SPI Inspection: Way to test solder paste printing

As the trend towards miniaturization of electronic devices continues to advance, electronic assembly faces various challenges. In order to enhance the reliability and functionality of devices, it is crucial to implement a comprehensive quality control process. During the SMT assembly process, automated solder paste printing equipment is used to apply solder paste onto the surface of the PCB. However, insufficient or excessive application of solder paste can lead to a decrease in PCBA quality. This is where SPI comes into play. In this article, FS Technology will delve deeply into the significance of SPI inspection in your project and explain why it is essential to incorporate it into the SMT production line.

SMT Assembly & SPI Inspection

SPI in SMT Process

In PCB assembly, SMT accounts for as much as 70% of the process, which means that almost all PCBA boards in electronic products rely on this process. Manufacturers typically use automated processes to complete the SMT process. PCB bare boards are fixed to the surface of the PCB after tin paste printing, component mounting, and reflow soldering, with SPI being used as a critical tool for detecting the quality of surface-mounted components prior to reflow soldering. It should be noted that SPI inspection of PCBs is considered mandatory in electronic manufacturing, as it helps to identify faults early, avoid production delays, and reduce manufacturing costs. During operation, inspection equipment uses high-resolution cameras and light sources to detect quality issues such as component position, direction, offset, and skew. If any solder paste deposits are detected, they are marked, and the operator corrects the errors to achieve efficient SMT assembly.

Composition of SPI Detection System

  • Solder Paste: Mixture of flux and solder that is used to join components on a circuit board.
  • Calibration Standards: Ensure the equipment is calibrated to project specifications and test accuracy is maintained.
  • Inspection Equipment: Comprised of a three-dimensional scanning system and an AOI unit for capturing images of electronic components on the surface of the PCB.
  • Light Sources: Provide illumination of electronic components to ensure proper capture by cameras.
  • Software: Utilized for image processing and analysis to detect faults, collect process data and generate reports.
  • Docking Stations: Transferring PCBs to the inspection area for processing.
  • Racks: Mounting equipment such as cameras, light sources, delivery systems, etc.
  • Controller: Controls the operation of the system, including the movement of the delivery system and the camera.

The Necessity of SPI Inspection in PCB Assembly


  • Enhance PCBA Quality: Ensure accurate and consistent deposition of solder paste to improve project quality.
  • Time Efficiency: Early detection of defects during assembly saves time in the production process by reducing the need for rework.
  • Cost-effectiveness:  SPI inspection PCB is an added feature to our SMT services and is provided to customers free of charge. It is an essential quality control measure that helps identify errors before soldering, minimizing the need for costly desoldering and repairs that may damage components.
  • Improved Customer Satisfaction: High-quality circuit boards ensure customer satisfaction and enhance the reputation of your business.


  • Limited flexibility: Typically used to verify the quality of solder paste printing and may not be effective for other aspects, such as the inspection of large components or high density interconnection PCB.
  • Lack of standards: Currently, there is no industry standard for SPI inspection, making it difficult to compare results between different software and machines.
  • Time-consuming calibration: Accurate results require machine calibration, which can be a difficult and time-consuming process.
  • Potential for false positives: There is a risk of false positives due to the presence of debris or dust on the board that may be mistaken for a defect.
  • Solder paste: The inspection of solder paste can be challenging due to its small size and variability, as well as the inability to inspect components beneath the solder paste layer.

Solder Paste Inspection Considerations

Types of SPI Device

Currently, PCBA companies use two types of SPI testing equipment – 2D and 3D – each with its own advantages and characteristics. When selecting equipment, the design complexity requirements must be taken into consideration.

  • 2D SPI inspection: Mainly used to inspect the height and volume of solder paste on the surface of the PCB. It operates faster and is less expensive, making it an ideal choice for PCBA companies. However, its capabilities are limited and it cannot detect certain types of failures that are not visible on the surface.
  • 3D SPI inspection: Provides a more detailed and accurate view of solder paste deposition on the PCB. This process is complex and time-consuming, but it measures the three-dimensional height and volume of solder paste. Therefore, it offers a more comprehensive inspection that can detect a wider range of defects.

Ensure Operation

  • Accuracy: The accuracy of the system is crucial in ensuring that the solder paste is deposited on the board with precision. The system should be able to detect even small variations in solder paste volume and height.
  • Speed: The process must balance speed and quality to provide high-quality PCBA boards with fast turnaround time.
  • Cost: It’s important to note that not all PCBA companies offer SPI services for free, and this cost may need to be factored into the budget. If you’re looking for affordable PCB assembly, FS Technology is the best choice.
  • Compatibility: The detection system should be compatible with the assembly method used in the circuit design to achieve optimal efficiency.
  • Data analysis and support: After the test is completed, perform a thorough analysis of the test results to identify trends and improvements in the assembly process. Additionally, consider the level of support provided by the supplier, such as technical support, training, and maintenance.

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