If you’ve ever looked at a solder flux label and felt like you were trying to decipher a secret code, you’re not alone. The string of letters and numbers—like ROL0, ORM1, or REL1—can seem cryptic at first glance. But these classifications aren’t just random; they’re a standardized way to communicate critical information about a flux’s chemistry, activity level, and reliability.
Understanding solder flux classification helps you make informed decisions about which flux best suits your application, whether you’re assembling high-reliability electronics or working with a no-clean formula for streamlined processing. In this post, we’ll break down the system behind these classifications so you can confidently choose the right flux for the job.
Decoding J-STD-004
The organization responsible for the four-character flux codes is the Institute for Printed Circuits, or IPC. They also like to give code names to their standards documents, and it is IPC’s J-STD-004 that outlines this system for classifying solder fluxes. In the next sections, we tackle each part of the code and give an overview of what the different designations mean for your flux selection.
First Two Characters: Flux Composition
The first two characters classify fluxes based on their composition—Rosin, Resin, Organic, or Inorganic. The composition of solder flux significantly influences soldering performance, affecting wetting behavior, joint integrity, and the need for post-soldering cleaning.
Rosin Flux (RO)
- Composition: Derived from pine sap, rosin flux is a naturally occurring substance. It is inert at room temperature, acidic at high temperatures, and non-acidic once cooled again. Because it is a plant product, it may be subject to more natural variation than its cousin, resin.*
- Function: Rosin flux acts by removing oxides from metal surfaces, enhancing the wetting process. Its mild nature requires minimal to no cleaning after soldering, depending on the specific formulation and application.
Resin Flux (RE)
- Composition: Resin fluxes are either modified rosin or completely synthetic material.* The performance of resin may be more consistent than that of rosin since it is less subject to natural variations.
- Function: Similar to rosin flux, resin fluxes facilitate the removal of oxides and improve the soldering process. They can be formulated to provide varying levels of activity and residue cleanliness, catering to diverse manufacturing needs.
* Note that some fluxes contain both rosin and resin and their designation is determined by which substance they contain more of.
Organic Flux (OR)
- Composition: Organic fluxes are composed of organic acids or other organic compounds. They are known for their stronger cleaning capabilities and are often water-soluble.
- Function: These fluxes remove oxides and contaminants from metal surfaces, ensuring excellent wetting. Because these fluxes generally do not deactivate until they reach high temperatures, they typically require thorough cleaning post-soldering to remove any residual active, corrosive residues.
Inorganic Flux (IN)
- Composition: Inorganic fluxes contain inorganic acids or salts. These are the most aggressive types of flux, used for applications requiring the removal of heavy oxides.
- Function: Due to their strong activity, inorganic fluxes are highly effective in preparing metal surfaces for soldering. They are mostly used in specialized applications where the removal of significant oxidation is necessary. Post-soldering cleaning is essential to prevent corrosion and damage to the assembly. IN flux is not used in electronics assembly.
Third Character: Activity Level
The activity level of flux directly influences the formation of solder joints by affecting the removal of oxides and impurities from the metal surfaces. Fluxes earn their activity level designations by falling within certain ranges on IPC specified tests, including copper mirror, surface insulation resistance (SIR), and quantitative halide tests. (Note that halide content, which is discussed in the next section, does not alone designate the activity level since many other additives also have an impact.)
Low Activity Fluxes (L)
- Definition: Designed for applications where minimal oxide removal is required. In addition to meeting other testing criteria, low activity fluxes contain less than 0.05% halides by weight when designated L0, and less than 0.5% halides by weight when designated L1.
- Impact: These fluxes are typically used in controlled environments where surfaces are already relatively clean. While they minimize the risk of damaging sensitive components, their mild action may not be sufficient for heavily oxidized surfaces.
Moderate Activity Fluxes (M)
- Definition: More aggressive than low-activity fluxes, moderate activity fluxes still manage to limit the risk of corrosion or damage. In addition to meeting other testing criteria, these fluxes contain less than 0.05% halides by weight when designated M0, and between 0.5 and 2.0% halides by weight when designated M1.
- Impact: Ideal for general soldering purposes, moderate activity fluxes ensure good wetting and bonding on surfaces with moderate oxidation. They strike a balance between cleaning effectiveness and post-soldering residue concerns.
High Activity Fluxes (H)
- Definition: These fluxes possess the highest cleaning strength, capable of removing significant amounts of oxide and contaminants from metal surfaces. In addition to meeting other testing criteria, these fluxes contain less than 0.05% halides by weight when designated H0, and more than 2.0% halides by weight when designated H1.
- Impact: High activity fluxes are essential for challenging soldering applications involving heavily oxidized metals or where strong cleaning action is needed. However, their aggressive nature requires careful handling and thorough post-soldering cleaning to prevent corrosion or residue-related issues.
Fourth Character: Halide Content
Halides are added to fluxes to improve their activity level, enhancing their ability to clean metal surfaces and promote wetting. While beneficial for soldering performance, halide containing residues may lead to corrosion, electrical failures, or reduced long-term reliability of the solder joint depending on the flux formula and application requirements.
Low Halide Content (0)
Indicated as “0” in flux designators, fluxes with halide content below 0.05% by weight are considered “halide-free.” These fluxes are designed to minimize the risk of corrosion and are suitable for applications requiring clean, reliable solder joints without extensive post-soldering cleaning.
Presence of Halides (1)
Indicated as “1” in flux designators, fluxes containing halides (above 0.05% by weight) are used for their enhanced activity. Halides boost the flux’s ability to remove oxides but will increase the flux residue ionic contamination potential.
A Note on Halides vs Halogens
Halides are ionic compounds typically used as activators and often associated with corrosion. IPC standards address test methods and thresholds for halide content. Non-ionic halogenated compounds, or halogens, are the subject of environmental regulation. European Environmental Standards (EN-14582) address test methods and thresholds for halogen content.
You may come across products described as “halide-free” or “halogen-free” and wonder if they mean the same thing, but they do not. Although they sound similar and are somewhat related, the test methods and test purpose for these two substances are entirely different. Halide content is measured using ion chromatography on the bulk flux product and assessment of the flux soldering and reliability characteristics. Halogen content is measured using oxygen bomb testing on the post-soldering flux residue and is an environmental/health and safety consideration.
The complete list of possible flux designations is summarized in the following table.
IPC J-STD-004/A vs IPC J-STD-004B/C/D
Between IPC J-STD-004 and its successors IPC J-STD-004B/C/D, the testing criteria for activity level and halide content designations underwent some changes. The most notable change was to the SIR testing, with updates to the test’s environment, electrical bias, and sampling frequency. The other major change was the elimination of the qualitative halide test.
The result is that some fluxes designated L0 under J-STD-004 now meet the criteria for L1 under J-STD-004B. In fact, it is possible that a newer flux labeled as L1 may have passed more stringent requirements than an older flux labeled L0.
Because of this, it is important to know which version of the J-STD a flux is classified under, and it may be worth considering L1 fluxes when you otherwise may not have if they have that designation under the newer standard.
The current standard, as of this writing, is IPC J-STD-004D. Testing requirements in this standard are largely the same as for IPC J-STD-004B, however, the newer standard provides specifications for additional optional SIR testing.
Conclusion
At AIM, we understand that selecting the right flux is just as important as choosing the right solder alloy. That’s why we offer a range of flux formulations designed to meet the diverse needs of modern electronics manufacturing. If you have any questions about which flux is best for your application, our team is always here to help.
Still have questions? Reach out to us—we’re happy to help you find the perfect flux for your process.
