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What is Solder Flux? | Soldering Basics

by: Mar 03,2023 1360 Views 0 Comments Posted in PCB Basic Information

Solder Flux

What is Solder Flux and How Does It Work?

Solder flux is a useful tool to promote bonding between the solder and the metal surfaces of the components during the soldering process. While solder paste is a mixture of solder alloy particles and flux, solder flux serves as a chemical cleaning agent to remove oxidation and other contaminants from metal surfaces before soldering. Without flux, oxidation may create a barrier between the metal and the solder, leading to weak and unreliable solder joints.


Types of Solder Flux

Rosin flux, no-clean flux, and water-soluble flux are the three main types of fluxes. Among these, rosin flux is the most commonly used due to its non-corrosive properties, effectiveness in the recommended soldering temperature range for most electronics applications, and less chance of flux residue. No clean flux is intended for applications where cleaning the flux residue is not necessary or feasible. No-clean flux typically leaves a minimal amount of residue that is not harmful to the PCB or the components. As for water-soluble flux, it is a glycol-based solution that is commonly used in automated PCB assembly processes where high volumes of boards need to be processed quickly. The water-soluble nature of the flux makes it easy to clean off the boards after soldering, using a simple water rinse.


There are various methods for cleaning flux residue, including using solvents, isopropyl alcohol, or specialized cleaning solutions. The specific method used will depend on the type of flux and the cleaning equipment available.


Considerations for Effective Soldering with Solder Flux

When using the solder flux in the soldering, two crucial factors must be considered. The first is the compatibility with the materials being soldered to prevent any potential damage or corrosion. For example, if the flux is incompatible with copper, it can lead to corrosion and failure of the solder joint. The other is the activity level of the flux. The flux should be active enough to remove any oxidation or contaminants from the surfaces being soldered but cannot be too active to damage the components or the solder joints. Finding the right balance between compatibility and activity level is essential to achieving strong and reliable solder joints.


In What Cases Is It Possible to Solder Without Using Flux?

While solder flux is generally recommended to achieve a successful solder joint, there are cases where it may not be necessary. For example, when you’re soldering small components like surface mount devices, the flux contained in the solder paste or pre-tinned pads may be enough for the solder to flow and bond with the metal. All in all, the decision of whether or not to use solder flux in the soldering process depends on specific applications, including the materials being soldered, the condition of the metal surfaces, the type of solder being used, and the desired strength and reliability of the solder joint.



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