# Soldering fluxes

Flux chemistry, electronic-assembly classifications, process control and residue cleaning, with distinct electronics and plumbing examples.

Reviewed: 2026-11-01

Tags: consumables, soldering, fluxes, joining

**Kester 951** is an organic **ORL0 no-clean** flux. **Kester 2331-ZX** is an organic **ORH1 water-soluble** flux requiring residue removal, with **54–66°C** the listed optimum cleaning-water range. 951 residue may remain after the intended soldering cycle where the assembly permits it. Qualify residue acceptance for the actual exposure. [MacDermid Alpha, Kester 951 product description](https://www.macdermidalpha.com/products/circuit-board-assembly/through-hole-assembly/liquid-flux/kester-951-liquid-soldering-flux) [2331-ZX TDS, pp. 1–3](https://www.kester.com/downloads?Command=Core_Download&EntryId=61845)

## Function and scope

Flux removes surface oxide to enable solder wetting. Select by substrate finish, solder alloy, heating route and residue requirements. Use electronics and plumbing products within their stated applications. [Indium, flux fundamentals](https://www.indium.com/blog/back-to-the-basics-flux/), [Oatey No. 95 technical specification](https://media.oatey.com/m/1fa683504281b5ed/original/03_No-95-Tinning-Flux_SUBMITTAL_001.pdf)

For electronic assemblies, chemistry and cleanup are separate descriptors. Rosin/resin, organic and inorganic describe formulation families; no-clean and water-soluble describe important process and residue characteristics. Liquid, paste-like and flux-cored delivery forms also need their own product instructions. Do not infer a cleaning method solely from consistency, color or an older shorthand such as “acid flux.” The exact product data sheet is the useful unit of selection. [Indium, flux fundamentals](https://www.indium.com/blog/back-to-the-basics-flux/)

## Reading an electronics flux classification

J-STD-004 designations combine a chemistry family with an activity class and halide indicator:

| Part of designation | Meaning |
| --- | --- |
| RO / RE | Rosin / resin formulation family |
| OR / IN | Organic / inorganic formulation family |
| L / M / H | Low / moderate / high flux and residue activity classification |
| Final 0 / 1 | Halide-free / halide-containing under the referenced standard's test definitions |

These are tested categories, not a universal ranking of joint quality. State the standard edition when it matters to a drawing or purchase specification. [Indium, classification overview](https://www.indium.com/blog/back-to-the-basics-flux/); [Indium, halide and halogen definitions](https://www.indium.com/blog/halide-free-vs-halogen-free-a-refresher/)

Halide-free does not mean the same thing as halogen-free. Indium explains that J-STD-004C's halide-free definition uses less than 0.05% combined halide ions by weight of flux solids, tested in unheated flux. A halogen claim addresses a different analytical scope and can involve processed residue. Consequently, neither the word “free” nor a final zero should be interpreted as a guarantee of absolute elemental absence. Ask for the applicable test definition when a customer specifies either term. [Indium, halide-free versus halogen-free](https://www.indium.com/blog/halide-free-vs-halogen-free-a-refresher/)

## No-clean and water-wash examples

Kester 951 is rosin-free and alcohol-based, with low solids intended to leave little non-tacky residue. [MacDermid Alpha, Kester 951 product description](https://www.macdermidalpha.com/products/circuit-board-assembly/through-hole-assembly/liquid-flux/kester-951-liquid-soldering-flux)

The 951 technical sheet, issued **July 16, 2026**, lists 2.0% theoretical solids and spray, foam or dip application. Its recommended deposition is **120–240 micrograms of solids per square centimetre**. It gives 93–115 °C preheat measured on the top/component side of the board, with typical wave dwell of 2–4 seconds for leaded alloys and 4–8 seconds for lead-free alloys. These are recommendations for that wave-soldering product, not soldering-iron temperatures or a general reflow profile. The measurement location and distinction between liquid volume and deposited solids are essential when transferring a process. [951 TDS, pp. 1–3](https://www.macdermidalpha.com/sites/default/files/2026-07/Kester-951-Soldering-Flux-LFX-TDS-GL-EN-16Jul2026.pdf)

Kester 2331-ZX’s **January 8, 2021** TDS cites **J-STD-004B**. It discourages high-mineral tap water, prefers cleaning within **eight hours** and gives a **48-hour maximum** for this product. [2331-ZX TDS, pp. 1–3](https://www.kester.com/downloads?Command=Core_Download&EntryId=61845)

## Residue reliability and process control

No-clean means residue can remain in suitable applications after the intended soldering process; it does not mean no residue exists or that any amount of unprocessed flux is harmless. Indium describes how some no-clean systems encapsulate activators during heating. If cleaning is necessary, use a compatible process that removes the residue fully: partial cleaning can expose active material and create corrosion or electrochemical-migration concerns. A quick solvent wipe should not be assumed equivalent to a validated cleaning process. [Indium, no-clean residue behavior](https://www.indium.com/blog/no-clean-flux/)

Kester says 951 residues need no removal in most applications and directs users to technical support when removal is required. For concentration control, it recommends acid-number titration and its specified 110 thinner. Substituting arbitrary solvent or controlling a low-solids flux only by appearance can change the process. [951 TDS, p. 3](https://www.macdermidalpha.com/sites/default/files/2026-07/Kester-951-Soldering-Flux-LFX-TDS-GL-EN-16Jul2026.pdf)

2331-ZX uses a different control recommendation: specific gravity measured with a hydrometer, with 4662 thinner for evaporative losses. Its sheet also calls for cleaning and replenishing a tank when debris accumulates. A recipe that works for one flux should not be copied to another merely because both are liquid. Retain product identity, concentration-control method, application settings and cleaning conditions in the process record. [2331-ZX TDS, p. 3](https://www.kester.com/downloads?Command=Core_Download&EntryId=61845)

## Plumbing flux is a separate selection

Oatey No. 95 is a petrolatum-based tinning flux containing solder powder. Its specification lists copper, brass and other named soft-solderable metals, but explicitly excludes aluminum, stainless steel and magnesium and says not to use it on electrical parts. Directions call for clean surfaces, a small amount inside the fitting and outside the pipe, controlled heating and removal of exterior residue with a damp cloth after cooling. This is an application-specific plumbing product, not an interchangeable substitute for an electronics no-clean flux. The linked specification carries a **1/2010** date mark; confirm current product instructions for the supplied package. [Oatey No. 95 specification](https://media.oatey.com/m/1fa683504281b5ed/original/03_No-95-Tinning-Flux_SUBMITTAL_001.pdf)

## Handling and storage

Hazards follow the actual formulation. Oatey's linked US No. 95 SDS, issued **August 30, 2021**, identifies serious eye damage and skin irritation and calls for protective gloves, eye and face protection. A lead-free label is consequently not a general safety assurance. Use the SDS for the supplied product and region, including its handling, ventilation and disposal requirements. [Oatey No. 95 SDS, §§1–2](https://media.oatey.com/m/26c52361ed9a1b0a/original/03_Oatey_No95TinningFlux_SDS_00341237.pdf)

Storage life is also product-specific: 951 lists one year at 10–25 °C with proper handling and a closed cap; 2331-ZX lists two years at the same temperature range. Both sheets identify flammability. Record manufacture date and storage conditions rather than treating all fluxes as indefinitely usable. [951 TDS, p. 3](https://www.macdermidalpha.com/sites/default/files/2026-07/Kester-951-Soldering-Flux-LFX-TDS-GL-EN-16Jul2026.pdf); [2331-ZX TDS, p. 3](https://www.kester.com/downloads?Command=Core_Download&EntryId=61845)

## Related references

- [Soft soldering](../../processes/welding-methods/soft-soldering.md)
- [SAC305 solder](sac305-solder.md)