Do You Need Flux With Rosin Core Solder?


Flux is used to clean metals to be joined by removing any oxidation and preventing any from forming. It removes surface oxides, grit, and grime from metal surfaces. Therefore, flux improves inter-metallic bonds’ quality; thus, it is necessary for soldering to be effective. It is best to use flux with rosin core solder when soldering surface mount components.  

Generally, rosin flux is mild and is designed to solder parts where flux residue cannot be removed. Components like electric circuit boards have no practical way of cleaning the flux residue after soldering. Rosin core solder is mainly used for electric purposes due to its material and modification. 

Rosin core solder is made of a metal alloy with a pitch-like organic compound substance that melts easily and adheres well to copper and other metals. Its properties enable it to form very strong bonds that are very good conductors of electricity. 

Mechanically, rosin core solder consists of a metal tube surrounding a rosin’s thin core. It is also fabricated with a hollow core inside the solder wire. Flux embedded into the solder’s core may not need any additional flux, although there is no danger in using extra. Rosin is, however, the best flux to use on electric appliances since they do not require very strong bonds due to their makeup materials. Rosin flux is also thin enough and flows easily through electric materials being soldered.

Besides, it is quite inactive in its solid-state and mostly works in liquid or paste form. However, solid rosin flux works with iron while the liquid and paste states are suitable for ware soldering.

Flux With Rosin Core Solder

Is Rosin Core Solder Toxic?

Rosin core solder is available in two types; lead- free and tin and lead alloy in the ratio of 60/40. Lead is quite poisonous on heating, and therefore rosin core solder made of it is toxic. Dust and fumes released while using rosin core solder containing lead are quite hazardous—fumes produced if inhaled cause occupational asthma and worsen existing asthmatic conditions. The fumes may also cause eye and upper respiratory tract irritation once inhaled.

Lead-free rosin core solder is safe to use as its fumes are quite harmless to people and the environment. Regular rosin core solder does not require a lot of cleaning; hence, little rosin flux is used in soldering. Both lead and lead-free rosin core solder conduct electricity but differ in their formation materials. 

Is Rosin Flux “No-Clean”?

Rosin-based fluxes pass no clean requirements due to the presence of rosin. “No clean” flux has benign, inert, and non-hygroscopic residues. Rosin flux does not need to be cleaned off after reflow as it is quite thin and harmless.

In the case where you use rosin core solder containing lead, and it is “no-clean,” you should take precautions like:

1. Ensure your workspace is well ventilated to dismantle and let go of the hazardous fumes produced quickly. Proper ventilation also involves the use of appropriate gear while soldering. Right equipment prevents inhalation of toxic fumes that have adverse effects on the human body.

2. Wash hands after use. In the absence of proper gloves and you are forced to work with rosin core solder bare hands, clean them off with water and soap after use. Please get rid of the material’s tiniest bits as it is quite toxic.

3. Stick to the original flux. There are two common flux types: acid core flux and rosin core flux. Rosin core solder works with rosin flux, and mixing it up with acid core flux can be dangerous. 

How Do You Use Rosin Flux?

Rosin flux exists in different states; hence the way of using them vary as explained below:  

Solid-State Rosin Flux 

Rosin flux in its solid state is quite useless. It does not flow freely and is therefore difficult to manipulate due to the permanent shape. Solid rosin is only used where the materials to be joined are of the same form as the rosin.

Paste State Rosin Flux


The paste can be manipulated easily compared to solid. You apply by following the steps below: 

Step 1: Scoop small amounts using fingers or a paintbrush. Since it cannot flow freely, you have to scoop it from the container for application.

Step 2: Spread out the rosin flux on the surface to be soldered. Use a paintbrush to ensure the layer created is thin and covers all the wires in electrical use. Ensure there are no creases that cause a difference in size.

Step 3: Put together the materials to be soldered and ensure they stick together to form a strong bond. Ensure the bond is smooth and cannot be dismantled easily. You can make the joint less visible if joining externally to make it more attractive.

Step 4: Clean off the extra flux. After joining the materials being soldered, excess flux overflows in the gaps and creates an ugly site. Clean off the excess flux to leave the appliances clean and attractive. 

Liquid State Rosin Flux

Rosin flows easily and very fast in its liquid state. Pour it out on the surfaces to be soldered in thin layers and smoothen with a paintbrush. This ensures the flux is evenly distributed and smooth. Since it is quite corrosive in a liquid state, avoid being in direct contact by wear gloves when soldering. 

The liquid state can be achieved by heating solid rosin to its melting point. Due to its comfortable flowing state, the excess flux may be poured out at once; hence cleaning off after soldering is essential. This is the most effective state of rosin for soldering.

Do I Need Separate Flux For The Rosin Flux Core?

Some components to be joined require more flux compared to others. If a lot of flux is required, the flux at the core of the rosin solder may not be enough. Different flux is purchased and applied. If the surfaces are thin, a flux pen is used to deliver the flux since it is sharp-pointed and can get through tiny spaces. For surface mount components like integrated circuits, extra flux must be added. Integrated circuits have numerous leads that increase the surface area to be soldered, hence requiring extra flux.

One of the primary purposes of flux is to wet the solder. The flux in rosin core solder may not be enough and may result to dry joints and bonds, which break easily. Electric bonds should not be dry, and flux must be in plenty. Metals mostly require less flux as a lot can corrode, causing them to wear out. Metals use acid core flux mostly hence the corrosion effect.

What Is The Difference Between Rosin And Flux?

Pure flux can either be organic or inorganic and differ from rosin that is mostly used on already cleaned surfaces. The surfaces are cleaned using a stronger chemical before rosin is used on them as it is less reactive. Rosin has the added advantage as it leaves no residue behind after use. Generally, rosin flux is mild and cannot clean off tough grit but only easy to remove dirt that gets completely dissolved, leaving no residue behind.

Organic flux is more reactive than rosin and can be used as the first chemical to clean a surface. Organic flux is insoluble in water hence a little less effective than inorganic flux, water-soluble. Organic and inorganic flux is raw as it is not processed as much as other flux types. Their rawness is an advantage and a disadvantage. They are most effective for cleaning surfaces and corrosive if they are not so dirty. These two should be used in small amounts and for the right surfaces. Inorganic flux is more common than organic ones since it dissolves in water, making it easier to manipulate.

How Can You Tell The Difference Between Acid Core Solder And Rosin Core Solder?

The difference between the two can be identified by heating them. On burning, if the smoke produced smells a bit sweet, that’s rosin. Rosin has a sweet smell because it is derived from conifers like pine which have a good scent. 

Acid core solder produces smoke that stings the eyes on burning. Acids are corrosive, and the fumes produced have a severe effect on burning. 

It is important to note the difference to avoid mixing up the flux and solder, which may result in hazardous situations when soldering.

Final Word 

Flux is best suited for use with the rosin core solder since it is mild compared to the rosin core. Some surfaces require more flux compared to others to solder. Sufficient flux must be used for soldering to be effective and create long-lasting bonds. Lead-free rosin core solder is the safest to use since the lead is quite poisonous. Rosin flux is used after cleaning the surface for long-lasting joints. 

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