Flux 101: How to Use Flux When Soldering Electronics

by Joost Nusselder | Updated on:  August 25, 2022
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Flux is a chemical agent used to reduce the surface tension of metals to aid in soldering. It’s applied to both the base metal and the solder to remove oxides and contaminants from the surfaces to create a uniform wetted surface.

In this article, I’ll explain what flux is, how it works, and why it’s crucial to successful soldering. Plus, I’ll share some of the different types available.

What is flux

Flux: The Mysterious Force That Makes Soldering Possible

Flux is a substance that is applied to metal surfaces before soldering to help the solder flow and bond properly. It’s a crucial component in the soldering process, as it helps to remove any oxide layers that may be present on the metal surface, allowing the solder to stick to the metal.

How Does Flux Work?

Flux works by reducing the surface tension of the solder, allowing it to flow more easily and evenly over the metal surface. It also helps to prevent oxidation by creating a barrier between the metal and the air.

Types of Flux

There are several different types of flux available, each with its own unique properties and applications. Some of the most common types include:

  • Rosin flux: This is the most common type of flux and is made from the resin of pine trees. It’s a good all-purpose flux that works well for most soldering applications.
  • Water-soluble flux: This type of flux is easy to clean up with water and is often used in electronics manufacturing.
  • No-clean flux: This type of flux leaves behind very little residue and is ideal for applications where cleaning is difficult or impossible.

Why Do You Need Flux?

Flux is essential for successful soldering because it helps to ensure a strong, reliable bond between the metal surfaces being joined. Without flux, the solder may not flow properly, resulting in a weak or unreliable joint.

How is Flux Applied?

Flux can be applied in several different ways, depending on the type of flux and the application. Some common methods include:

  • Brushing: Flux can be applied using a small brush or applicator.
  • Spraying: Some types of flux can be sprayed onto the metal surface.
  • Dipping: The metal can be dipped into a container of flux.

Common Mistakes When Using Flux

While flux is a crucial component of the soldering process, it’s easy to make mistakes when using it. Some common mistakes include:

  • Using too much flux: This can result in a messy, difficult-to-clean joint.
  • Using the wrong type of flux: Using the wrong type of flux can result in a weak or unreliable joint.
  • Not cleaning the flux residue: Flux residue can be corrosive and can damage the joint over time if not properly cleaned.

Understanding the Terminology of Flux

Flux is a term that is used in various fields, including electromagnetism, transport, and calculus. The word “flux” comes from the Latin word “fluxus,” which means “to flow.” In physics, flux is a vector field that describes the transfer of a quantity through a surface. The concept of flux is fundamental to the analysis of many physical systems, and it has contributed to the development of major theories in physics.

The Key Differences in the Definition of Flux

The definition of flux can vary depending on the field in which it is used. Here are some of the key differences in the definition of flux:

  • In electromagnetism, flux refers to the integral of the magnetic field over a surface. This is defined by Maxwell’s fundamental theorem of electromagnetism.
  • In transport, flux describes the transfer of a quantity, such as mass or energy, through a surface. This is defined by the corresponding density gradient.
  • In calculus, flux is based on the concept of the derivative and refers to the changing rate of a quantity through a surface. This is defined by the gradient of the function.

The Seminal Contribution of James Clerk Maxwell

James Clerk Maxwell was a Scottish physicist who made seminal contributions to the field of electromagnetism. In his treatise “A Dynamical Theory of the Electromagnetic Field,” he defined the concept of flux and derived expressions for the integral of the magnetic field over a surface. His work laid the foundation for the development of modern electromagnetic theory.

The Conflicting Definitions and Interchangeability of Terms

The definition of flux can be conflicting and interchangeable depending on the field in which it is used. For example, in nontechnical contexts, the terms “flux” and “flow” are often used interchangeably to describe concrete phenomena. However, in technical contexts, the terms have distinct definitions and cannot be used interchangeably.

The Integration of Flux in Calculus

In calculus, flux is integrated over a surface to derive expressions for the changing rate of a quantity. This is done using the fundamental theorem of calculus, which states that the integral of a function is equal to the difference between the values of the function at the endpoints of the integration. The integration of flux is a fundamental concept in calculus and is used in many applications, including fluid dynamics and heat transfer.

Flux: The Secret Ingredient for Perfect Soldering

Flux is a chemical agent used in soldering to promote the wetting of metal surfaces by molten solder. It helps to prevent the formation of oxides on the surface of the metal, which can inhibit the flow of solder and cause poor adhesion between the solder and the metal. Flux also protects the exposed metal surfaces from air, which can cause the formation of oxide films, altering the surface and making it difficult to solder.

The Purpose of Flux in Soldering

The purpose of flux in soldering is to aid in the formation of a uniformly wetted surface between the solder and the metal components being joined. Flux helps to clean the surface of the metal, removing any oxides or other contaminants that may prevent the solder from adhering properly. It also promotes the flow of solder by reducing the surface tension of the molten solder, allowing it to spread more easily and uniformly over the metal surfaces.

Choosing the Right Type of Flux for Your Soldering Process

Using the right type of flux is essential to ensure the best possible performance of your electronic components. Here are some reasons why:

  • Using the wrong type of flux can result in poor soldering performance and even damage to your components.
  • Using the right type of flux can increase the lifespan of your components and prevent the need for costly repairs.
  • Using the right type of flux can ensure that your soldering process is completed correctly and efficiently.

Cleaning Flux Residue from Electronics

When you’re done soldering your electrical parts, you may notice that there’s some excess flux left on the board. Leaving this residue on the board can cause electrical problems and even short circuits. Therefore, it’s essential to clean the flux residue from your PCB board to improve the overall quality of your product.


So, there you have it- a brief introduction to flux and why it’s so important when you’re soldering. Flux helps remove oxide layers from the metal and makes the solder flow more easily. It’s a crucial component of the soldering process and helps you get the job done right. So, don’t forget to use it the next time you’re working with metal.

I'm Joost Nusselder, the founder of Tools Doctor, content marketer, and dad. I love trying out new equipment, and together with my team I've been creating in-depth blog articles since 2016 to help loyal readers with tools & crafting tips.