Choosing the Right Flux for Soldering

On the whole, flux does one and only one thing for an electronics assembly – it ensures that the two surfaces being joined together are free from all kinds of impurities that might hinder the formation of a good and dependable solder joint. So, your choice of flux can have both direct and indirect implications for your PCB assembly. From the performance of your solder joint to its reliability, conductivity and overall quality, everything will be impacted with the selection you will make here, whether you are going for wave soldering or selective soldering. This article is here to tell you everything you need to know about choosing the right solder flux for your project.

Understanding types of fluxes

The main criteria for selecting the right kind of flux are its activity levels, material type and solids content. These attributes also determine whether or not you will have to clean the circuit board after the soldering process is complete. You also need to examine whether or not the solder flux will be able to wet the solder material, the board and the hole (for through hole application) properly. Following are the basic types of solder fluxes that you can choose from on the basis of your specific requirements:

• Water soluble fluxes: These provide the best soldering joint possible and have the best activity levels that can readily clean off the metal surfaces that you are planning to join together. However, these are aggressive chemicals and might end up corroding the surfaces they are in contact with or lead to dendrite growth and therefore, they require rigorous cleaning with water after the soldering process is complete.

• Full rosin fluxes: These also have a good metal cleaning action and can help create reliable solder joints. They can also last throughout the soldering process and will not corrode the surface they come in contact with. However, they do leave a residue behind which is a larger threat to the integrity of the circuit boards than the reliability issues of water soluble fluxes. Cleaning rosin residue will require the use of a special solvent which comes with its own set of issues.

• No-Clean flux: These are essentially like rosin based solder flux but with considerably smaller amounts of rosin. However, the activity levels of these fluxes are also lower and hence, their cleaning ability for metal surfaces is slightly questionable. But at the end, the reduced rosin content ensures that the added board cleaning step in the soldering process is eliminated entirely.

Designation of solder fluxes

Flux is usually categorized and marked in accordance with its content, namely rosin (RO), resin (RE), organic (OR), or inorganic (IN). Its activity levels are also referred to as low (L), medium (M), or high (H). The last letter of flux designation denotes the presence (1) or absence (0) of Halides. So if you are considering a flux denoted by ROL1, it means it is a rosin-based, low-activity flux that contains halides. This designation can therefore give you all the information you need to know before selecting the right flux type for your needs.

Selecting the right solder flux

Your final choice of solder flux for your assembly process will eventually depend on the following factors:

• The cleaning activity you desire from the flux,

• The presence or absence of Halides,

• Whether you wish to invest your money/time for cleaning circuit boards,

• Suitability to your soldering technique – Wave soldering (Rosin or water soluble flux) or Selective soldering (No-clean flux).

Choose wisely!