What is soldering flux

Which flux is right for me?

Flux overview

A flux is essential for a good solder joint. There are many products on the market, so that an inexperienced buyer can lose track of things relatively quickly. There are products from well-known German manufacturers as well as no-name products from China. I have been soldering myself for 20 years, 7 of them in an industrial environment. During that time I have tried many products and would like to share my knowledge with my readers here. But first of all, some theory:

What happens when soldering?

Put simply, we have different workpieces when soldering, which we connect mechanically and electrically with a soldered joint. In the simplest case, this can be the individual wires of a stranded wire that we connect to each other or Through The Hole, SMD, BGA components that we solder onto a circuit board.

During soldering, the function of the solder is to fill the space between the workpieces and to connect them to one another mechanically, electrically and thermally. The solder forms an alloy with the metal on the surfaces of the workpieces. In practice, however, there is always a problem. The workpieces are often in the air for a long time. The surface oxidizes in the process. And as soon as you heat the surface with the soldering iron, it oxidizes even more, within seconds. The oxidation layer prevents the solder from alloying with the surface of the workpiece. Then you have soldering results, as can be seen in the picture on the right.

This is exactly where the flux comes in. The main function of a flux is to remove the oxidation layer through a chemical reaction and to ensure that no new oxidation layer is created during soldering.

Another function of fluxes is to reduce the interfacial tension. This then creates better and more durable solder joints.

Do you need extra flux?

A modern solder wire today has at least one core (often even several cores) with flux, usually 2.5%. If you lead the solder wire to the components that you want to solder, the solder melts and releases the flux. The flux then burns or evaporates with a plume of smoke.

This is exactly where the problem becomes apparent. If the flux is burnt, it no longer works because it is no longer there. The oxidation problem is still there though. Especially when soldering SMD components or when soldering with certain soldering techniques (e.g. drag soldering), you usually lack flux. And that's exactly where additional flux comes into play. This is then simply given directly to the soldering point. And as soon as you get near the spot with the soldering iron, the heat activates it.

Clean or No-Clean Flux?

As you have already learned above, a flux removes the oxidation layer on the components or boards that you want to solder. It happens through a chemical reaction. And so that you don't have to wait long for the oxidation layer to be removed when soldering, a flux has to be aggressive. To do this, it contains various chemical components.

As a rule, a flux cannot completely evaporate or burn, but leaves residues or traces on the circuit board or the component. These residues then remain permanently on the circuit board if the circuit board is not cleaned. Some fluxes are so aggressive or corrosive that over short or long periods - depending on the flux - they can attack and dissolve the soldered connection, components or the conductive layers on the circuit board. To prevent this from happening, the flux must be removed. We'll learn how to do this later in this article.

In contrast to clean fluxes, no-clean fluxes can remain on the board because they are not corrosive. Nevertheless, it makes sense to remove flux as much as possible, although this is not so easy with BGA chips.

Is rosin a flux?

What is rosin Rosin is a tree resin, it is sold in different forms. Either in a block or as granules. Unfortunately, this form makes it difficult to distribute rosin on a circuit board. Therefore, rosin in a block is actually only suitable for wetting conductors with solder.

Alternatively, rosin can be dissolved in an alcohol. The liquid can then be used as a flux. Unfortunately, this homemade flux is hardly comparable to the modern flux available today. We therefore advise against making such a flux yourself.

Which flux should you buy?

The flux market is unfortunately very confusing. In addition, counterfeits from China are sold on the usual platforms. Fluxes are usually sold with a shelf life of approximately one year. A branded flux can also be used without any problems.

The first thing to consider is what you want to use the flux for. A flux for soldering BGA chips has to perform at its best when running through a soldering profile. Depending on the profile, the flux must remain activated for up to 400 seconds without boiling and lifting the chip. A flux for through-the-hole soldering, however, does not have to be. Accordingly, it is significantly cheaper in price.

Another differentiating factor is the base of the flux - the "active ingredient". Flux is classified according to J-STD-004 or DIN EN 61190-1-1

J-STD-004 types of flux
Rosin (natural resin) RO
L Low (activated low)
Resin (synthetic resin) RE
M Moderate (moderately activated)
Organic (resin-free organic flux) OR
Inorganic (inorganic flux) IN

There is also a division into types, with the first letter standing for the activity:

  • L0: Absolutely no corrosion, no-clean
  • L1: Absolutely no corrosion, but contains halogens, no-clean
  • M0: Minor Corrosion, Clean and No-Clean
  • M1: Minor corrosion, contains halogens, clean and no-clean
  • H0: Major Corrosion, Clean
  • H1: Major corrosion, contains halogens, clean

When soldering electronics it is common to use either low to moderately activated fluxes. Modern high-performance fluxes for BGA soldering consist of a large number of components and can only be divided into categories with difficulty.

Our recommendation:

As with solder, everyone has their preferences when it comes to flux. After extensive tests in recent years, three no-clean fluxes have emerged that we can recommend. That’s the Ersa FMKANC. The FMKANC is a white paste, it leaves barely visible, transparent residues that are non-corrosive and easy to remove.

Another flux is the AMTECH NC-559. The NC-559 is a light yellow paste that also leaves barely visible transparent residue. When exposed to heat for a long time, the residues turn slightly brown.

The third flux is from German manufacturer Martin. It is a slightly yellowish flux that also leaves behind barely visible, transparent residues.

All of these fluxes are not cheap, but they can be used for many purposes. A syringe with about 5ml of flux costs about 20-30 €. Especially with SMD soldering and rework, these fluxes score points compared to the flux pens, which also cost around 7-10 €. However, there is much more content in such a syringe. Because only small amounts are used when soldering, such a 5ml syringe will often last for years. And anyone who has ever worked with such a modern flux will most certainly not want to use another one.

Are flux vapors hazardous to health?

It is a very difficult subject. The fact is that during soldering, the vapors of the flux - whether in the soldering wire or something extra - are created and the soldering end inhales them. I don't like to judge whether these vapors are hazardous to health or even carcinogenic or not. Certainly these vapors will not be beneficial to health.

In industrial soldering, we use an extraction system that sucks solder fumes from the workstations and cleans them through a filter. Surely you can't expect a hobby electronics technician to buy such a device. However, there are quite effective alternatives to this. In the picture below you can see a simple self-built device, consisting of two fans from a decommissioned PC. These fans can be operated on a laboratory power supply unit via normal laboratory cables with banana plugs. The air flow ensures that the soldering fumes get away from the workplace. Although these are still in the room, the person soldering does not inhale them directly.

Homemade blower


Flux from China

Unfortunately, there are not only the original products on the market, but also counterfeits from China. There is now even plagiarism of plagiarism. Such plagiarism is characterized by the fact that the effect is significantly worse than the original. A syringe with plagiarized flux costs around USD 1 in China. You can therefore tell from the price whether you are ordering an original product or not.

Another problem is the hazard from the flux. There are certain chemicals in a flux. When soldering, the vapors of these chemicals rise into the air and are inhaled. With an original product, you can always use the safety data sheet to see exactly what is in the flux. With plagiarism, you don't know what you are breathing in when you work with the flux.

Therefore, please do not use any fake flux from China!