The primary difference between SPI (Solder Paste Inspection) and AOI (Automated Optical Inspection) lies in what they inspect and when they are used during the SMT (Surface Mount Technology) assembly process.
In essence, SPI focuses on the quality of the solder paste print before components are placed, while AOI inspects the placement of components and the quality of solder joints after components are placed and soldered. As the provided reference states, "AOI is mainly used to detect assembly quality and solder connection quality, and SPI is mainly used to detect solder paste printing quality."
Understanding SPI (Solder Paste Inspection)
SPI systems are critical for ensuring the foundation of a good solder joint: the solder paste itself. This inspection happens immediately after the solder paste is printed onto the bare printed circuit board (PCB) pads, but before any components are placed.
What SPI Checks:
- Volume: Is the correct amount of solder paste deposited on each pad?
- Area: Is the footprint of the solder paste correct?
- Height: Is the thickness of the solder paste deposit within tolerance?
- Shape: Is the paste blob correctly formed without bridging or smearing?
- Registration/Alignment: Is the solder paste accurately aligned with the pads on the PCB?
Why SPI is Important:
Checking solder paste quality early is crucial because over 60-70% of SMT defects can be traced back to issues with the solder paste printing process. Catching these problems at this stage prevents wasted components and rework later in the assembly line.
Understanding AOI (Automated Optical Inspection)
AOI systems use cameras and lighting to automatically scan the PCB for various defects. Unlike SPI, AOI is typically performed after components have been placed (pre-reflow AOI) or, more commonly, after the soldering process is complete (post-reflow AOI).
What AOI Checks:
- Component Presence/Absence: Are all required components on the board?
- Component Placement: Are components correctly positioned on their pads?
- Component Polarity: Are polarized components (like diodes or electrolytic capacitors) oriented correctly?
- Solder Joint Quality (Post-Reflow):
- Bridging (unwanted solder connections between pads)
- Insufficient/Excessive Solder
- Lifted Leads or Components
- Opens (missing solder joints)
- Tombstoning (components standing on end)
Why AOI is Important:
AOI verifies the correct assembly and soldering of components. It's a versatile inspection method used at different stages to catch defects introduced during component placement and the soldering process.
Key Differences Summarized
Here is a comparison table highlighting the main distinctions between SPI and AOI:
| Feature | SPI (Solder Paste Inspection) | AOI (Automated Optical Inspection) |
|---|---|---|
| Primary Focus | Solder Paste Printing Quality | Component Placement & Solder Joint Quality |
| Inspection Stage | After Solder Paste Printing (Before Component Placement) | After Component Placement & Soldering (Typically) |
| What is Checked | Solder paste volume, height, area, shape, registration | Component presence, placement, polarity; solder joint quality |
| Main Defects | Insufficient/Excessive Paste, Paste Misalignment, Bridging Paste, Paste Shape issues | Missing/Misplaced Components, Wrong Polarity, Solder Bridges, Opens, Insufficient/Excessive Solder |
| Purpose | Ensure proper solder paste deposit for reliable connections | Verify correct assembly and successful soldering |
In summary, while both are optical inspection technologies crucial for SMT quality control, SPI is an upstream process focused purely on the solder paste, whereas AOI is a downstream process examining the assembled board and solder connections.
