A flux is a substance added during soldering , which causes better wetting of the workpiece by the solder . It removes the oxides on the surface through a chemical reaction. The same applies to oxides that are created during the soldering process by the oxygen in the air. Fluxes also reduce the interfacial tension .
Mode of action
During soldering, the flux reduces the hot surface of the workpieces to be joined and the liquid solder. In addition, the surface tension of the liquid solder is reduced.
The requirement is the same for all fluxes to be used: They must secure oxide-free soldering surfaces or ensure that they are oxide-free. Fluxes vary in their composition, depending on the type of parts to be soldered and the temperature of the solder used. For both soft and hard soldering, there are special fluxes that take into account the melting point of the solder in terms of composition and concentration.
- For work on oxidized joining partners, acidic fluxes are used (soldering water based on hydrochloric acid , salicylic acid , acetylsalicylic acid , adipic acid , soldering grease). Residues of acidic fluxes must be removed as they lead to corrosion of the solder joint in the long term.
- Activators ( zinc chloride , ammonium chloride or organic salts) are used to improve the alloying process .
- For the solder connection, the activators generate a higher concentration of mobile ions in the flux film on the workpiece surface.
- Organic solvents are used for greasy, soiled parts to be joined.
- The flux, which melts during use, leaves behind partially chemically converted residues on the soldering point and on neighboring insulating materials. Depending on the type, these residues are corrosive, conductive or hygroscopic . This can lead to malfunctions in the electronics due to insufficient insulation or electrical connections that corrode away over time. The soldered joints made with these fluxes must therefore be cleaned with a suitable solvent after soldering . The residues of many fluxes can be removed with isopropanol . Others can only be cleaned with chlorofluorocarbons . Some are soluble in water. There are also fluxes especially for soft soldering in electronics, which are chemically so passive at room temperature that cleaning is not necessary. These fluxes are advertised as " No-Clean ".
- For aluminum there are now alternative soldering processes that make the use of flux superfluous.
- Chrome-nickel steel and stainless steel can also be soldered with special fluxes.
In special cases, such as when renewed oxidation of the soldering point by the atmosphere must absolutely be avoided and no flux is available, soldering is carried out without such under protective gas or in a vacuum. This prevents oxidation of the surfaces to be connected during the soldering process.
Fluxes have different compositions for different tasks. They only reach full reactivity at the melting temperature of the solder. But they can also pose a chemical hazard at room temperature. Depending on the type, they are in contact
- corrosive ( hydrochloric acid , adipic acid ) and / or
- corrosive ( zinc chloride , ammonium chloride ) and / or
- harmful to health ( halogenated hydrocarbons and / or fats and / or colophony ).
Therefore, when using a flux, the relevant safety data sheet with the H and P statements contained therein must be observed. This is especially true for the vapors generated during the soldering process with many fluxes.
Type identification of flux used for soldering
The type identification consists of the letter F (derived from the term flux), followed by two more letters. The first indicates the material to be soldered : S ( heavy metal ), L ( light metal ); the second the soldering process: H ( hard soldering ) , W ( soft soldering )
The tools can be used for both hard and soft soldering.
The fluxes used differ according to their effective temperature and the corrosion behavior of the residues. Corrosive residues should be washed off or stripped off after the soldering process because they chemically attack the soldering point or the workpiece.
- for heavy metals
- F-SH 1: effective temperature 550 ° C to 800 ° C, boron compounds , complex fluorides , corrosive
- F-SH 1 a: effective temperature 550 ° C to 800 ° C, boron compounds, complex fluorides, chlorides , corrosive
- F-SH 2: effective temperature 750 ° C to 1100 ° C, boron compounds, non-corrosive.
- F-SH 3: Working temperature 1000 ° C to 1250 ° C, boron compounds, phosphates , silicates , non-corrosive.
- F-SH 4: Working temperature 600 ° C to 1000 ° C, chlorides, fluorides without boron compounds, corrosive.
- for light metals
- F-LH 1: effective temperature 500 ° C to 600 ° C, hygroscopic chlorides and fluorides, corrosive
- F-LH 2: effective temperature 500 ° C to 600 ° C, non-hygroscopic fluoride, non-corrosive.
For soft soldering
Flux residues cause, depending on the type of flux, different degrees of or only slight corrosion . The solid residues of the applied flux must be carefully removed after soldering. In addition, there are practically residue-free fluxes, so-called No Clean Flux , which are increasingly used in electronics production. Most fluxes are hygroscopic, so they absorb humidity and encourage corrosion.
EN ISO 9454
Fluxes for soldering applications are specified in the EN ISO 9454 standard . They are identified by a four-digit code consisting of three digits followed by an optional letter with the following meaning:
|Flux type||Base||Activator||Physical state|
Old standard DIN 8511
Before the introduction of DIN EN 29454-1 in 1994 (in Germany) fluxes were designated according to the DIN standard DIN 8511. The scheme for soft solders was F-SW-xx , where xx stands for two digits. DIN 8511 followed the following classification:
Residues cause corrosion
- F-SW 11: liquid. An aqueous solution of zinc , chlorides of other metals and / or ammonium chloride . The active substance in activation is hydrochloric acid , sulfuric acid , nitric acid or hydrofluoric acid . For heavily oxidized surfaces, e.g. gutters made of pure zinc.
- F-SW 12: The basis is zinc chloride or other heavy metal chlorides and / or ammonium chloride / salmiac. Flux residues must be carefully washed off with water. Use as a liquid in radiator construction , for plumbing work on copper, dip tinning , among other things. As a powder for covering solder and tin baths. As a solder / flux mixture for soft tinning, for example of cast iron , bronze or stainless steel .
- F-SW 13: The basis is phosphoric acid or its chemical derivatives . Residues must be removed using suitable cleaning methods. Use as a liquid for working on copper, copper alloys or stainless steel.
Residues have a limited corrosive effect . As a carrier, u. a. Alcohol , mineral oils and fats, higher alcohols, organic solvents and emulsions are used.
- F-SW 21: effective temperature 140 ° C to 450 ° C, zinc and / or ammonium chloride in organic preparation (e.g. higher alcohols, fats ), conditionally corrosive, for copper and copper alloys.
- F-SW 22: Active temperature 200 ° C to 400 ° C, zinc and / or ammonium chloride in organic preparation (e.g. higher alcohols, fats), without ammonium chloride, conditionally corrosive, for copper and copper alloys, drinking water installations.
- F-SW 23: effective temperature 200 ° C to 400 ° C
- F-SW 24: effective temperature 200 ° C to 400 ° C
- F-SW 25: effective temperature 200 ° C to 400 ° C
- F-SW 26: effective temperature 140 ° C to 450 ° C
- F-SW 27: effective temperature 140 ° C to 450 ° C
- F-SW 28: effective temperature 140 ° C to 450 ° C
Residues are not corrosive . Residues can remain on the solder joint without the risk of corrosion. With electrical measuring devices u. the electrical and mechanical properties can be impaired; it is therefore recommended to remove brittle resin residues. Alcohol or tri ( trichlorethylene ) are used as solvents .
- F-SW 31: Working temperature 200 ° C to 400 ° C, based on natural or modified resin (rosin) without additives. Flux residues can remain on the solder joint. Use in electrical engineering, electronics as a powder to cover solder baths. or incorporated into the solder wire as a flux core.
- F-SW 32: effective temperature 200 ° C to 300 ° C, basis like F-SW 31, but with organic, halogen-free activation additives, for example stearic , salicylic , adipic acid , without amines , diamines or urea . Use as a powder, incorporated into solder wire as a “soul”. or as a mixture of solder and flux in electrical engineering, electronics , miniature technology ( SMD ), circuit boards .
- F-SW 33: Based on synthetic resins with organic, halogen-free activation additives, but without amines , diamines or urea. Application like F-SW 32.
- F-SW 34: Basis are organic, halogen-free acids and natural resin (rosin) without amines, diamines or urea. Use as a liquid or as a "soul" incorporated into the solder wire in electronics, miniature technology (SMD), circuit boards.
Comparison of the two standards
|Corrosion behavior of
|DIN 8511||ISO 9454-1|
IPC-J-STD-004B, EN 61190-1-1
In the industry, a classification based on the specification published by the IPC according to IPC-J-STD-004B is increasingly being made (largely identical to EN 61190-1-1). This specification describes the respective flux with three letters and a number:
|Base||RO (sin) - RE (sin) - OR (ganic) - IN (organic)|
|effectiveness||L (ow) - M (oderate) - H (igh)|
|Halide content <0.05% (500 ppm)||0 (yes) - 1 (no)|
All combinations are possible, such as ROL0 , REM1 or ORH0 .
Occasionally you can still find English / American abbreviations with which the type and activator of a flux is roughly described, especially RA and RMA :
|OA||Organic acid||with organic acid, very active|
|R.||Rosin||pure rosin without activator, very weakly active|
|RA||Rosin activated||Colophony with a strong activator|
|RMA||Rosin mildly activated||Colophony with a mild activator|
|SA||Synthetic activated||Colophony with synthetic activator, very active|
|SRA||Superactivated raisin||Colophony with a very strong activator|
|WW||Water white||purest rosin, very weakly active|
Literature on soldering aids
- Reinard J. Klein Wassink: Soft soldering in electronics . 2nd Edition. Eugen G. Leuze, Saulgau 1991, ISBN 3-87480-066-0 .
- Wolfgang Scheel (Hrsg.): Assembly technology of electronics . Verlag Technik u. a., Berlin a. a. 1997, ISBN 3-341-01100-5 .
- History of the flux. In: Peter CA Schel: Small Encyclopedia of the German Middle Ages
- Solders and flux , wiki of electrical contacts
- | Patent specification (German Patent DE4415527) on the use of salicylic acid or acetylsalicylic acid in soft soldering.
- Soft soldering of aluminum without flux. Video of the MDR television; accessed on February 8, 2019.
- Standards for soldering fluxes ( Memento of the original from July 19, 2011 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. DIN EN 29454-1 (ISO 9454-1) Soldering Fluxes, Technical Information (English) accessed on March 11, 2011.
- IPC J-STD-004B (PDF; 113 kB) Requirements for Soldering Fluxes, IPC, 2008 (English)