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Flux, a term often heard in electronics soldering, is an auxiliary material that enables solder to wet and bond properly.
This page explains what soldering flux is, including its roles, main components, and types, and also introduces solder paste, which consists of solder powder and flux.
Pre-flux, also called an organic solderability preservative (OSP), is another term used in electronics assembly. However, it is different from the soldering flux discussed on this page.
Pre-flux is a thin protective coating formed on the copper surfaces of PCB lands and pads to protect them against contamination and oxidation before soldering. Various OSP treatments are used depending on PCB and assembly requirements.
Soldering is a joining process in which a solder alloy is melted by heat and solidified at the interface between the materials to be joined, forming a thin reaction layer and creating a joint.
Soldering is one of the most widely used joining methods in electronics assembly.
Many electronic components in devices such as smartphones, game consoles, tablets, and PCs are soldered to printed circuit boards.
Photo 1 shows an example of soldering with a soldering iron.
Photo 1: Example of soldering
Metal surfaces are normally covered with oxide films. Simply melting solder with heat is therefore not enough to achieve proper wetting and bonding to the base material*1.
Even after an oxide film is removed, the exposed metal surface can oxidize again when it reacts with oxygen in the air.
This is where flux plays an important role.
Rosin, a naturally derived material obtained from pine resin, is a well-known flux base material. Photo 2 shows a lump of rosin.
Photo 2: Lump of rosin
Flux supports soldering through functions such as:
✓ Removing oxide films and surface films from solder and the metal surfaces to be joined, helping the solder wet the surface
✓ Helping protect cleaned metal surfaces from reoxidation during soldering
These functions help molten solder wet and form a reliable joint.
*1 Base material: The material being joined. In electronics assembly, examples include component terminals and electrode pads on printed circuit boards.
This section focuses on resin-based fluxes commonly used in electronics assembly.
For the different flux categories, see “Types of Soldering Flux and Their Characteristics” below.
Resin-based flux generally consists of a base material such as rosin or synthetic resin, activators, and, where required, solvents. The main components are explained below.
The main base material in resin-based flux is a resin such as rosin or synthetic resin.
During heating, the flux system helps remove or control oxide films on solder and the surfaces being soldered and helps protect the cleaned surfaces against reoxidation during the soldering process.
Activators are additives that enhance the flux's ability to remove oxide films from solder and the surfaces being joined. Depending on the solder alloy and soldering temperature and time, a flux formulation may contain more than one type of activator.
Alcohol-based solvents and other suitable solvents are used to dissolve resin components and adjust flux viscosity.
They also support uniform application to printed circuit boards, wetting of the base material, and penetration into narrow gaps.
In addition to resin-based fluxes such as rosin-based flux, organic and inorganic fluxes are used for different soldering applications.
The table below summarizes representative base materials and activators.
| Flux Category | Constituents | |
|---|---|---|
| Base Material | Activator | |
| 1. Resin-Based | 1. Rosin a. Modified rosin 2. Synthetic resin |
1. None 2. Amine halide salts 3. Organic acids and amine organic-acid salts |
| 2. Organic | 1. Water-based materials 2. Solvent-based materials |
|
| 3. Inorganic | a. Water-soluble materials b. Water-insoluble materials |
a. Ammonium halides b. Zinc halides c. Tin halides d. Phosphoric acid e. Hydrohalic acids |
Rosin-based fluxes are widely used in electronics assembly and are commonly referred to as rosin fluxes.
Among organic fluxes, formulations based on water with organic activators are commonly referred to as water-soluble fluxes. Other organic flux formulations use alcohol or other solvents as the carrier.
Inorganic fluxes include formulations based on water-soluble materials and formulations based on water-insoluble materials. Their residues can remain chemically active after soldering and may cause corrosion if not adequately removed. For this reason, inorganic fluxes are generally not used for ordinary printed circuit board soldering.
Fluxes are also available in formulations designed for specific environmental or process requirements, including halogen-free, low-residue, and low-VOC types.
As described above, many electronic components in devices such as mobile devices and computers are soldered to printed circuit boards. Surface-mount assembly commonly uses solder paste for this purpose.
Solder paste is a mixture of powdered solder and paste-form flux. It is also called solder cream in some contexts.
Solder powder consists of fine, generally spherical solder particles. The particle size is selected according to stencil aperture size, component pitch, printing requirements, and the applicable solder-paste powder classification. Finer solder powders are used for increasingly small and fine-pitch components.
The flux portion of solder paste consists of components such as resin, activators, solvents, and rheology-control additives.
Thixotropic or rheology-control additives help prevent separation of the solder powder and flux and provide the printing characteristics required for stencil printing.
Flux is essential to the soldering process, but after soldering, flux-derived material may remain on the assembly as flux residue. Depending on the flux formulation, residue amount, product design, operating environment, and reliability requirements, flux residue can contribute to electrical, physical, or process-related problems.
| The following page explains flux residue in more detail. | |
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Technical Solutions
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Standard Micro-Soldering Technology, 3rd Edition
Edited by the Micro Soldering Education Committee, Japan Welding Engineering Society
The Nikkan Kogyo Shimbun, Ltd.
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Published Technical Article
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Technical Solutions
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