What is solder paste and how to print it on PCBs in SMT assembly

Posted by

Introduction to Solder Paste Printing

Solder paste printing is a crucial step in the Surface mount technology (SMT) assembly process. It involves depositing a precise amount of solder paste onto specific areas of a printed circuit board (PCB) where electronic components will be placed. The accuracy and consistency of solder paste printing greatly impact the quality and reliability of the final assembled PCB.

What is Solder Paste?

Solder paste is a mixture of tiny solder particles suspended in a flux medium. The solder particles are typically made of a tin-lead (SnPb) or lead-free (e.g., SAC305) alloy, while the flux helps to remove oxides from the metal surfaces and improves the wetting and bonding of the solder to the PCB Pads and component leads.

The composition of solder paste can vary depending on the specific application and requirements. Some common types of solder paste include:

Type Alloy Composition Melting Point Features
SnPb 63% Sn, 37% Pb 183°C Good wetting, low cost
SAC305 96.5% Sn, 3% Ag, 0.5% Cu 217-220°C Lead-free, good mechanical strength
SN100C 99.3% Sn, 0.7% Cu, Ni 227°C Lead-free, good wetting, low cost

Solder Paste Properties

To achieve optimal printing and assembly results, solder paste must have the following properties:

  1. Viscosity: The paste should have a consistent viscosity that allows it to be easily dispensed through the stencil apertures while maintaining its shape on the PCB pads.
  2. Tackiness: The paste should be sticky enough to hold the components in place during the placement process.
  3. Slump resistance: The printed paste should maintain its shape and not spread or slump, especially during the time between printing and component placement.
  4. Flux activity: The flux in the paste should effectively remove oxides from the metal surfaces and promote good solder wetting and bonding.

Solder Paste Printing Process

Stencil Design

The first step in solder paste printing is designing a stencil that matches the PCB layout. The stencil is typically made of stainless steel and has apertures (openings) that correspond to the pads on the PCB where solder paste needs to be deposited.

Factors to consider when designing a stencil include:

  • Aperture size and shape
  • Stencil thickness
  • Aperture wall smoothness
  • Fiducial marks for alignment

Stencil Printing Machine Setup

A solder paste printing machine, also known as a screen printer, is used to apply the solder paste to the PCB. The machine consists of the following main components:

  1. Screen holder: Holds the stencil in place above the PCB.
  2. PCB support: Supports the PCB and provides a stable surface for printing.
  3. Squeegee: A blade that moves across the stencil to force the solder paste through the apertures and onto the PCB pads.
  4. Vision system: Cameras that help align the stencil with the PCB and inspect the printed solder paste.

To set up the printing machine, the following steps are typically followed:

  1. Mount the stencil onto the screen holder and secure it in place.
  2. Place the PCB on the support and align it with the stencil using the vision system and fiducial marks.
  3. Adjust the squeegee pressure, speed, and angle according to the solder paste manufacturer’s recommendations and the specific PCB design.
  4. Dispense a bead of solder paste along one edge of the stencil.

Printing Process

Once the printing machine is set up, the actual printing process can begin:

  1. The squeegee blade moves across the stencil, forcing the solder paste through the apertures and onto the PCB pads.
  2. As the squeegee moves, it also collects the excess solder paste from the stencil surface.
  3. After printing, the stencil is lifted away from the PCB, leaving the printed solder paste on the pads.
  4. The PCB is then removed from the support and inspected for print quality.

Print Inspection

After printing, it is essential to inspect the PCB to ensure that the solder paste has been deposited correctly. This can be done visually or using automated solder paste inspection (SPI) systems.

Visual inspection involves checking for the following defects:

  • Insufficient or excessive solder paste
  • Bridging between pads
  • Solder paste smearing or slumping
  • Misaligned or missing prints

SPI systems use cameras and image processing algorithms to assess the print quality more accurately and consistently. They can measure the volume, height, and area of the printed solder paste and compare them to predefined tolerances.

Factors Affecting Solder Paste Printing Quality

Several factors can influence the quality of solder paste printing, including:

  1. Stencil design: Aperture size, shape, and wall smoothness can affect the amount and distribution of solder paste deposited on the pads.
  2. Solder paste properties: The viscosity, tackiness, and slump resistance of the solder paste must be suitable for the specific PCB design and printing process.
  3. Printing parameters: Squeegee pressure, speed, and angle, as well as the separation speed between the stencil and PCB, can impact the print quality.
  4. Environmental conditions: Temperature and humidity can affect the solder paste’s rheological properties and the printing process’s stability.

To optimize the solder paste printing process, it is essential to carefully control these factors and regularly monitor the print quality using visual inspection and SPI systems.

Troubleshooting Common Solder Paste Printing Defects

Despite best efforts to control the printing process, defects can still occur. Some common solder paste printing defects and their possible causes include:

  1. Insufficient solder paste
  2. Clogged stencil apertures
  3. Low squeegee pressure
  4. Incorrect solder paste viscosity
  5. Excessive solder paste
  6. Oversized stencil apertures
  7. High squeegee pressure
  8. Slow separation speed between stencil and PCB
  9. Bridging between pads
  10. Excessive solder paste
  11. Incorrect stencil aperture design
  12. Poor stencil-to-PCB alignment
  13. Solder Paste Slumping
  14. Insufficient paste tackiness or slump resistance
  15. High ambient temperature
  16. Prolonged time between printing and component placement

To troubleshoot these defects, consider the following actions:

  • Check and clean the stencil apertures regularly
  • Adjust the printing parameters (pressure, speed, angle) according to the paste manufacturer’s recommendations
  • Verify the solder paste properties and storage conditions
  • Optimize the stencil design for the specific PCB layout
  • Control the environmental conditions (temperature, humidity) in the printing area
  • Minimize the time between printing and component placement

Best Practices for Solder Paste Printing

To achieve consistent, high-quality solder paste printing, follow these best practices:

  1. Select the appropriate solder paste: Choose a solder paste with the right alloy composition, particle size, and rheological properties for your specific application.
  2. Design an optimal stencil: Work with your stencil supplier to create a stencil with the appropriate aperture sizes, shapes, and thickness for your PCB layout.
  3. Control the printing environment: Maintain a stable temperature and humidity in the printing area to ensure consistent solder paste performance.
  4. Set up the printing machine properly: Adjust the squeegee pressure, speed, and angle according to the paste manufacturer’s recommendations and the specific PCB design.
  5. Perform regular maintenance: Clean the stencil, squeegee blades, and other printing equipment regularly to prevent contamination and ensure consistent print quality.
  6. Monitor the print quality: Use visual inspection and SPI systems to regularly assess the solder paste print quality and identify any defects or trends.
  7. Train operators: Provide adequate training to the operators responsible for solder paste printing to ensure they follow the correct procedures and can troubleshoot common issues.

By implementing these best practices, you can minimize solder paste printing defects, improve the overall quality of your SMT assembly process, and increase the reliability of your final products.


Solder paste printing is a critical step in the SMT Assembly process that directly impacts the quality and reliability of the final assembLED PCB. By understanding the properties of solder paste, designing an optimal stencil, setting up the printing machine properly, and regularly monitoring the print quality, you can achieve consistent, high-quality solder paste deposits on your PCBs.

Troubleshooting common printing defects and implementing best practices can further optimize your solder paste printing process, reducing rework and improving overall production efficiency.

Frequently Asked Questions (FAQ)

  1. What is the shelf life of solder paste?
    Solder paste typically has a shelf life of 6 to 12 months when stored under refrigerated conditions (2-10°C). Always check the manufacturer’s recommendations for specific storage requirements and shelf life information.

  2. How often should I clean my stencil?
    Stencils should be cleaned regularly to prevent solder paste buildup and ensure consistent print quality. The frequency of cleaning depends on the volume of production and the specific solder paste being used. As a general guideline, clean the stencil every 4-8 hours of continuous printing or whenever print quality issues are observed.

  3. Can I reuse solder paste?
    It is generally not recommended to reuse solder paste that has been removed from the stencil or PCB. The solder paste may have been contaminated or exposed to environmental conditions that can degrade its properties. Always use fresh solder paste for each printing session.

  4. What is the optimal stencil thickness for solder paste printing?
    The optimal stencil thickness depends on the specific PCB design and component sizes. As a general rule, the stencil thickness should be 80-90% of the smallest component lead or termination height. For example, if the smallest component has a 0.5mm lead height, the stencil thickness should be around 0.4-0.45mm.

  5. How do I store unused solder paste?
    Unused solder paste should be stored in an airtight container under refrigerated conditions (2-10°C) to prevent drying out and contamination. Before using refrigerated solder paste, allow it to reach room temperature and mix it thoroughly to ensure consistent properties. Always follow the manufacturer’s storage and handling recommendations.