The Essential Guide to Soldering Electronics with Flux
When joining two metals in a soldering process, such as in PCB assembly, flux is essential to achieve a true metallurgical bond. This ensures that the solder joint remains durable and resistant to cracking or coming loose, even with everyday wear and tear. This article discusses the types of fluxes available, their advantages and disadvantages, and options for flux removal.
Flux aids in the soldering and desoldering processes by removing oxide films that form on the surface of metals being soldered. It enhances the wetting ability of the solder, allowing it to flow uniformly over surfaces without balling up (dewetting).
In electronic soldering, flux plays a crucial role by facilitating the process and ensuring reliable connections between components. Soldering involves joining metal surfaces using a molten alloy known as solder. However, impurities, oxides, and contaminants present during soldering can impede the formation of a strong bond. Flux addresses these issues by cleaning the metal surfaces, enabling a strong and reliable metallurgical connection.
What Is Flux & How Does It Work?
Flux is a chemical compound that prepares metal surfaces for soldering by removing oxides, promoting wetting, and enhancing solder flow. It is available in various forms, including paste, liquid, or as a core within solder wire. The active ingredients in flux, such as rosin or organic acids, react with oxides on the metal surface.
When heated, flux activates and removes oxides, preventing them from interfering with the soldering process. It also promotes wetting, the ability of molten solder to spread and adhere to metal surfaces. By reducing the surface tension of the solder, flux ensures it flows smoothly and evenly, creating strong and reliable solder joints.
Moreover, flux prevents the reformation of oxides during soldering by creating a protective barrier on the metal surfaces. This barrier shields the freshly cleaned metal from the atmosphere, preventing rapid oxidation and ensuring a clean, reliable solder joint.
Types of Fluxes
Various types of flux are available for different soldering applications. Some fluxes are formulated for specific metals, like copper, while others are suitable for a broad range of uses. Additionally, fluxes come with different activity levels, ranging from mild to highly active, depending on the degree of oxidation or contaminants present on the metal surfaces.
IPC J Standard Flux Classification
The IPC J Standard (Joint Industry Standard) flux classification system has replaced the military's soldering standards previously defined under QQ-S-571 and MIL-F-14256. Fluxes are categorized as RO (rosin), OR (organic), IN (inorganic), and RE (resin/synthetic resin). The activity level of the flux solution is rated as L (low activity or <0.5% halide), M (medium activity or 0 to 2% halide), and H (high activity or 0 to >2% halide). Fluxes are classified based on halide (Cl- or Br-) content as either 0 (no halides) or 1 (some halides). For instance, an ROL0 flux would be a rosin flux with low activity and zero halides. An RMA flux could be classified as an ROM1 if it contained 0.5 to 2.0% halide content under this classification scheme.
Rosin (type R) Flux
The most fundamental soldering flux, employed for centuries, is natural rosin derived from pine tar resin. Pine tar resin undergoes dissolution in a solvent and subsequent distillation to produce clear, water-white rosin utilized in soldering flux. Rosin comprises naturally occurring acids, primarily abietic acid and its homologs. In soldering flux, clear rosin is dissolved in a solvent, typically isopropyl alcohol, without the addition of acid activators, classifying it as type R rosin flux.
To enhance the flux's ability to dissolve heavier oxide films, especially those formed at the elevated temperatures needed for lead-free solder alloys, activators are incorporated into soldering flux. Activated fluxes can be mildly activated, labeled as RMA (rosin - mildly activated), or fully activated, denoted as RA (rosin -activated). Commonly used activators include organic acids, halogenated compounds (containing chlorine or bromine), amides, and monobasic and dibasic organic salts. These activators are corrosive and must be removed from the circuit board to ensure long-term reliability.
Activated and mildly activated rosin fluxes may leave behind chloride ions and other corrosive residues, necessitating their removal from the printed circuit board post-soldering or desoldering to prevent long-term corrosion-related failures. Residues from these fluxes can also be tacky, attracting dust containing conductive elements that could lead to shorts and other electrical failures on the board. With the increasing prevalence of lead-free solder alloys in manufacturing, the use of highly activated fluxes to counteract oxidation film formation at higher soldering temperatures will likely rise. Consequently, thorough cleaning after soldering or desoldering with lead-free alloys will become imperative.
No-Clean Flux
No-clean fluxes can be formulated with either natural rosin or synthetic resins. Rosin-based no-clean flux solutions are similar to rosin (R Type) fluxes but typically contain natural gum rosin at a lower concentration than R Type (R, RMA, and RA) flux solutions. True synthetic no-clean fluxes utilize synthetic resins that offer the same desirable properties as natural rosin. Additionally, no-clean flux solutions may include extra activators, and the residues they leave behind can lead to corrosion.
Designed to enable circuit board manufacturers to avoid the time and cost of post-soldering cleaning, no-clean fluxes leave significantly less residue compared to conventional R type fluxes. This reduced residue amount usually does not interfere with board operation or cause long-term corrosion-related failures.
However, residues from no-clean fluxes may be adhesive and attract dust, affecting the appearance of the circuit board. Therefore, cleaning may be necessary to meet appearance or operational standards. If the circuit board is to be conformal coated for circuitry protection during operation, the board surface must be free of flux residues, even minimal residues left by no-clean flux, to ensure proper adhesion of the conformal coating.
Moreover, the increased need for activated (corrosive) flux when soldering with lead-free alloys may necessitate flux residue removal, potentially diminishing the advantages of using no-clean fluxes.
Water Soluble (Aqueous) Flux
Water-soluble fluxes typically utilize water-soluble resins, and their residues require removal through a water rinse. Some water-soluble fluxes are formulated as water-based solutions, eliminating the need for alcohol-based flux solutions. This presents an opportunity to reduce VOC emissions, particularly for board manufacturers operating under strict environmental regulations. Acid activators commonly found in water-soluble fluxes include organic acids, halogenated compounds (containing chlorine or bromine), amides, and monobasic and dibasic organic salts. All these activators are corrosive and should be eliminated from the circuit board to ensure long-term reliability.
Flux Formats & Packaging Options
Various soldering flux formats are available, including liquid flux, tacky flux, flux core, and flux in solder paste, each offering unique advantages suitable for different soldering applications. The choice of flux format depends on factors such as soldering type, joint accessibility, preferred application method, and specific process requirements.
Liquid flux, commonly thinned with isopropyl alcohol (IPA), is a prevalent form of soldering flux. Packaged in bottles, syringes, or pens for easy application, liquid flux is applied to solder joints or components before soldering. It cleans surfaces, enhances solder flow, and improves joint quality, making it essential for wave soldering in electronics manufacturing. Before wave soldering, liquid flux is selectively applied using spray, foam, or a flux applicator to ensure precise and controlled coverage.
Tacky flux, also known as sticky flux, possesses a thicker consistency than liquid flux, with a sticky or gel-like texture that prevents running or dripping. It adheres well to surfaces during soldering, making it suitable for vertical or overhead joints where flux retention is crucial.
Flux core solder features a hollow core filled with flux, combining solder and flux into a single product. As the solder wire melts during soldering, the flux cleans surfaces and aids soldering, offering convenience and efficiency.
Flux in solder paste consists of solder alloy particles and flux in a semi-solid or paste form. Widely used in surface mount technology (SMT) applications, solder paste cleans solder pads and components, promotes solder wetting, and holds solder in place before reflow soldering.
Flux plays a vital role in electronic soldering by removing oxides, promoting wetting, enhancing solder flow, and preventing re-oxidation. These functions collectively contribute to the creation of strong and reliable solder joints. Selecting the right flux for each soldering task ensures effective joining of electronic components, resulting in optimal electrical and mechanical connections.
