Choosing The Best Flux for High Quality Electronic Soldering

When joining two metals in a soldering process, like used in PCB assembly, flux is required to achieve a true metallurgic bond. That ensures the solder joint doesn’t crack or come loose even with the day-to-day wear-and-tear. This article covers the types of fluxes available, the advantages and disadvantages of each, and options for flux removal.

Flux aids in soldering and desoldering processes by removing oxide films which form on the surface of metals being soldered. It increases the wetting ability of the solder, causing it to flow more uniformly over surfaces without balling-up (dewetting). 

Flux plays a crucial role in electronic soldering, facilitating the process and ensuring reliable connections between components. Soldering involves joining metal surfaces using a molten alloy known as solder. However, during the soldering process, various impurities, oxides, and contaminants can hinder the formation of a strong bond. This is where flux comes into play.

What is Flux & How Does It Work?

Flux is a chemical compound that helps prepare the metal surfaces for soldering by removing oxides, promoting wetting, and enhancing the flow of solder. It typically comes in the form of a paste, liquid, or core within the solder wire. The flux compound contains active ingredients, such as rosin or organic acids, that react with the oxides on the metal surface.

When heated, the flux activates and starts to remove the oxides, preventing them from interfering with the soldering process. The flux also promotes wetting, which is the ability of the molten solder to spread and adhere to the metal surfaces. By reducing the surface tension of the solder, flux ensures that it flows smoothly and evenly, creating strong and reliable solder joints.

Additionally, flux helps prevent the re-formation of oxides during soldering by creating a protective barrier on the metal surfaces. This barrier shields the freshly cleaned metal from the surrounding atmosphere, preventing rapid oxidation and ensuring a clean and reliable solder joint.

Classifications of Fluxes

There are different types of flux available for various soldering applications. Some fluxes are designed for specific metals, such as copper, while others are suitable for a wide range of applications. Additionally, there are fluxes with different activity levels, from mild to highly active, depending on the amount 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 under QQ-S-571 and MIL-F-14256.. Fluxes are rated as RO (rosin), OR (organic), IN (inorganic) and RE (resin/synthetic resin). The activity 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 for halide ( Cl- or Br-) content as 0 (no halides) or 1 (some halides). The Under this classification scheme an ROL0 flux would be a rosin flux with low activity and zero halides  An RMA flux could be classified under this scheme as an ROM1, if it contained 0.5 to 2.0% halide content. 

Rosin (type R) Flux

The most basic soldering flux, one that has been used for over a thousand years, is the natural rosin derived from pine tar resin. Pine tar resin is dissolved in solvent and then distilled to yield the clear, water-white rosin used in soldering flux. Rosin is a collection of naturally occurring acids, chiefly abietic acid and its homologs. When used as a soldering flux, the clear rosin is dissolved in a solvent, usually isopropyl alcohol. When used in this manner, without the addition of acid activators, it is referred to as type R rosin flux. 

Activators are added to soldering flux to increase the ability of the flux to dissolve heavier oxide films, especially those produced at the higher soldering temperatures required for lead-free solder alloys. Activated fluxes can be either mildly activated or type RMA (rosin - mildly activated) or RA (rosin -activated). Activators commonly used include organic acids, halogenated (containing chlorine or bromine) compounds, amides, and monobasic and dibasic organic salts. All of these activators are corrosive and should be removed from the circuit board to ensure long term reliability.

Activated and mildly activated rosin fluxes can leave behind chloride ions and other corrosive residues and therefore must be removed from the printed circuit board after soldering or desoldering to prevent long term corrosion related failures. The residues of these fluxes are also sometimes tacky and attract dust which may contain conductive elements that can cause shorts and other electrical failures on the board. As lead-free solder alloys become more prevalent in manufacturing, the use of highly activated fluxes, to overcome oxidation film formation at higher soldering temperatures, will increase. Thorough cleaning after soldering or desoldering, when using a lead-free alloy, will become mandatory.

No-Clean Flux

No-clean fluxes can be either made with natural rosin, or contain synthetic resins. Rosin-based no-clean flux solutions are essentially the same as the rosin (R Type) fluxes, but usually contain natural gum rosin at a much lower concentration than that used in the R Type (R, RMA and RA) flux solutions. True synthetic no-clean fluxes contain synthetic resins that impart the same desirable properties to the flux as does the natural rosin product. No-clean flux solutions can also contain additional activators, and the residues they leave behind can lead to corrosion. 

No-clean fluxes were designed to help circuit board manufacturers skip the time and expense of cleaning the board after soldering. No-clean fluxes leave much less residue behind than the conventional R type flux, and this smaller amount of residue will usually not interfere with the operation of the board or cause long-term corrosion-related failures.

The residues left by a no-clean flux may be sticky and attract dust or otherwise detract from the appearance of the circuit board and therefore can require removal (cleaning) to meet appearance or operation standards. If the circuit board is to be conformal coated to protect the circuitry during operation, the board surface must be free of flux residues, even the minimal residues left by no-clean flux, to ensure good adhesion of the conformal coating.

The need to use more activated (corrosive) flux when soldering with lead-free alloys may also make removing flux residues a necessity, further reducing the benefits of using no-clean fluxes.

Water Soluble (Aqueous) Flux

Water soluble fluxes usually employ water-soluble resins, whose residues should be removed using a water rinse. Some water-soluble fluxes are water-based solutions, which eliminates the need for using an alcohol-based flux solution. This is one way in which VOC emissions can be reduced, for those board manufacturers operating under stringent environmental regulations. Acid activators commonly used in water soluble fluxes include organic acids, halogenated (containing chlorine or bromine) compounds, amides, and monobasic and dibasic organic salts. All of these activators are corrosive and should be removed from the circuit board to ensure long term reliability.

Flux Formats & Packaging Options

There are various types of soldering flux formats available, including liquid flux, tacky flux, flux core, and flux in solder paste. Each soldering flux format has its advantages and is suitable for different soldering applications. The choice of flux format depends on factors such as the type of soldering, joint accessibility, desired flux application method, and the specific requirements of the soldering process.

Here's an explanation of each format:

1. Liquid flux - Liquid flux is a common form of soldering flux that comes in a liquid consistency, often thinned with isopropyl alcohol (IPA). It is typically contained in a bottle, syringe, and even pen packaging for easy application. Liquid flux is applied to the solder joints or the components being soldered before the solder is applied. It helps to clean the surfaces, improve solder flow, and enhance the quality of the solder joint.
   
   Liquid flux is an essential material used in wave soldering, a process commonly employed in the electronics industry for soldering through-hole components onto printed circuit boards (PCBs). Before the PCB enters the wave soldering machine, liquid flux is applied to specific areas where soldering is required. This is typically done by using a spray, foam, or selective flux applicator. The flux is applied in a controlled manner to avoid excessive application and ensure it is only present in the required regions.

2. Tacky flux - Tacky flux, also known as sticky flux, has a thicker consistency compared to liquid flux. It has a sticky or gel-like texture that helps it adhere to the surfaces being soldered, preventing it from running or dripping. Tacky flux is often used in situations where there is a need for the flux to stay in place during the soldering process, such as when soldering vertical or overhead joints.

3. Flux core - Flux core solder is a type of solder wire that has a hollow core filled with flux. As the solder wire melts during the soldering process, the flux is released and helps clean the surfaces and facilitate soldering. Flux core solder is convenient to use as it combines both the solder and flux into a single product, eliminating the need for separate flux application.

4. Flux in solder paste - Solder paste is a mixture of solder alloy particles and flux. It usually comes in a semi-solid or paste form. Solder paste is commonly used in surface mount technology (SMT) applications, where components are placed on circuit boards and then soldered. The flux in solder paste helps to clean the solder pads and components, promote solder wetting, and hold the solder in place before reflow soldering.

Flux is an essential component in electronic soldering. It removes oxides, promotes wetting, enhances solder flow, and prevents re-oxidation, all of which contribute to the formation of strong and reliable solder joints. By using the appropriate flux for the specific soldering task, electronic components can be effectively joined together, ensuring optimal electrical and mechanical connections.

Chemtronics Flux Products

CircuitWorks Flux Dispensing Pens provide controlled and exact applications that are generally compatible with most materials in the electronics industry. Dispense flux on printed circuit boards, heat sinks, chip carriers, switches, sockets and much more.

CircuitWorks Flux Dispensing Pens are fast drying, completely portable and have minimal excess flux waste. Instant wetting action provides thorough deoxidation of metallic surfaces providing the best possible surfaces for hand soldering.

CircuitWorks Flux Pens are designed specifically to apply each type of flux with precision control:

• Rosin Flux Dispensing Pen quickly applies a noncorrosive type R flux. This flux meets MIL-F-14256 E and F.
• No Clean Flux Dispensing Pen precisely applies a patented noncorrosive, halide-free, organic low solid flux. This flux meets Bellcore TR-NWT-000078 and IPC SF-818 for surface insulation resistance.
• Lead-Free Flux Dispensing Pen quickly applies a noncorrosive, halide-free, No Clean flux which meets Bellcore TR-NWT-000078 and passes IPC SF-818 for surface insulation resistance.
• Water Soluble Flux Dispensing Pen is designed specifically to apply water soluble flux with precision control. The Water Soluble Flux consists of a neutral pH organic water soluble flux compatible with most solder masks. The high activity organic product cleans easily with water, reducing cleaning cost.

CircuitWorks Tacky Flux is a Type ROL0 formulation designed for BGA rework requiring high-reliability, stability and cleanliness. CircuitWorks Tacky Flux gel composition holds the BGA component in position even with board movement. Its lower viscosity allows easy application and contains no ionic material. CircuitWorks Tacky Flux is suitable for clean room applications.

For soldering tips, inclucing soldering and desoldering techniques, and the care and maintenance of soldering irons, check out the Techspray article "Ultimate Guide to Electronic Soldering".

Chemtronics Flux Removers

Flux removers are recommended for post-solder operations, PC boards, sensitive circuit components, component leads, SMD pads, chip carriers, plugs, sockets and heat sinks, and Thru-hole and SMT devices.

Proper maintenance is critical to ensure reliable circuitry. Not only unsightly, some flux residue can promote short circuits and corrosion, degrading or destroying the printed circuit board. The type of flux to be removed, solvent compatibility and ease of application are all factors to consider when choosing the appropriate product. Flux-Off® is available in aerosol, BrushClean™ System, and liquid form for vapor degreasers, spray systems, ultrasonics, and dip tank applications.

Flux-Off products are specially formulated to remove all types of rosin (types R, RA, and RMA), no-clean, water-soluble and synthetic (type SA) flux. Flux remover spray can spray in any direction, even upside down.

CircuitWorks Flux Remover Pens are designed specifically to remove each type of flux:

• Rosin Flux Remover Pen quickly cleans type R, RMA, and RA flux residues.
• No Clean Flux Remover Pen precisely removes both organic and synthetic low solid no clean fluxes.
• Lead-Free Flux Remover Pen quickly cleans type R, RMA, and RA rosin flux residues as well as organic and synthetic no clean fluxes used in higher heat lead-free applications.

For more information on the best cleaning methods of all types of fluxes and electronic assemblies, check out "Ultimate Guide to Cleaning Electronics" or contact your Chemtronics application specialist at info@itwcce.com or 770-424-4888.