What is flux, the leading actor of soldering ?

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What is "flux", which is often heard of in the soldering process, and what does ”flux" do?

In this article, we will explain what flux is, its role, components, types, etc., as well as problems caused by flux residue left after soldering with solder paste, which consists of solder powder and flux, and countermeasures against such problems.

The Role of Flux in Soldering

Soldering is a technique in which an alloy called "solder" that melts at a temperature of 450°C or lower is melted by heat and solidified at the interface between the materials to be joined, forming a very thin reaction layer.
Soldering is the most commonly used joining method in the electronics field.
A variety of electronic components are soldered to circuit boards in our everyday electronic devices such as smartphones, gaming devices, tablets, and PCs.
Photo 1 shows an image of soldering with an iron (soldering iron).

Photo 1: Image of soldering

Normally, the surface of a metal is covered with an oxide film, so simply melting the solder with heat is not enough to join to the base metal*1.
Moreover, even if the oxide film is removed once, it can quickly form again when it is combined with oxygen in the air.
This is where flux comes into play.

Rosin, a naturally occurring substance, is well known as a flux. Photo 2 shows a clump of pine resin.

Photo 2: A lump of pine resin

By using flux, you can
✓ Remove the oxidized film or surface film on the surface of the solder powder and the metal surface of the base material, and make it ready for soldering (this is called "wetting").
✓ Prevents re-oxidation of the metal surface and keeps the metal surface clean.
and thus flux plays a important role in enabling soldering.

*1 Base metal: Something to be joined. In the case of soldering in the mounting process, it is the electrode of the component or the electrode pad of the circuit board.

Components of flux

This section describes fluxes classified as "resin-based," which are commonly used in the mounting process.
For more information on flux types, please refer to "Flux Types" below.
Resin-based fluxes consist of a main agent such as pine resin (hereinafter referred to as rosin), an activator, and a solvent if necessary.
Let's take a look at them one by one in detail.

Main agent

The main agent in flux is a resin (rosin, synthetic resin, etc.).
It is heated to remove the oxidized film on the surface of the solder or adherend, and at the same time, it protects the solder and adherend surface from re-oxidation.


An activator is an additive that increases the ability to remove the oxide film from the surface of the solder and adherend. Depending on the type of solder and the soldering temperature and time, it may contain more than one type of activator.


Solvent is an alcohol-based solvent that dissolves the solid resin that is the main agent.
It also adjusts the viscosity of the flux and helps to ensure that the flux is applied evenly to printed circuit boards, wets the base material, and improves penetration into gaps.

【Reference】Types of Flux

In addition to resin-based fluxes, organic fluxes are sometimes used for soldering in the mounting process.
The table below shows the constituent materials of each type.

Flux Classification Component Materials
Main Agent Activator
①Resin-based 1. Rosin
a. Modified rosin
2. Synthetic resin
1. No additives
2. Halogenated salts of amines
3. Organic acid, amine organic acid salt
②Organic system 1. Water-based substance
2. Solvent-based substance

・Rosin-based fluxes are commonly used in electronics packaging in Japan.
・Organic fluxes include a type called "water-soluble flux," which is often used overseas.
・In recent years, halogen-free, low-residue, and low-VOC fluxes have been developed with the environment in mind.

Solder paste

As mentioned above, various electronic components are soldered to circuit boards in the electronic devices around us (mobile devices, computers, etc.), and solder paste is used for soldering in these devices.
Solder paste is a mixture of powdered solder and paste-like flux, also known as cream solder.

Solder powder

Solder powder is generally spherical with a particle size of 20~30um. Recently, solder paste containing solder powder with a particle size of 5-15um has been introduced to the market for the mounting of small and micro components such as 0201 size.

Flux in paste form

A paste consists of flux (rosin, activator, solvent) and a thixotropic agent.
Thixotropic agents are added to prevent the powdered solder from separating from the flux, and to improve printability when printing solder paste.

Flux after soldering

Flux is necessary for soldering, but once soldering is completed, the flux is no longer needed as it burns away. Burnt flux is called flux residue, and it can cause problems with printed circuit boards.

Flux residues are explained in the following page.
Trouble solutions
What is flux residue?
Flux residue, which is burned flux after soldering, can cause electrical and physical problems on printed circuit boards. This page introduces problems caused by flux residue and their solutions.


Standard Micro Soldering Technology, 3rd Edition
Japan Welding Engineering Society, Micro Soldering Education Committee [ed.]
Nikkan Kogyo Shimbun

Related articles/products

Technical notes
Technical Document ~ Flux Cleaning ~
Excerpt from Micro Packaging and Bonding Technology (2012).
Describes the evolution of flux cleaning agents, the purpose and original significance of flux cleaning, as well as current technologies and future issues, with a focus on glycol ether-based cleaning agents, which are the most commonly used flux cleaning agents in current mounting sites.
Trouble solution
Cleaning in the Electronics
An explanation of the mounting process and the related cleaning is given, using the example of the double-sided mounting process, in which inserted components are mounted after surface mounting.
Product information
Flux cleaning system
KAKEN TECH was one of the first companies to establish its own cleaning evaluation technology, cleaning agents, and cleaning equipment for flux cleaning for electronic circuit mounting, and has accumulated a great deal of experience in flux cleaning. We were one of the first companies to predict that flux cleaning would become more difficult with the introduction of lead-free and halogen-free solder, and our pre-developed products have already been well received in many markets.


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