Flux: what is this product that is used in welding?


If you are learning how to micro-weld Electronic components, or for reballing, surely you have verified that you need a paste called flux. This product is a complete stranger to many, since for the usual soldering with tin this element is not usually used, on the other hand, for those other solders it is of great help.

In this guide you can learn everything you need to know about this paste, such as its composition, purpose, how to apply it, types, etc.

What is flux?

The English word flux can be translated as flux, and comes from the Latin «fluxus» which means «flow». It is not only used in soldering for electronics, it is also used in many other types of soldering as a flux or purifying agent, as well as in metal production in the metallurgical industry, that is, they usually have more than one function.

The first fluxes they were made with lime, potash, sodium carbonate, borax, lead sulfate, coke, etc. All of them used in foundries to purify metals. Instead, as a cleaning agent or to facilitate welding fluxes, generating a better quality weld and eliminating oxidation, it would begin to be used later.

Focusing on the welder, which is the subject that interests us here, it is basically an element (paste, liquid or powder) that can prevent the rust of the joints, acting as an insulator from the contact of the air while the fusion takes place. For this, chemical mixtures such as fluorides, borates, borax or boric acid are used.

In addition to this protective effect, in tin solders, it also serves to make the easier process, receiving the metal and making it more manageable, while making the components adhere well. And, of course, it improves the quality of the welding, avoiding dirt and debris produced during the process.

Another effect of this flux is to favor the lower temperature welds. This decreases the probability of damage to the welded components with the temperature generated during the process.

Flux applications

As for the application flux, is often used a lot in the soldering of electronic devices such as:

  • Repair of micro-welds, where it is more complicated to proceed due to the small size of the components.
  • SMD soldering on PCBs.
  • Reballing for BGA.
  • Cleaning of remains or rust residues.

Types of flux

Several types of flux in the market for welding, each with different prices and characteristics.

  • Ammonium bromide without heavy metals: it is an aqueous solution of organic ammonium bromides with free hydrobromic acid. They do not usually leave a trace of residue in the weld, and if they do, it does not cause the formation of holes (pitting).
  • Zinc chloride: another aqueous solution of ammonium chloride and zinc chloride mixed with free hydrochloric acid. It stands out for its ease of use and effectiveness, although removing the residue implies cleaning it to avoid the formation of holes (pitting).
  • Zinc bromide: another type of liquid solution of zinc bromide and ammonium bromide with free hydrochloric acid. Like the previous one, it is very easy to use, but it is also easy to remove the residue by washing it with water. Even if the residue remains in the solder, it will not form holes.
  • Home: Although not recommended, some makers experiment with creating their own homemade flux. To do this, they use a little bit of pitch that they crush and mix with alcohol. However, this type of flux will not achieve the same results as commercial products.

Differences between flux and solder paste

One of the frequent doubts is if it is the same flux as the solder paste or if there are some differences. The truth is that it is complicated, and many times the products themselves are confusing in their description, using both terms as synonyms. Some make a distinction, and claim that there is only slight differences:

  • Flux: chemical substance applied to metals so that they heat up uniformly and improve the quality of the weld, or also for desoldering.
  • Solder paste: used for welding to facilitate the adherence of the most difficult metal parts.

But, as I comment, for practical purposes, you can take it as synonymous. In fact, in some areas one term is used and in others another. Just make sure when you buy the product that it is not a flux or paste for plumbing or for other applications, and that it is specific for electronics ...

How to use flux

Using the flux, or flux, is simple if you follow a series of steps. By doing this, you can get the best out of this product. In addition, you should use a series of precautions, such as using it in a ventilated room as it is a chemical that can generate vapors, wearing glasses and a mask, as well as gloves.

Hen/Stag instructions to use the flux for welding is:

  1. Clean any dirt that may be in the welding area, if there is one.
  2. Apply a thin layer of flux to the area or both. It does not matter that parts are filled where or there will be welding. Also, if the area is somewhat hot, the better.
  3. Then the welding is carried out as it would normally be done, depending on whether it is a SMD welding, reballing, etc.
  4. Lastly, clean up any remaining flux.

Regarding this last point, there is a controversy over whether to clean or not. In fact, it is not only a doubt for hobbyists (or manual welding), it is also a doubt in the industry (automated welding). Many manufacturers neglect the post-production part by not having automated tools for it and by relying on the efficiency of the machinery used to generate the joints with enough cleanliness.

Instead, this false sense of security can lead to neglect of the electrochemical contamination generated in certain components and that apparently does not generate negative effects, although it would end a long-term failure.

When it is being used flux core solder wire or flux, this flux is usually a solid resin with a higher melting point than the metal that surrounds it. In these types of welds, when the tip of the welder touches the wire, the flux becomes liquid and spreads over the workpiece. In this way, the molten metal follows the heat and flux, forming the bond. As can be deduced, since it requires heat to melt, the risks of contamination are very low ...

On the other hand, this is not the case in other soldering procedures where much more flux is applied due to the characteristics of the solder, such as SMD. However, there are products «No Clean» liquids that do not require cleaning, but do need to be exposed to heat to render them inert.

There are various rinse or cleaning products, such as solvents such as isopropyl alcohol (IPA), as well as wipes, swabs, etc. You should carefully read the flux manufacturer's directions for use and recommendations to determine the correct way.

For example, when doing a full wave soldering, that is guaranteed, but not in other techniques where selective point-to-point soldering or reballing is used. In those cases, localized heat may not be sufficient to break down triggers of the chemical and make it inert. And that's a problem for leftovers outside the weld zone or spread under or over components.

How to store flux

Once you finish using the flux, so that it is properly preserved you should:

  • Leave the flux in its original bottle and close it well.
  • The container must always be vertical, avoiding storing it upside down.
  • Keep in a cool place, the lower the temperature, the better. Leaving it in dry places or at high temperatures should be avoided at all costs.
  • If you have stored it in a place at a low temperature (5-6ºC) as some manufacturers advise, before using it again you should leave it at room temperature for about 6 hours to reach the optimum temperature for use.

Disadvantages and precautions for the use of flux

Fluxes or flux are not free of drawbacks, even though their benefit makes them worth using. For example, some chemicals are usually somewhat aggressive and cause corrosion in the elements. Others can generate some interference with the components, being somewhat insulating.

It can also be the case of contamination sensitive parts, such as some optical sights, laser diode facets, MEMS mechanisms, switches, etc. Another problem is that certain chemicals in water-soluble fluxes, such as polyethylene glycol, cause deterioration of the dielectric properties of the layers of printed circuits.

In high frequency circuits, flux residues could also cause certain problems. It has even been found that in some cases it can cause electromigration of the connections and formation of whiskers by ionic residues, the surface moisture they cause and the bias voltage.

Before I also warned about the protection before using these chemicals, and that is that volatile organic compounds can also have adverse health effects. Solvents required for cleaning are also harmful, which also have an environmental impact.

I repeat that it is important wear goggles, mask and gloves for handling. Failure to do so could lead to health problems from prolonged exposure to, for example, rosin fumes. That can cause asthma in more sensitive people.

In the eyes or skin it can also cause problems. In fact, these fluxes adhere to the skin and can transfer heat better, causing burns.

Where to buy flux?

Finally, if you want to buy flux, you can find it at a good price in several specialized electronics stores. These are some recommendations:

Be the first to comment

Leave a Comment

Your email address will not be published. Required fields are marked with *



  1. Responsible for the data: Miguel Ángel Gatón
  2. Purpose of the data: Control SPAM, comment management.
  3. Legitimation: Your consent
  4. Communication of the data: The data will not be communicated to third parties except by legal obligation.
  5. Data storage: Database hosted by Occentus Networks (EU)
  6. Rights: At any time you can limit, recover and delete your information.