What is the Difference Between Solder Paste And Flux?

Solder paste and flux are mixtures used to essentially ‘glue’ metals together. If you are working in metallurgy, plumbing, electric circuit boards, and much more, it is important to know the difference between solder paste and flux. 

Solder Paste is a product used to glue different parts of metal alloy to each other. In contrast, Flux refers to an ingredient that is added to the surface before or added to the solder paste. Flux acts as a cleaning agent during the soldering process, making the solder paste more effective.

Knowing the difference between the two options can make or break your project. Read on to understand their differences on what they are, what they’re made of, and how to use them both in creating circuit boards!

What is Solder Paste?

Solder paste, also known as solder cream, is a mixture used for bonding parts of metals together. Solder can be used in projects from metallurgy to making electric circuit boards to even connecting your license plate to your car. 

‘Soldering’ is a term used to describe the process of combining two separate metals together. Once heated and molten, solder acts as a kind of glue to conjoin metal surfaces. This point of conjunction where solder paste holds two surfaces together is frequently referred to as the ‘solder joint.’

Instead of using regular old glue, solder paste is much more effective for connecting electrical and mechanical parts. This is because the molten solder allows for the transmission of electric signals, allowing for wires and other objects to transmit electricity uninterrupted.

What is Solder Paste Made Out Of?

Solder paste is composed of powdered metal, creating a thick consistency, just like paste! To think of it plainly, it’s basically like tiny bits of metal that are melted down in a concoction to make a glue to combine other metal objects. 

The paste typically has a sticky, tacky consistency allowing for it to be a great tool to connect electrical and any other parts to a circuit board. It also allows for electric continuity since it is made out of metal. Meaning it will not interrupt the flow of electricity in necessary components of electrical circuit boards.

What Other Factors Go Into Choosing A Solder Paste?

The properties of solder pastes can vary dramatically due to a number of factors. Generally, these properties are referred to as ‘rheological,’ which refers to the various ways the paste can flow in its ‘soft liquid’ state. 

Differences in the following properties are key to understanding the variations of solder paste mixtures:

  • Slump is the term that defines how much or how little the paste spreads after its initial application to a surface. If the mixture has a high degree of slump, it might be a bit harder to manage since its walls will not perfectly straight and even. 

A high degree of slump is not desirable in soldering projects because it risks spreading into another ‘bridge’ of paste. This would be bad because such a joining of pastes would create a short circuit. 

  • Tack is basically how long and how well the paste can remain properly tacky. A device with solder joints that have strong tack properties is more likely to last longer and work more effectively over time.
  • Viscosity refers to how much or how little the paste flows. This is the most common variant in different solder pastes on the market. Be sure to do some research on your project beforehand to determine which viscosity of paste is needed.  
  • Response-to-pause or RTP is a measurement used for determining the time it takes for the solder paste to dry, thus determining how long the overall soldering process will be. 
  • Working life refers to how long a solder paste can last on a stencil or your intended location before spilling over and affecting the printing properties of your circuit.

How do You Use Solder Paste?

To use solder paste, it must be mixed with the right ingredients; then, it must be melted down to exactly the right temperature. 

Depending on what type or brand of solder paste you use, the temperature point needed for the paste to melt may vary. Be sure to check online or with the manufacturer before melting your pre-made solder paste.

The solder paste only temporarily holds the part onto the board. Now is the time to get out the heating device!

In order to permanently bond the metal electrical pieces together, the board needs to be heated at a controlled temperature. This allows the solder paste to melt while bonding the parts electrically and also mechanically. 

Solder paste can be applied to circuit boards using a number of different processes. Such processes include syringe, stencil printing, or jet printing. 

Specific components can then be meticulously soldered by hand or by using an automated machine.

Solder paste is most commonly melted down using a soldering iron or other heating devices. See section ‘How to Apply Flux?’ below for more detail on the different heating processes. 

What is a ‘Solder Iron’?

A soldering iron is the handheld device used to apply solder to the two surfaces you intend to glue together. It has iron plating around its exterior to protect the interior, which is typically made of some variation of copper alloy that acts as a thermal conductor. 

The iron has a metal tip that acts as the applicator of the solder paste. The soldering iron is either plugged into an outlet or a soldering machine, allowing for a part of the device to heat up to the temperature needed for soldering to work.

Other solder irons are heated up by placing them against a separate heating device, opposed to it internally producing heat from electricity. 

Finally, the soldering iron has a handle, which is insulated from the hot temperatures produced within the device. Its design is somewhat similar to the design of a curling iron used for hair. 

The tip of the iron is then heated up in order to melt the solder paste. Choosing which kind of iron to use depends on how long and how controlled you wish for the soldering iron to be in temperature. 

Once heated to the correct temperature, the tip then makes contact with the solder paste. For a short time at the right temperature, the soldering iron is able to arrange the paste to cover the desired locations.

What is Flux?

Flux is an important component of solder paste, which is added to add temporary adhesiveness to the solder paste mixture. 

In solder paste, flux acts also as a cleaning agent that can be applied before or during the soldering application process. 

The main function of flux is to prep metal objects or surfaces you desire to adhere to one another. 

What is Flux Made Out Of?

Flux can be made from one or more different ingredients. There are also different varieties of flux available depending on what your intended use is. 

Different ingredients of flux include zinc chloride, rosin, or hydrochloric acid. Another kind of flux, called rosin flux, can even be made at home using pine cones!

Flux typically appears to have a grey color and has a putty-like consistency. Once added to solder paste and heated up properly, you are ready to bond some metals together!

Why Do You Need Flux In Solder Pastes?

Flux is the most essential ingredient to smolder paste because it is the main agent in ensuring a smooth and solid bond. Flux helps you achieve a clean and solid adhesion of two objects. 

Flux allows for the oxide layers on metal surfaces to dissolve, then allowing for the solder to “wet” or prep the surface for adhesion. Wetness, or a state of liquidus, allows for molten solder to be more adhesive. 

By altering the surface tension of metal’s molten solder using flux using this process, you are then able to make a bond or solder joint. Therefore flux ‘cleans’ the metal surfaces, allowing for strong solder joints to form.

Flux is also crucial for protecting your metals from reoxidizing after the soldering process. Over time, and after exposure to oxygen, metal objects form oxides that prevent the formation of solid solder joints. 

Flux is thus great at preventing the alteration of the solder joint or solidified solder paste from the process of oxidation. Oxidation is undesirable on metal because it breaks down molecular structures of the surfaces. 

This causes corrosion to occur, which morphs your metals and potentially breaks solder joints. For example, if your joints within the plumbing pipes to your bathroom sink are oxidized and eroded, you could have a major problem and mess to clean up.

Thus flux is key to preserving solder joints by preventing oxidation and corrosion.

What Types of Solder Pastes With Flux Are Out There?

There are several different kinds of flux that can be added to solder paste, each used for different uses. It can come in the form of a solid, liquid, or paste consistency. Knowing the differences in the bases of each kind of flux is important to know before starting the soldering process. 

The categories of flux are described as the following:

  • Rosin based pastes are considered to be the safest substances to use on circuit boards since it is extremely easy to clean up. This is because it is made out of rosin, which is a natural ingredient that is extracted from pine trees.

In addition to pine tree sap extract, rosin also typically contains petroleum jelly, which is the main pasty ingredient in products such as Vaseline. It will also often contain acids that have active ingredients, such as abietic acid. 

  • Water-soluble fluxes are best used in plumbing projects since it is particularly good at bonding copper and copper alloy pipes that need soldering. 

It is also good for most other soldering applications that use copper piping, such as air conditioning, heating, and water sprinkler systems. 

As the name suggests, water-soluble fluxes are easily removed by simply applying water to the applied area. 

In order to activate water-soluble flux mixtures, you need to use stronger activating ingredients, such as acids. 

  • A No-Clean flux refers to mixtures that are made with resins and other different levels of solid debris. 

As the name suggests, the paste is claimed to not need to clean up after the solder paste becomes molten.

No-Clean flux formulas require extra cleaning of the metal surfaces you intend to combine together. 

Can You Use Solder Paste Without Flux?

In short, the answer is no. It would be nearly impossible to adhere surfaces together with Solder paste lacking flux. 

This is because the flux is the cleaning agent in solder paste, allowing for adhesivity to form. Without flux, your solder joints would also be at risk of oxidation over time.

How Do You Apply Solder Paste With Flux?

In order to properly adhere your desired surfaces together, you must first properly clean the surfaces before applying any paste. To ensure that you achieve a solid metallurgical bond, you must thoroughly wipe away any dirt and grease from the surfaces. 

Generally, the application process of flux begins with mixing or purchasing a pre-mixed container of Solder paste with flux. The flux can be applied by hand or by machine. 

If you are applying the solder paste by hand, you can apply it with a pen or a solder wire that technicians use. You can also use a small paintbrush or your fingers. 

The flux must be spread evenly over the area you intend to solder. Make sure to cover your wires or other surfaces evenly so the process works as intended!

Automated machines can use a broader range of a few different methods for applying flux to surfaces. 

While applying flux to electronic surfaces, you can apply the mixture in the following methods: 

  • Solder Reflow is a method that is used for surfaces or circuit boards in the process of solder reflow. Solder reflow means that the solder paste and flux will be heated into a molten mixture by using a reflow oven, infrared heat, or even a special desoldering hot air device, shaped like a pencil.

This method is most commonly used in adhering small electrical parts together, such as applying surface-mount technology (SMT) to a printed circuit board (PCB). 

Such a process requires careful attention to control the preheating, soaking, and reflow heating stages.

  • Wave Soldering is the preferred method of soldering lead objects into parts of SMTs that are adhered to circuit boards. 

The process typically consists of more solvents than other application methods. The flux is sprayed on the surface prior to the solder wave. Once applied to the surface, the flux serves to clean the soldered parts, removing any unwanted oxidized layers. 

If you choose to use a less aggressive mixture of flux, you have to prep the surface extra well. Consider cleaning the surface more than one time before wave soldering.

  • Selective Soldering refers to solder that is applied to a surface by being dropped or sprayed. 
.

How Do You Clean Up or Remove Solder Paste With Flux From Unwanted Surfaces? 

Clean-up of solder paste and flux can be an important factor to consider before deciding on which type of paste to use. 

If you misplaced or accidentally spilled your flux onto an unwanted surface, it may be dissolvable depending on what kind of flux it is. This is particularly useful when you need to clean up your electric circuit board, which often has tiny crevices that need molten solder paste removed. 

The different categories of flux and how easily they are removed from surfaces are as follows:

  •  Rosin Based flux needs to have specific chemical solvents that generally have fluorocarbons. Fluorocarbons are chemical compounds that contain only fluorine and carbon. 

There are many organic materials that contain fluorocarbons. Thus these materials can make for great removers of rosin-based flux. 

  • Water Soluble flux requires many cleaning ingredients or agents to remove it. Such ingredients include detergents and deionized water. 

Therefore you can remove this type of flux a lot of the time simply by using hot water, and sometimes a saponifier is needed to accelerate the cleaning time. 

Saponifiers refer to alkaline and corrosive mixtures. When they are mixed with solder residue, they form a soap-like consistency. This is then easily washed away once again by applying hot water. 

  • No-clean flux mixtures claim to require no cleaning in the aftermath and can be left on the surface and the objects it was applied to.

It is important to note that many users of no-clean flux have difficulties in removing the flux substance if need be. Some find the residue nearly impossible to remove. 

Be sure to consider this part of no-clean flux before using it. If you mess up and misplace the flux, it may just stay there permanently! 

Sources 

https://en.wikipedia.org/wiki/Solder_paste#:~:text=A%20solder%20paste%20is%20essentially,gray%2C%20putty%2Dlike%20material.

https://en.wikipedia.org/wiki/Printed_circuit_board

https://www.tempoautomation.com/blog/understanding-soldering-part-4-how-to-use-flux-when-soldering-electronics/

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If you liked this article, have a look at my other articles I wrote about the topic!

Alexander Berk

A bit about myself: I am a certified international welding engineer (IWE) who worked in different welding projects for TIG, MIG, MAG, and Resistance Spot welding. Most recently as a Process Engineer for Laser and TIG welding processes. To address some of the questions I frequently got asked or was wondering myself during my job, I started this blog. It has become a bit of a pet project, as I want to learn more about the details about welding. I sincerely hope it will help you to improve your welding results as much as it did improve mine.

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