Solder Paste Something You Need to Know

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What is Solder Paste?

Solder paste is a thick, paste-like material composed of tiny solder alloy particles, typically ranging from 25 to 45 microns in diameter, suspended in a flux medium. The solder alloy is usually a combination of tin, lead, and sometimes silver, with varying ratios depending on the specific application and desired properties. The flux medium serves to remove oxides from the metal surfaces, promote wetting, and protect the solder from oxidation during the reflow process.

Composition of Solder Paste

The composition of solder paste can be broken down into two main components:

  1. Solder Alloy Particles
  2. Tin (Sn)
  3. Lead (Pb)
  4. Silver (Ag)
  5. Copper (Cu)
  6. Antimony (Sb)
  7. Bismuth (Bi)

  8. Flux Medium

  9. Rosin-based flux
  10. Water-soluble flux
  11. No-clean flux

The choice of solder alloy and flux medium depends on various factors such as the desired melting temperature, compatibility with the components and PCB materials, environmental considerations, and cleaning requirements.

Solder Paste Properties

To ensure the success of the soldering process, it is crucial to understand the key properties of solder paste and how they affect its performance.


Viscosity is a measure of the solder paste’s resistance to flow. It is an essential property that determines the ease of application and the ability of the paste to maintain its shape after being dispensed onto the PCB. The viscosity of solder paste is influenced by factors such as the particle size distribution, flux composition, and temperature.

Viscosity Range Characteristics
Low Easier to dispense, may cause bridging or slumping
Medium Balanced dispensing and shape retention
High Difficult to dispense, better shape retention


Slump refers to the tendency of solder paste to spread or flow after being dispensed onto the PCB. Excessive slump can lead to bridging between adjacent pads or components, causing short circuits. Slump resistance is influenced by the particle size distribution, flux composition, and the rheological properties of the paste.


Tackiness is the ability of solder paste to adhere to the PCB and components during the placement process. Adequate tackiness ensures that components remain in place until the solder paste is reflowed. The tackiness of solder paste is affected by the flux composition and the ambient temperature and humidity.

Shelf Life

Shelf life is the period during which the solder paste remains usable after being manufactured. Solder paste has a limited shelf life due to the gradual degradation of the flux medium and the oxidation of the solder particles. Proper storage conditions, such as refrigeration and sealed containers, can help extend the shelf life of solder paste.

Solder Paste Application Methods

There are several methods for applying solder paste onto PCBs, each with its advantages and limitations.

Stencil Printing

Stencil printing is the most common method for applying solder paste in high-volume production. It involves using a stencil, which is a thin metal sheet with apertures that correspond to the pads on the PCB. The solder paste is deposited onto the stencil and then squeegeed across the surface, forcing the paste through the apertures and onto the pads.

– High-speed and consistent application
– Suitable for fine-pitch components
– Excellent control over solder paste volume

– Requires a custom stencil for each PCB design
– Stencil cleaning and maintenance required
– Limited flexibility for low-volume or prototype production


Dispensing involves using a syringe or automated dispensing system to apply solder paste directly onto the PCB pads. This method is often used for low-volume production, prototyping, or rework processes.

– Flexibility in applying solder paste to specific areas
– No stencil required
– Suitable for odd-shaped or large components

– Slower than stencil printing
– Less consistent solder paste volume
– May require specialized equipment for automated dispensing

Jet Printing

Jet printing is a relatively new technology that uses a piezoelectric or pneumatic jetting system to dispense solder paste onto the PCB. It offers a combination of the advantages of stencil printing and dispensing.

– High-speed and flexible application
– No stencil required
– Suitable for fine-pitch components and complex PCB designs

– Higher equipment cost compared to stencil printing or dispensing
– May require specialized software for generating jetting patterns

Solder Paste Printing Process

The solder paste printing process involves several key steps to ensure the accurate and consistent application of solder paste onto the PCB.

  1. Stencil Alignment
  2. The stencil is aligned with the PCB using fiducial marks or alignment pins
  3. Proper alignment is critical for accurate solder paste deposition

  4. Solder Paste Application

  5. Solder paste is dispensed onto the stencil
  6. A squeegee blade is used to spread the paste across the stencil surface

  7. Solder Paste Transfer

  8. The squeegee blade forces the solder paste through the stencil apertures and onto the PCB pads
  9. The stencil is then lifted away from the PCB, leaving the solder paste deposits on the pads

  10. Inspection

  11. The printed PCB is inspected for any defects or inconsistencies in the solder paste deposits
  12. Common defects include insufficient paste volume, bridging, or misalignment

  13. Cleaning

  14. The stencil is cleaned to remove any residual solder paste
  15. Regular cleaning maintains the quality and consistency of the printing process

Solder Paste Reflow Profile

After the solder paste is applied and the components are placed, the PCB undergoes a reflow process to melt the solder and form permanent connections. The reflow profile is a critical factor in achieving reliable solder joints and minimizing defects.

A typical reflow profile consists of four stages:

  1. Preheat
  2. The PCB is gradually heated to activate the flux and remove any moisture
  3. Typical preheat temperatures range from 150°C to 180°C
  4. The preheat rate should be controlled to prevent thermal shock to the components

  5. Soak

  6. The PCB is held at a stable temperature to allow the flux to remove oxides and prepare the surfaces for soldering
  7. Typical soak temperatures range from 180°C to 200°C
  8. The soak duration depends on the size and thermal mass of the PCB and components

  9. Reflow

  10. The temperature is rapidly increased to melt the solder particles and form solder joints
  11. The peak reflow temperature depends on the solder alloy composition
  12. Common peak temperatures range from 220°C to 250°C for lead-free solder alloys

  13. Cooling

  14. The PCB is cooled down to solidify the solder joints
  15. The cooling rate should be controlled to prevent thermal stress and warpage
Reflow Profile Stage Temperature Range Duration
Preheat 150°C – 180°C 60 – 120 seconds
Soak 180°C – 200°C 60 – 120 seconds
Reflow 220°C – 250°C 30 – 60 seconds
Cooling 250°C – Room Temp 60 – 120 seconds

Solder Paste Defects and Troubleshooting

Despite careful process control, solder paste defects can still occur. Identifying and addressing these defects is crucial for ensuring the reliability and functionality of the assembLED PCBs.

Common Solder Paste Defects

  1. Insufficient Solder
  2. Occurs when the solder paste volume is too low, resulting in weak or open connections
  3. Causes: Clogged stencil apertures, low paste viscosity, incorrect stencil thickness

  4. Bridging

  5. Occurs when solder paste spreads and connects adjacent pads or components
  6. Causes: Excessive paste volume, high slump, incorrect stencil design

  7. Solder Balls

  8. Small spheres of solder that form on the PCB surface or component leads
  9. Causes: Excessive paste volume, high reflow temperature, contamination

  10. Tombstoning

  11. Occurs when a component stands up on one end during reflow, causing an open connection
  12. Causes: Uneven paste volume, component misalignment, rapid heating during reflow

Troubleshooting Solder Paste Defects

  1. Adjust Solder Paste Volume
  2. Ensure the correct stencil thickness and aperture size for the desired paste volume
  3. Monitor the paste viscosity and replace the paste if it becomes too thin or thick

  4. Optimize Stencil Design

  5. Ensure adequate clearance between apertures to prevent bridging
  6. Use appropriate aperture shapes and sizes for the specific Component Types

  7. Control Reflow Profile

  8. Follow the recommended reflow profile for the solder paste and PCB Assembly
  9. Avoid excessive peak temperatures and rapid heating or cooling rates

  10. Maintain Cleanliness

  11. Keep the stencil, PCB, and components clean and free from contamination
  12. Use appropriate cleaning methods and solvents for the specific solder paste and flux type

Best Practices for Solder Paste Handling and Storage

To maintain the quality and performance of solder paste, it is essential to follow proper handling and storage practices.

  1. Storage Temperature
  2. Store solder paste in a refrigerator at a temperature between 2°C and 10°C
  3. Allow the paste to reach room temperature before opening the container to prevent condensation

  4. Shelf Life

  5. Monitor the shelf life of the solder paste and use it within the specified period
  6. Rotate the inventory to ensure that older paste is used first

  7. Handling

  8. Use clean, lint-free gloves when handling solder paste to prevent contamination
  9. Avoid excessive stirring or agitation, which can introduce air bubbles and affect the paste consistency

  10. Sealed Containers

  11. Keep the solder paste in sealed containers when not in use to prevent drying and contamination
  12. Use a syringe or cartridge system for dispensing to minimize exposure to air

  13. Disposal

  14. Follow proper disposal guidelines for expired or contaminated solder paste
  15. Consult local regulations for the safe disposal of solder waste

Frequently Asked Questions (FAQ)

  1. What is the difference between leaded and lead-free solder paste?
  2. Leaded solder paste contains lead (Pb) in the solder alloy, typically in a 63Sn/37Pb composition. Lead-free solder paste uses alternative alloys, such as Sn/Ag/Cu (SAC), to comply with environmental regulations. Lead-free solder paste generally has higher melting temperatures and requires different reflow profiles compared to leaded solder paste.

  3. Can solder paste be reused?

  4. It is not recommended to reuse solder paste that has been dispensed onto a stencil or PCB. Once exposed to air, the solder paste can start to dry out and lose its desired properties. Reusing solder paste can lead to inconsistent results and defects in the soldering process.

  5. How do I choose the right solder paste for my application?

  6. When selecting a solder paste, consider factors such as the solder alloy composition, particle size, flux type, and compatibility with the components and PCB materials. Consult with the solder paste manufacturer or supplier for recommendations based on your specific application requirements.

  7. What is the shelf life of solder paste?

  8. The shelf life of solder paste varies depending on the specific product and storage conditions. Typically, solder paste has a shelf life of 6 to 12 months when stored at the recommended temperature (2°C to 10°C). Always check the manufacturer’s guidelines and expiration date on the solder paste container.

  9. How do I clean solder paste residue from PCBs?

  10. The cleaning method for solder paste residue depends on the type of flux used. Water-soluble flux can be cleaned using deionized water and a mild detergent. Rosin-based flux requires the use of solvents, such as isopropyl alcohol or specialized Flux Removers. Follow the manufacturer’s recommendations for the specific solder paste and flux type.


Solder paste is a critical component in the electronics manufacturing process, enabling the reliable and efficient assembly of PCBs. Understanding the properties, application methods, and best practices for handling solder paste is essential for achieving high-quality solder joints and minimizing defects.

By selecting the appropriate solder paste composition, optimizing the printing process, and controlling the reflow profile, manufacturers can ensure the success of their PCB assembly operations. Regular monitoring and troubleshooting of solder paste defects, along with proper handling and storage practices, further contribute to the overall quality and reliability of the assembled products.

As technology advances and new materials and processes emerge, staying informed about the latest developments in solder paste technology is crucial for staying competitive in the fast-paced world of electronics manufacturing.