During my first few weeks as a Technical Support Engineer at Indium Corporation, I researched extensively and consulted with my team to find out when flux should be used and why it’s essential to making a solder joint.
Flux is a compound that, when heated, helps remove oxides off solderable surfaces, which is critical in allowing them to bond to each other. As almost every metal and alloy will grow an oxide layer over time, flux is one of the most important materials if you are soldering by-hand or in a reflow oven. Because of this, many people want to know: what type of flux should we be using?
There are two major kinds of fluxes to choose from: no-clean and water-soluble.
No-clean fluxes use rosin or resin as part of the flux composition. They are typically lower-activity fluxes. After the soldering stage, the rosin/resin in the flux encapsulates the remaining flux activator and renders it inert. This action removes the need to clean the flux residues, hence the name "no-clean". This does not mean it cannot be cleaned, but cleaning the flux residue typically requires the use of a chemically compatible cleaning agent to fully clean off the flux residues. Partial cleaning of the flux could lead to reliability concerns in the life cycle of the product.
Water-soluble fluxes are typically highly-active and do a fantastic job of removing oxides (due to their activity) but require a full cleaning after being used. As the name suggests, the flux can be cleaned off with deionized water. This is a great flux to use if you have heavily-oxidized materials, or if your project requires the parts fully cleaned after soldering.
Knowing what type of flux to choose is the first major step. You may also need pick a flux based on the classification, i.e. you are building to a print or a flux classification on a callout. Fluxes are classified according to industry standards such as J-STD-004. There are 3 critical components to classify a flux: flux composition, flux activity level, and halide content
Flux Composition:
- Rosin (RO): Typically comprised of natural rosins found in conifer trees that have been refined. These types of fluxes are typically no-clean fluxes and are used for easy-to-solder surfaces.
- Resin (RE): Made of natural or synthetic resin mixed with other organic compounds to help boost fluxing activity. Due to its ability to be mixed with other organic compounds, this allows for resins to be designed for specific application needs.
- Organic (OR): Does not have any resin or rosin. Quick and efficient at cleaning oxides but leaves behind conductive residue that must be cleaned off. Can be water soluble, meaning they can easily be cleaned with DI (deionized) water.
- Inorganic (IN): Highly corrosive and used to solder very challenging metals such as stainless steel. They are not typically used for electronic assemblies. This flux always needs to be cleaned fully.
Flux Activity Level & Halide Content:
Flux activity is based on the corrosive activity of the flux, which can be caused by halide content. Large quantities of halides cause high levels of corrosion, and a higher flux designation is a good indicator that a flux must be cleaned. The metric used to determine flux activity is the copper mirror test, which shows how the flux interacts with the copper mirror. For this test, Low (L) shows no breakthrough at all, Medium (M) shows no more than 50% breakthrough, and High (H) being anything greater than 50% breakthrough or complete removal.
If you have further questions about which flux to choose, contact our Technical Support Engineering team for assistance!