down fluxes as having a High Solids Content or Low Solids Content, and whether it contains rosin or not. You will also notice a four-character designator starting with RO, RE, OR, etc. The IPC instituted this designator system to more clearly classify, or identify, the activity of fluxes as Low, Medium, or High, based on tests outlined in IPC-J-STD-004. All flux manufacturers perform these tests and identify each flux with the appropriate designator. How the designator applies is illustrated in the table below.

Here are some general representations of flux classification. The list below shows the details of the abbreviation used in the designator:

CAPLINQ manufactures solder pastes with rosin, no-clean and washable flux options. One of the commonly known products offered is LINQALLOY SP-SAC305, a Pb-free, no-clean, solder paste that has been designed for surface mount technology.

SP-SAC305 is available in 4 different options: 

• LINQALLOY SP-SAC305-NC: Designed for applications where post-soldering cleaning is unnecessary, providing minimal, non-conductive residues.
• LINQALLOY SP-SAC305-RMA: Ideal for applications where minimal residue is acceptable, without the need for rigorous cleaning.
• LINQALLOY SP-SAC305-RA: Offers good wetting properties and versatility for a wide range of applications.
• LINQALLOY SP-SAC305-WS: Developed to clean flux residue after the reflow process, crucial for demanding applications.

Learn more about our soldering products here. Contact us with your application requirements to see how we can help you move forward toward a more efficient and sustainable future.