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What is Solder Paste in Electronics?

Solder Paste

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In electronics manufacturing, it is essential to bond every circuit board component to ensure the device works smoothly. Thus, solder paste can make a strong bond between the components without hampering the electrical connection. Solder paste is like glue that holds each electronic component together, providing solid connections between boards and components during soldering.


In this article, we will see about solder paste, its use in circuit boards, the proper method of using paste, and more.

What is Solder Paste?

It is a mixture of paste-like flux and solder powder, which is used in electronics to join the components in PCB. It is viscous in state and is basically gray. This paste is applied to the areas where components need to be soldered onto the PCB. 

When heated during the soldering process, the flux helps clean the metal surfaces, and the solder particles melt, creating a strong electrical and mechanical connection between the components and the board. 

It is typically applied using a stencil or a dispenser machine before components are placed onto the PCB and then heated in a reflow oven to complete the soldering process.

What is the Use of PCB Solder Paste?

solder paste for smd

It is used in surface mount technology (SMT) soldering. SMT is the process of creating a PCB from SMD components or mounting the components in the pads of the PCB. 

It is used for its ability to create solid electrical connections between electronic components and PCBs.

Below are some key reasons why PCB paste is used in electronics manufacturing:

Bonding Components

It is a bonding agent that attaches the electronics components like capacitors, resistors, connectors, and integrated circuits. It creates a secure bond between the leads of components and the PCB pads. 


It has excellent electrical conductivity. It efficiently transmits electrical signals between the components and ensures electronic devices’ proper functioning.

Thermal Conductivity

It helps to dissipate the heat generated by electronic components during the operations. 

Mechanical Strength

Solder joints created by solder paste provide strong mechanical support to electronic components. This mechanical strength is crucial for withstanding mechanical shocks, vibrations, and other environmental stresses.


With the trend toward miniaturization in electronics design, solder paste offers a precise and efficient method for attaching small and delicate components to densely populated PCBs. It allows for assembling complex electronic circuits in compact and space-constrained devices.

Types of Solder Paste


The lead-based pcb paste contains the lead as a major component in the solder alloy. However, its use is declining because of environmental worries about lead toxicity. It was once widely utilized but is now being phased out in many applications in preference for lead-free substitutes.


Unlike lead-based pcb paste, this variety has no lead. It’s growing increasingly popular as environmental rules limit the use of lead in electronic items. Lead-free solder paste complies with these rules while also providing dependable solder junctions for electronic components.


After the soldering procedure, this kind of solder paste leaves very little residue. Electronics can operate normally with this residue because it is not conductive or acidic, requiring no special cleaning procedures. No-clean solder paste is useful in situations where thorough cleaning could be challenging or not feasible.


After soldering, water can be used to remove water-soluble solder paste readily. However, it is critical to ensure that the solder joints are thoroughly dried thereafter to avoid corrosion. While water-soluble fluxes make cleaning more accessible than other types, adequate drying techniques are required to ensure the integrity of the electronic assembly.

Rosin Based

This solder paste contains rosin flux made from natural pine resin. Rosin-based fluxes are preferred for general-purpose soldering because of their high wetting qualities and ability to remove surface oxides from metal surfaces. However, they leave a sticky residue after soldering that may need to be cleaned to avoid long-term dependability difficulties.


How is Solder Paste Applied to PCB?

what is solder paste

Applying solder paste to PCB is typically done in a few steps:

  1. Preparation: First, the PCB is prepared by cleaning its surface to remove any contaminants or oxidation that could interfere with the soldering process.
  2. Stencil Printing: A PCB stencil, usually made of stainless steel or polyester, is placed over the PCB. This stencil has openings, called “apertures,” that align with the locations where components will be soldered onto the board.
  3. Solder Paste Application: It is then applied onto the stencil. It is done manually using a squeegee to spread the paste over the stencil openings, or it can be done automatically using a solder paste dispenser machine.
  4. Squeegeeing: If applied manually, a squeegee is used to press the paste through the apertures in the stencil, ensuring that the paste is evenly distributed and fills the openings properly.
  5. Component Placement: After applying PCB paste, electrical components such as resistors, capacitors, integrated circuits, and connectors are carefully put on the PCB and aligned with the matching PCB paste deposits.
  6. Reflow Soldering: The PCB, complete with components and PCB paste, is then placed in a reflow oven. The temperature in the oven is carefully managed to slowly bring the PCB and PCB paste to a specific temperature, usually around the solder alloy’s melting point.
  7. Soldering: As the PCB paste reaches its melting point, the solder particles liquefy and form metallurgical bonds between the component leads and the PCB pads. The flux helps to remove oxides and contaminants from the metal surfaces, ensuring strong and reliable solder joints.
  8. Cooling and Inspection: After soldering, the PCB is cooled to solidify the solder joints. It then undergoes inspection to verify the quality of the solder connections. Automated optical inspection (AOI) and X-ray inspection are commonly used techniques to check for defects such as insufficient solder, solder bridges, or misalignment.

Choosing the Right Solder Paste for Electronics

PCB Component Testing in PCB Quality Contorl

Choosing the suitable PCB paste for electronics involves a few key steps:

  • First, understand your project requirements. Consider factors like component types, operating environment, and industrial regulations or standards. 
  • Then, check the composition of the paste, including the solder alloy types and the flux formulations. Ensure that you are going for lead-based or lead-free solder. Also, evaluate the flux properties like activity level, reliability, and residue type. 
  • Ensure the method of paste application, including dispensing, stencil printing, or jetting. Consider the PCB paste compatible with the chosen application method, ensuring good printability and solder joint quality. 
  • Determine the compatibility of the PCB paste with your assembly process, including reflow soldering conditions and any post-soldering cleaning needs. Ensure the solder paste can resist the temperature profile of your reflow oven and that any residues can be easily removed if cleaning is required.
  • Evaluate the solder paste’s reliability and performance, such as solder joint strength, wetting ability, and resistance to mechanical and thermal stressors. Select a solder paste that will work reliably and consistently during the estimated life of the electronic assembly.
  • Finally, examine the solder paste supplier’s reputation and assistance. Choose a reliable manufacturer with a track record of creating high-quality solder pastes and giving technical support and assistance as needed.


Thus, solder paste is a need for electronics. Your manufacturer will choose the required paste based on your application requirements.

Contact ElePCB today for making your PCB strong by using Solder Paste. Elevate your PCBs with ElePCB.


How to use Solder Paste?

Solder paste is used with the following methods:

  1. Prepare surfaces.
  2. Apply paste.
  3. Place components.
  4. Reflow solder.
  5. Cool.
  6. Inspect.

How is Solder Flux different from Solder Paste?



Solder Flux

Solder Paste


Typically, liquid or gel substance is used to clean and prepare metal surfaces for soldering.

A mixture of flux and solder particles, providing both fluxing and bonding properties.


Applied separately onto the metal surface before soldering.

Pre-mixed and applied directly onto the components to be soldered.


Cleans metal surfaces, removes oxides and promotes solder flow.

Facilitates soldering by holding components in place and enhancing conductivity.


Usually liquid, gel, or solid form.

Semi-solid consistency, similar to toothpaste.

How Strong is Solder Paste?

The strength of PCB paste varies based on how it’s made and applied. But usually, people don’t pick solder paste for its strength in holding things together. Its main job is to make sure electricity flows smoothly in electronic devices. The connections it forms are usually strong enough to keep circuits stable. If extra strength is needed, people might use things like screws or glue along with soldering.


What is the difference between lead-free and lead-based solder paste?

Lead-based paste includes lead in the solder mixture, while lead-free versions align with environmental standards by omitting lead. 

The shift towards lead-free options is driven by environmental considerations, offering dependable solder connections without using lead.

How can I ensure the reliability of solder joints made with solder paste?

To ensure the reliability of solder joints  with solder paste, consider the following steps:

  • First, try to follow the recommended soldering profiles and guidelines provided by manufacturers.
  • Then, perform a visual inspection or use automated inspection techniques to verify solder joint quality.
  • Conduct reliability testing, like thermal cycling or vibration testing. This ensures the board’s long-term performance.

What are common issues encountered when using solder paste, and how can I troubleshoot them?

While applying PCB paste, various issues could be encountered, such as:

  • Insufficient deposition of solder paste
    • For this, you can check the stencil thickness and designs and adjust printing parameters. 
  • Bridging between the solder
    • To address this error, you can optimize the stencil design, adjust the volume of solder paste, and improve the alignment of components, too.
  • Poor solder joint formation:
    • Always verify the quality of the solder paste before using it. Also, check for the placement accuracy of the components. 

How do you ensure the proper application of solder paste?

  • Ensure the PCB surface is clean and free of contaminants.
  • Use a high-quality stencil or dispenser machine for precise application.
  • Ensure even distribution of solder paste on PCB pads and proper alignment of components.

What is the difference between no-clean and water-soluble solder paste?

  • No-clean solder paste leaves minimal residue after soldering. It doesn’t affect electronics operation and doesn’t require cleaning, too.
  • Water-soluble solder paste can be easily removed with water after soldering. Thus, thorough drying is necessary to prevent corrosion.


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About the Author
I am an Electrical and Electronics Engineer, and I have 5 years of work experience with electronics and communication jobs. I'm the full time content creator of ELE company.
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