What is Solder Paste Inspection (SPI)?

Key Takeaways

• - Approximately 70% of SMT defects originate from the solder paste printing stage—making Solder Paste Inspection (SPI) the critical first line of defense in PCB assembly quality control.
• - 3D SPI technology significantly outperforms traditional 2D methods: it doesn’t just check coverage and X/Y offset—it accurately measures solder paste volume, height, and 3D shape, which is essential for high-density components like BGAs and ultra-miniature 01005 packages.
• - NextPCB implements advanced 3D SPI with a closed-loop feedback system, enabling real-time automatic adjustments to the printer. This ensures consistent print quality and maximizes First Pass Yield (FPY).
• - SPI does not replace AOI—they are complementary: SPI validates the “foundation” (solder paste), while post-reflow AOI inspects the final “structure” (component placement, solder joints, shorts, etc.).
• - Advanced 3D SPI supports warped or flexible PCBs: using warpage compensation algorithms, NextPCB’s systems map the actual board surface to ensure accurate height and volume measurements—even on non-flat substrates.

Whether you're a hardware engineer, startup founder, or high-volume manufacturer, understanding how SPI prevents defects, reduces rework costs, and enhances long-term reliability is essential for successful PCB assembly.

1. Table of Contents
2. What is SPI? The Core Quality Control Point of the SMT Line
3. Comparison of 2D vs. 3D SPI Technology
4. The 5 Key Defects SPI Detects
5. How NextPCB Enhances Manufacturing Reliability via SPI
6. Frequently Asked Questions (FAQ) - Latest Insights
7. Conclusion

Introduction

In the field of electronics manufacturing, prototype failure often results in high costs in terms of both time and money. While engineers sometimes blame soldering issues on chip failures or improper reflow temperature profiles, industry data reveals a critical fact: approximately 70% of PCB assembly defects originate from the very first step of the SMT process—solder paste printing.

For this reason, Solder Paste Inspection (SPI) has become a crucial link in ensuring manufacturing quality. At NextPCB, we insist on process transparency to build trust with our customers. This guide will analyze the core functions of SPI in detail, the necessity of 3D technology compared to traditional 2D technology, and how NextPCB utilizes this process to safeguard your project from hidden defects.

What is SPI? The Core Quality Control Point of the SMT Line

Solder Paste Inspection (SPI) is an automated process in the SMT (Surface Mount Technology) assembly line used to monitor the quality of solder paste deposited on PCB pads in real-time.

In an SMT production line, the process flow typically follows this sequence:

1. - Solder Paste Printing
2. - SPI (Critical Inspection Point)
3. - Pick and Place (Component Mounting)
4. - Reflow Soldering
5. - AOI (Automated Optical Inspection)

SPI is located after printing and before component placement. Specialized machines scan the printed solder paste to evaluate its geometric properties, ensuring that the volume and position of the solder paste are precise. If defects generated during the printing stage (such as insufficient solder) are not intercepted by SPI, components will be placed on an incorrect base and solidified through reflow soldering. Fixing this at a later stage requires heating, desoldering, and cleaning. This rework process is not only costly but may also damage the reliability of the circuit board.

By intercepting errors in real-time before component placement, the production line can simply wash and reprint the affected boards, thereby eliminating mass quality issues at the source.

>> Recommend reading: How to Create a Centroid (Pick and Place) File Instantly?

Comparison of 2D vs. 3D SPI Technology

As electronic components evolve towards miniaturization (such as 0201 and 01005 packages) and high density (such as HDI and BGA), traditional inspection methods can no longer meet accuracy requirements.

The Limitations of 2D SPI

Early 2D SPI systems used a top-down camera perspective. They primarily measured the coverage area and X/Y axis offset of the solder paste. However, 2D inspection cannot acquire height data. In actual production, even if the solder paste covers the entire pad, if the thickness is extremely thin (insufficient volume), 2D inspection will still judge it as a "Pass." However, this is highly likely to form a weak solder joint or an open circuit after reflow.

3D SPI: The NextPCB Technical Standard

To eliminate this blind spot, NextPCB fully adopts 3D SPI technology in our production lines. Using Phase Shift Profilometry (Moiré patterns) or laser triangulation, 3D systems can reconstruct a precise three-dimensional model of the solder paste.

Its inspection dimensions include:

• Volume: The most critical indicator for evaluating solder joint reliability.
• Height: Preventing solder paste that is too thick (risk of bridging/short circuits) or too thin (risk of insufficient strength).
• Geometric Shape: Identifying abnormal shapes such as peaks, slumps, or "dog ears" to prevent tombstoning.

For BGA (Ball Grid Array) packages, the solder joints are hidden underneath the component and are difficult to observe directly after reflow. Therefore, using 3D SPI to ensure the initial solder paste volume meets specifications is the only effective preventive measure to guarantee BGA soldering reliability.

The 5 Key Defects SPI Detects

According to the IPC-7527 (Requirements for Solder Paste Printing) standard, SPI primarily monitors the following failure modes:

1. 1) Insufficient Solder: The solder paste volume is below the set threshold (e.g., less than 70% of the aperture volume). This is usually caused by clogged stencil apertures or insufficient squeegee pressure, leading to insufficient mechanical strength or electrical open circuits.
2. 2) Excessive Solder: Solder paste volume or height exceeds the standard. During reflow, excess solder will overflow to adjacent pads, causing short circuits (bridging).
3. 3) Bridging: Solder paste on adjacent pads connects before soldering. This is typically caused by excessive printing pressure or incomplete stencil cleaning.
4. 4) Offset: The printed position of the solder paste deviates from the center of the pad. Severe offset causes uneven forces on the component during soldering, resulting in tombstoning or shifting.
5. 5) Shape Deformity: The solder paste does not present a normal "brick" shape but appears as peaks or irregular shapes, which affects wetting during the reflow process.

How NextPCB Enhances Manufacturing Reliability via SPI

NextPCB treats SPI not just as a means to filter defects, but as a data source for optimizing production processes:

Closed-Loop Feedback System

Our 3D SPI machines are interconnected with fully automated solder paste printers. When the SPI detects a minor trending offset in printing position (which has not yet exceeded the standard but shows signs of deterioration), the system automatically feeds compensation data back to the printer. This realizes real-time correction of printing parameters, preventing defects before they occur.

Quality Support for High-Precision Designs

For complex projects utilizing 0.4mm pitch, HDI technology, or micro-via designs, the tolerance for solder paste volume is extremely low. 3D SPI can precisely control micron-level deposition, ensuring high-yield production for difficult, high-precision electronic products at NextPCB.

Reducing Comprehensive Costs for Customers

By intercepting problems at the front end of the process, we significantly reduce the need for later rework. Boards that have not undergone rework possess better thermal stability and electrical integrity. The application of SPI allows customers to obtain high-quality hardware that meets "First Pass Yield" standards in a shorter cycle.

Frequently Asked Questions (FAQ) - Latest Insights

Conclusion

Solder Paste Inspection is not a simple visual check, but a quality assurance tool based on volumetric measurement. As circuit board design density continues to increase, the era of relying on manual or 2D inspection has passed.

By applying advanced 3D SPI technology, NextPCB ensures that the foundational step of PCBA manufacturing—solder paste printing—achieves excellence. This allows designers to focus their energy on product innovation without worrying about hidden failures in the manufacturing process.

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About the Author

Julia Wu - Senior Sales Engineer at NextPCB.com

With over 10 years of experience in the PCB industry, Julia has developed a strong technical and sales expertise. As a technical sales professional, she specializes in understanding customer needs and delivering tailored PCB solutions that drive efficiency and innovation. Julia works closely with both engineering teams and clients to ensure high-quality product development and seamless communication, helping businesses navigate the complexities of PCB design and manufacturing. Julia is dedicated to offering exceptional service and building lasting relationships in the electronics sector, ensuring that each project exceeds customer expectations.

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