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.
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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.
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.
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.
The IPC J Standard (Joint Industry Standard) flux classification system has replaced the military’s soldering standards under QQ-S-571 and MIL-F-.. 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.
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 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 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.
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.
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Here's an explanation of each format:
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.
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.
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.
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.
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:
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".
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:
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I figured out I'm in need of taking flux seriously;Yes, indeed, flux is essential for soldering, but good solder has the flux already included. Lots of SMT soldering techniques do require adding extra flux though. Solder wick also contains (powdered) flux, but adding more flux often helps there too. There are lots of good (and also bad and ugly) video's on youtube about the use of (extra) flux during soldering. Go watch them to see what a good soldering process looks like. (Hint: "GreatScott" is terrible at soldering. Avoid him at all costs when it's the soldering you are interested in.)
I even have literal rosin but it doesn't seem great if I just ground it and mix it with alcohol; I have another "soldering grease" flux which appears to work (to achieve wetting) but I fear it might be unhealthy with its fumes and it might not be the best possibleRosin isn't healthy either. The only solution I know is to use filtering and/or ventilation.
so I wonder if you have suggestions on what to buy from ali (or elsewhere) that has the highest quality possible for the purpose.Quality from Ali? I wish yo lots of luck.
I figured out I'm in need of taking flux seriously; I even have literal rosin but it doesn't seem great if I just ground it and mix it with alcohol; I have another "soldering grease" flux which appears to work (to achieve wetting) but I fear it might be unhealthy with its fumes and it might not be the best possible so I wonder if you have suggestions on what to buy from ali (or elsewhere) that has the highest quality possible for the purpose.If you want to take flux seriously (which I recommend), then don't bother with Aliexpress, etc. Even if you get a good product now, you have no idea whether the next batch will be the same.
And how exactly does wetting work I wonder; I know it seems to split the solder when there is the plastic of the PCB between the different metal contacts; but why does the solder not just keep connecting to itself between those metal contacts since it has already connected to itself before flux and it seems to separate instead into different parts between the PCB contacts after adding flux?Wetting is name for the solvent action of molten solder on the metal of the components/boards. The surface tension of solder is very high, so it wants to pull back. Solder mask (the colored lacquer on PCBs) is specifically designed to make it hard for solder to bridge across it.
In general, you do not need additional flux, use cored solder which contains flux.I don't think that's the best advice, since especially with SMD work, there are situations where external flux is essential. (Even when THT was king, there were still situations where external flux was useful.)
In close to 40 years in the service industry working for a multinational manufacturer on their products, mostly through hole boards (SMD was a board swap), we never used additional flux.In general, you do not need additional flux, use cored solder which contains flux.I don't think that's the best advice, since especially with SMD work, there are situations where external flux is essential. (Even when THT was king, there were still situations where external flux was useful.)
No. The quality of the solder has nothing to do with whether it has a flux core or not. There is terrible quality flux-core solder, and there's top-quality solder with no flux core.I figured out I'm in need of taking flux seriously;Yes, indeed, flux is essential for soldering, but good solder has the flux already included.
Lots of SMT soldering techniques do require adding extra flux though.Indeed. It's essential to certain techniques. (Heck, that's even true in THT soldering, for example in mass production. It also helps in manual THT work if your components are a bit more tarnished.)
Solder wick also contains (powdered) flux, but adding more flux often helps there too.I thought it was made by coating it in liquid flux and drying it. I always assumed the powdery appearance was because of the wick getting flexed after the flux was dry, causing it to fracture.
There are lots of good (and also bad and ugly) video's on youtube about the use of (extra) flux during soldering. Go watch them to see what a good soldering process looks like. (Hint: "GreatScott" is terrible at soldering. Avoid him at all costs when it's the soldering you are interested in.)Absolutely! I love GreatScott's projects and explanations, but his soldering is awful!
Wetability is the same principle as with water.It's actually not like that at all!
If you pour a glass of water over a T-shirt it gets wet (and sometimes other nice side effects).
If you pour a glass of water over a raincoat it rolls of.
With soldering, there is always a bit of dirt and oxidized stuff on everything you want to solder.Which is why it's always a good idea to clean your boards (and to the extent possible, components) with solvent (like alcohol) to remove contaminants. Then the flux can take care of oxidation.
In practice, the solder sticks to the clean other metals, but not to the dirt and rust, and a part of the job of the flux is to remove the dirt.Solder doesn't "stick" like glue. It dissolves the substrate, creating an intermetallic layer between the substrate and the solder itself.
I think the flux also lowers the surface tension of the solder (but I'm not entirely sure). But if it does, it is (a bit) comparable to mixing in a bit of soap with the water before you pour it over the T-shirt. (Although the flux does not dissolve into the solder, so it's a mediocre analogy).I think it lowers the surface tension, but I think more in the sense that oxides (which form on the solder itself, too) have extremely high surface tension, and by eliminating the oxides, the flux allows the solder itself to contact the joint. The following users thanked this post: helius
And? Just because you never used it doesn't mean it might not have made some things easier. (It is possible to do something the wrong way for decades, after all! )In close to 40 years in the service industry working for a multinational manufacturer on their products, mostly through hole boards (SMD was a board swap), we never used additional flux.In general, you do not need additional flux, use cored solder which contains flux.I don't think that's the best advice, since especially with SMD work, there are situations where external flux is essential. (Even when THT was king, there were still situations where external flux was useful.)
Don’t forget the more flux you add, the more you end up having to clean off afterwards.Very true.
Adding flux is easy and it does no harm, it’s not easy to clean off.Depends on the flux (e.g. no-clean fluxes tend to be much harder to remove than "traditional" rosin fluxes), and how the flux was handled after application (e.g. overcooked flux is far harder to remove than flux heated briefly).
Those who say there is no need to clean flux off should look at commercial boards in new equipment, they are spotlessly clean.I don't think you can make that claim, as a blanket statement. Many boards are cleaned, and many are not. Some use carefully controlled reflow soldering processes using no-clean flux, such that the amount of flux residues is so small you don't notice with the naked eye. Some use water-soluble fluxes that are then removed in hot water. And, frankly, it's not uncommon to see boards that have been cleaned, but not spotlessly.
Indeed, despite some people's claims that rosin in alcohol makes a "great" flux, rosin without added activators is a very mild flux that is good only for parts/boards that are very clean already. It can't cut through serious oxides the way activated fluxes can.
"Soldering grease" is plumbing flux. Do not use it on electronics, as it will cause corrosion.
In metallurgy there are two states for metal surfaces: active and passive. An active surface has metal bonds exposed that can react with other substances. A passive surface has its metal bonds protected by a thin layer of non-metallic material, and will not react.Yeah pure rosin does not live to its reputation at all: I had to replace the surface-connected oled screen of a ts100 soldering iron and pure rosin-alcohol made a mess; a cheap 'soldering grease' felt almost perfect compared to it; but it triggered me to make this thread because obviously it wasn't the best material for the purpose.
The solvent action of solder is to react with an underlying metal and form bonds to it, as tooki said. This is analogous to the solvency of something like water, which surrounds and bonds to a solute like table salt to form a clear and homogeneous mixture called a solution. Solder surrounds and bonds to metals such as copper and tin to form a tightly adhering, strong joint after the solder cools. The heart of the joint is a solid solution of metal atoms from both the copper and solder mixed homogeneously, around a micron thick. This is called "wetting". If the solder doesn't form this solid solution layer, the joint is a defective "dry joint".
For the solder to react with the metal and bond to it in this way, the surface of the metal must be active. If the metal is passivated with oxides etc, the solder cannot get "close enough" to form metallic bonds. So the reason flux is used is to activate the metal surface: to dissolve and remove the oxides that form the passivating layer on the metal. The activity of flux is a measure of how strong its ability to dissolve oxides is. Pure rosin has some activity but is relatively weak. Most electronic flux has activators added in a few tenths of a percent that increase the flux's activity. The chemical properties of the activators are what determine the suitability of the flux to the application.
There is also a very important aspect which is the disposition of the activators after soldering is complete. If the activators remain in the dried flux and don't continue their chemical reaction, post-process cleaning is not necessary. If the activators continue to react, the flux must be cleaned off or corrosion will occur. Zinc chloride is an example of an activator that continues to react and so it must be cleaned off. In fact it is corrosive at such trace amounts that it should never really be used on electronics at all. Zinc chloride is the activator in plumber's flux. Other activators include halides: organic salts of fluorine, chlorine, or bromine; phosphates; citrates; imides, etc. The J-STD-004 category reveals details about what activators are present. Usually you want a ROL0 type flux if the board won't be cleaned. That means ROsin base, Low activity, less than 0.5% halides.
Some metal surfaces are very strongly protected by passive layers, such as aluminum or stainless steel. In order to solder to these metals very strong specialized activators are needed, such as orthophosphoric acid. Obviously cleaning must be meticulous after they are used.
Once you understand the purpose of flux, you also will see how it should be used: applied thinly to the component leads and the pads of the board. Never apply flux to the soldering iron!