Any type of electronic project involving PCBs or printed circuit boards requires soldering, whether it uses through-hole components or surface-mount components. Soldering anchors the electronic components mechanically to the PCB, while also providing good electrical conductivity between the component terminals and the copper pad on the board. The role of flux in the soldering process is one of reduction. Over time, exposure of the copper pad and the component terminals leads to the formation of oxides. This layer of oxides hinders proper wetting of the soldering surfaces by molten solder, ultimately leading to a dry joint. Not only is a dry joint mechanically weak, but it also has poor electrical conductivity.
The Role of Flux in Soldering:
Keeping the solderable surface coated with flux helps reduce the oxides when applying molten solder. This allows the solder to interact with the solderable surfaces on the board and on the component terminals. Upon cooling, the solder bonds with these surfaces, producing a strong and clean mechanical joint and good electrical conductivity.
Why does Flux Matter?
As a chemical compound, you will typically apply the flux to the metal surfaces before soldering. The application of heat causes the flux to clean the metal surfaces by removing impurities and oxides. This also allows the molten solder to flow smoothly over the surface, while preventing new oxides from forming during the soldering process. Without flux, the solder joint could be weak, dry, and even fail.
Types of Flux Available
Depending on the melting metal, flux ensures the joint is not only strong, but clean and electrically reliable. However, there is a large variety of flux types available, and picking the right one can feel overwhelming.
In this article, we at Rush PCB UK explain the types of flux available, their properties, and when you should use a specific type of solder.
1. Rosin Flux:
Rosin flux is the most commonly used flux in electronics work. Made from pine sap, it is refined into a sticky liquid or amber-colored paste. Depending on their activity level, rosin flux is subdivided into three subtypes. Note that the more active the flux is, the more aggressively it will clean the metal surface.
– Non-Activated (R)
This type of flux is barely active and mostly used on clean, shiny surfaces. Common examples of use are on pre-tinning wires that require minimal cleaning.
– Mildly Activated (RMA)
This type of flux offers more cleaning power and is best suited for soldering general electronic circuit boards.
– Fully Activated (RA)
This type of flux is super aggressive and used for highly oxidized or dirty metal surfaces. Common examples are old rusty surfaces. It is also sticky and highly corrosive, and requires thorough cleaning with alcohol after application.
2. Water-Soluble Flux:
Water-soluble flux not only provides strong cleaning of the solderable surfaces, but is also easy to clean up after soldering. This is because it is typically made from chemicals like acids or amines that dissolve easily in water. Being more aggressive than rosin flux, water-soluble flux is commonly used for metals with a greasy or heavily oxidized surface.
As water-soluble flux is very corrosive, it is necessary to clean the surface after soldering to prevent damage. Fortunately, cleaning requires only a brush and warm water.
3. No-Clean Flux:
No-clean flux is a modern option meant to save time. This is a synthetic or a rosin-based flux. It produces a minimal residue that is also non-corrosive, and you may safely leave it on the project. Much less aggressive compared to water-soluble flux, the no-clean flux works well for clean or lightly oxidized surfaces.
No-clean flux is perfect for modern high-density and high-volume electronics, where it is impractical to stop to clean every joint.
However, as the no-clean flux is not aggressive enough, it is not very effective for heavily oxidized metal surfaces. Moreover, the residue can look untidy when visible in decorative projects.
4. Acid-Based Flux:
Made with strong acids, the acid-based flux, also known as inorganic flux, is the muscle of the industrial world. Used extensively in industrial soldering, it is designed for soldering tough metals such as heavily oxidized copper, brass, steel, stained-glass, and in plumbing.
Being highly corrosive, it is essential to clean acid-based flux with water or a neutralizer to prevent the flux residue from corroding your project. Moreover, due to its corrosive nature, it is not advisable to use acid-based flux on electronic projects, as it is likely to destroy circuits.
Type of Flux to Use
Fluxes are available in different types. Each type is suitable for specific materials, environments, or soldering methods. Depending on your application, you can use:
1. Flux-Core Solder Wire
This is a beginner-friendly method to apply flux while soldering. The solder comes in the form of a hollow wire, and the hollow space contains liquid flux. As you melt the solder, it automatically releases the flux, which flows out from the wire to clean and prepare the surface for a good joint.
However, the flux in the solder wire is not very corrosive, and therefore, the combination does not work for PCB pads or components that are heavily oxidized. For such cases, you must add extra flux separately.
2. Flux Paste
This flux has a thick consistency and is a paste of solder and flux. This combination is mostly suitable for SMD components and other precision soldering jobs. You can apply it with a toothpick, brush, or syringe to control the amount and the location.
In the electronic industry, it is common to use it with an automatic dispenser or apply it through a silk screen, before placing the component on it. The flux holds the tiny components in place until the assembly passes through the reflow oven, when the solder melts and anchors the components on the board.
3. Liquid Flux
Liquid flux is useful for general-purpose electronic soldering and precision repair work. Easy to apply using a cotton swab, syringe, pipette, or even a small brush, this type of flux is very versatile for manual work.
4. Flux Pens
This is another variation of the liquid flux, available in a convenient pen form. Using it is very simple, as you can apply the flux exactly where needed, and it will not create a mess or waste. This is very useful for small manual touch-ups and quick jobs on your circuit.
Related: In-Circuit Testing vs. Functional Testing: Key Differences and Applications
Conclusion
At Rush PCB UK, we recommend using flux when soldering—it is necessary. However, you must pick the right one for the purpose, and our article should help you to do that.