Let us share our solder paste vs. flux insights to help you understand the soldering process’s specifics.
Learning to solder is essential as you will need the technique when installing components to any circuit board.
One of the ways to create a solid and dependable electrical connection between the parts is to incorporate solder paste into the solder joint.
But many cannot tell this material from solder flux, so we have prepared this insightful article. Take a look.
What is Soldering?
A repairman is soldering a circuit board.
It is the process through which we combine two metals to create a solder joint that allows an electrical connection.
A solder is a molten substance that acts as a binder between the two metal parts (the circuit board and the component pin).
What is Solder Paste
Soldering paste in a red container.
It is a thick, sticky, and creamy substance that features small bits of solder and filler material.
Rather than using glue to connect metal parts when soldering, we apply solder paste as its connection allows the transmission of electrical signals.
Hence, the solder paste mixture serves two main roles in the soldering process; it acts as a connector and allows electrical contact between the two metal parts.
What is the composition of Solder Paste?
Real solder paste comprises three main components:
- Metal solder powder
- Flux
- Filler material
The solder powder provides a conductive bridge for an electrical bond between the component and the electric circuit board.
On the other hand, flux withdraws heat from the soldered parts ensuring they don’t burn up/melt.
Finally, the filler material holds the solder powder and the flux (acts as media) while reinforcing the soldered parts’ bond strength.
Pros of Using Soldering Paste
Applying soldering paste on a PCB.
Below are the main advantages of soldering paste.
- It improves the strength of the joints, thus enhancing the connection between the parts. And this is imperative in ensuring a durable connection of electric wires to the circuit board.
- Also, it is useful in enabling uninterrupted signal transmission thanks to the solid electric bond. It improves the reliability of your joints.
- Thirdly, it has excellent viscosity, which makes it an effective glue.
- It doesn’t affect the operation of the electrical circuit board; hence there’s no harm in using it.
Cons of Using Soldering Paste
- Applying it requires special equipment and tools, which are pretty pricey.
- Beginners may experience challenges applying the paste in soldering electrical wires to PCB.
What Other Factors Go Into Choosing A Solder Paste?
Electric soldering iron with solder paste and wire.
Here are some of the critical considerations when settling on a solder paste.
Paste Slump
The feature measures the degree of the paste spread after applying it initially on the PCB.
A high slump paste provides significant challenges in managing primarily because its walls are distorted.
Avoid solder paste that easily flows out when choosing an ideal slump, as it might spread to unintended joints, leading to a short circuit.
Paste Tack
It’s a measure of the duration in which the paste remains tacky. Hence, excellent tack properties mean a stronger bond.
Viscosity
While it is similar to slump, viscosity is different as it measures how thick or fluid the paste is. Viscosity determines how convenient/easy it is to apply the paste.
Response-to-pause (RTP)
It is a parameter that highlights the duration for the solder paste to dry after application.
Working life
This feature determines how long the PCB/ stencil can retain the solder paste before it spills over to other parts.
How Do You Use Solder Paste?
Soldering paste in a syringe.
Here’s a step-by-step guide on how to apply solder paste on your PCB.
- First, thoroughly clean your PCB and the parts you want to solder with a compatible solvent. Ensuring the components are dried adequately before the soldering process is also essential.
- Next, draw the paste from its pack, preferably using a syringe, and apply it to the PCB and the surface of the component you’re soldering.
- Lay a hot soldering iron on the applied solder paste to melt it. It will lead to the formation of a soldering bond.
- It is also essential to allow some time for the solder to dry. Finally, shake the component gently to remove the excess solder paste. This final step is also critical in ensuring you have made a strong bond that is unlikely to break.
Solder Paste vs. Solder Wire
Soldering components.
The two materials serve the same role, with their primary difference being how you use each.
To apply solder paste, melt it with a soldering iron and apply the molten material to your joint.
Conversely, to make a joint with a solder wire lay it on the surfaces you’re connecting and melt it by heating it.
A solder paste is useful in projects with insufficient space on the circuit board.
On the other hand, we often use a soldering wire when making connections capable of retaining their strength even under extreme temperatures of the PCB application.
What is Flux
An example of a soldering flux.
It’s a mixture of acids with cleaning properties whose fundamental role is strengthening the bond between the circuit board and the attached components.
Using it is vital in enhancing the solder bond strength, as explained in another section below.
What is the Composition of Flux?
Different kinds of flux exist, and each type has a different composition.
Rosin based pastes
Rosin is a material from the pine tree stem that is the primary material in rosin-based fluxes.
Also featuring petroleum jelly, and abietic acid, a rosin-based paste is an effective flux that is the easiest to clean up after application.
Water-soluble fluxes
Tool and accessories for soldering.
They are water-soluble, meaning you can scrape them off after application by simply washing the surface with water.
The fluxes are ideal for soldering copper and its alloys, which makes them common in plumbing applications.
The other common applications of water-based fluxes include water sprinklers, plumbing, and air conditioning units.
You must activate water-soluble fluxes with an acid activator to enhance their effectiveness.
No-Clean flux
If you don’t fancy the hassle of cleaning up the flux after application, this is the go-to type of flux.
This flux is from resins, and although it doesn’t require cleaning, you must apply it on a thoroughly clean circuit board surface for it to work.
Why Do You Need Flux In Solder Pastes?
A syringe with flux solder.
Here are the primary roles of flux:
- Flux operates by dissolving contaminants and metal oxides, which would interfere with strong bond formation if not removed. Therefore, during soldering, we apply flux before applying solder paste to clean the surfaces.
- It also wets the surface after removing the oxides, which is vital in enabling the adhesion of the molten solder.
- Thirdly, flux is significant in curtailing the reoxidation of the metal parts after soldering is complete. And this is critical in preventing the oxidation of metals leading to the weakening of the solder joint.
- Flux is also an effective heat transfer medium between the two parts forming the bond. Hence it fastens the bond formation process.
- Lastly, it lowers the solder melting point, which enhances the melting and bond formation process.
Pros of using soldering flux
- It eliminates oxides and improves bonding by thoroughly cleaning the binding metal surfaces.
- Also, it reduces any danger that can happen during the soldering process by curtailing oxidation.
Cons of using soldering flux
- It may hinder the functioning of your soldering wire/ paste components. It would be detrimental to the soldering process.
- Sometimes flux distorts the temperature of the electric circuit board, which may, in turn, affect the soldering temperature.
What Other Factors Go Into Choosing a Flux?
- The main consideration is choosing flux compatible with the metal parts you’re soldering. Therefore, research whether combining a particular flux with your circuit board and its components can have any effects.
- Also, consider if the flux is best suited to the role you are using it for and the solid paste type you’re using.
Alternatives to Soldering Flux
Petroleum jelly is an alternative to soldering flux.
You can use either petroleum jelly or vaseline if you don’t have flux, as it is equally effective in hindering corrosion by blocking the oxidation of metals during soldering.
However, flux is often the best for soldering; therefore, only opt for the alternatives as a last resort.
How Do You Use Flux?
Applying flux.
Follow the steps below for a fruitful flux application:
- First, you must thoroughly clean the surfaces and remove all the dirt, dust, and grease that could react with the flux, thus reducing its efficiency.
- Once clean, now apply the flux mixed with solder paste. Most solder pastes come premixed with flux, so you don’t have to buy the two components separately.
You can apply the paste with a small brush or a soldering wire.
- Ensure you apply an evenly spread layer over the part you’re soldering.
Below are the standard flux application techniques based on the materials you want to bond.
Solder Reflow
The reflow process is a simple method that involves using a reflow oven to heat the flux and soldering mixture to a molten paste, then applying it to the joint.
You can use the reflow soldering technique when dealing with small components that are hard to solder via a wire. Hence, the method is handy in surface-mount technology (SMT).
Other solder flux application techniques include wave soldering and selective soldering.
The former involves making a flux spray on the metal surface before applying a solder wave paste.
Selective soldering is a generic term for any solder application via spraying or dropping the mixture on a metal surface.
Solder Paste vs. Flux FAQs
Solder paste vs. flux: Is soldering paste the same as flux?
No, they are different. The soldering paste is an accumulation of different materials, including flux, soldering powder, and filler material, which are imperative in creating a solder joint upon melting.
Conversely, flux comprises a mixture of organic acids and other components that clean the metal surfaces to facilitate effective bonding during soldering.
Can You Use Solder Paste Without Flux?
You can use solder paste without flux.
However, the absence of flux is a precursor to oxidation, reducing solder bond strength.
Further, this also encourages the corrosion of the metal joints, which, if significant, can lower the reliability of the PCB and the electronic component.
Therefore, it is always important to use flux during soldering.
How Do You Clean Up or Remove Solder Paste With Flux From Unwanted Surfaces?
The cleaning formula is dependent on the flux type you are using. Here are the removal techniques for different types of flux spills.
| Type of Flux | Cleaning Solution |
| Rosin-based fluxes | Apply fluorocarbon-rich chemical solvents. |
| No clean flux | No need to clean them as they are harmless. |
| Water-based fluxes | Deionized water, general household detergents, or hot water. It would also help to add saponifiers to improve cleaning. |
Summary
Soldering parts on a PCB is a straightforward process, but it requires some expertise to ensure you create solid and long-lasting bonds. This process’s most significant element is applying flux with soldering pastes.
Thankfully, we have made a distinction between the two and also elaborated on the significance of each. You can now blissfully do a quality soldering job.