Understanding the Role of Flux When Soldering
Anyone who’s ever tried soldering components with untinned pads without flux will know the problems to go along with the process. The solder will hardly stick to the pads or the components, and even if it does, the solder joints are often brittle and of bad quality. That’s often due to a mix of factors. For example, the copper pads on the PCB start to oxidize as soon as they’re exposed to air after the etching process. This thin oxidization layer builds up on the surface, preventing the solder from sticking to the copper properly.
This is where flux comes in, as this chemical product helps prepare the surfaces by removing oxidization and wetting the surface. Further, flux helps create a true metallurgic bond between the PCB’s pads and component legs, ensuring that solder joints won’t crack when exposed to thermal and physical stress. Finally, flux also helps prevent new oxidization from forming during the high heat soldering process which would normally accelerate oxidization.
The Three Main Types of Soldering Flux in Electronics
Like solder, you can distinguish the types of flux commonly available for use in electronics assembly by the base chemical used to create the flux. The three main types are rosin-based, no-clean, and water-soluble flux. Alternatively, it’s also possible to classify the flux according to its delivered form, such as a spray, liquid, paste, or core embedded within the solder.
The first type, rosin-based flux, is made from pine tree sap and then dissolved in a solvent, usually isopropyl alcohol. The resulting product is slightly acidic, which helps dissolve built-up gunk and oxidization. In this state, the flux is called type-R rosin flux. However, manufacturers can add additional acid activators to make the flux more aggressive and increase its ability to dissolve heavier oxide films, which may be required when soldering at higher temperatures, such as lead-free solder. Depending on the activation level, the flux is categorized into RMA (rosin mildly activated) and RA (rosin activated). Due to its acidic nature, this flux must be cleaned off the PCB after assembly to prevent it from corroding the board over time.