MENTO Builds Three-Gate SMT Inspection Lines: 3D SPI, Pre-Reflow AOI and Post-Reflow 2D/3D AOI

Three Chances to Catch a Defect — and Only One Cost Curve

In SMT production the cost of a defect climbs with every metre of conveyor it travels. A paste deposit that is 30% short costs cents to fix right after printing; after reflow it means rework; at functional test it means scrapping a board that already carries hundreds of components. MENTO's SMT line architecture is built around that arithmetic: three optical gates, each placed where the defect of the previous stage is still cheap to correct.

The layout reads left to right: solder paste printer → SPI → pick-and-place → pre-reflow 2D/3D AOI → reflow oven → post-reflow 2D/3D AOI → board collection machine. Everything between the printer and the unloader is measured at the moment it can still be influenced.

Gate 1: SPI Immediately After the Printer

Solder paste is the single largest source of SMT defects and the only one that can be measured volumetrically before any component is committed. The SPI station sits directly behind the printer and measures height, volume, area and offset for every pad.

On the K32 series, dual DLP structured light with phase-shift technology and a red/green/blue light source reconstructs the paste deposit in 3D, while a 5 MP colour industrial camera provides the 2D reference image. Representative specification:

Bridging, insufficient paste and miss-print are reported within the same print cycle, making closed-loop paste control practical rather than theoretical.

Gate 2: Pre-Reflow 2D/3D AOI After Placement

After the pick-and-place machine, the second gate checks what placement actually produced: presence, position, rotation, polarity and wrong part. Because the joints are still paste, 3D height data separates a component sitting on its pads from one perched on a paste ridge that will tombstone in the oven.

• Placement accuracy — X/Y offset and rotation measured against the CAD reference.
• Polarity and orientation — diodes, LEDs and electrolytic capacitors verified before they become unserviceable.
• Wrong or missing parts — feeder and nozzle failures caught inside the line, not at final test, including across high-mix changeovers.

Reflow: One Way Through

The reflow oven is not modified and its profile stays the customer's process recipe. What changes is the information the process engineer holds: boards entering are verified twice, boards leaving are measured against the same reference data set.

Gate 3: Post-Reflow 2D/3D AOI Before the Unloader

The final optical gate, placed between the oven and the board collection machine, judges the finished joint. This is where 3D measurement does the work that 2D alone cannot:

1. Insufficient solder, excess solder and non-wetting
2. Solder bridging and shorts
3. Lifted leads, poor fillets and void-like depressions
4. Component shift, tombstoning and billboarding
5. Missing or wrong components after reflow
6. Solder ball and splatter detection

MENTO's 3D AOI platforms use RGB LED illumination and structured-light height reconstruction, with inspection areas configurable up to 510×510 mm and automatic alignment for fast product changeover. Systems support SMEMA line communication, automatic PCB width adjustment and left-to-right or right-to-left conveyance, so they can be retrofitted into an existing line without changing the material flow. Units are produced under ISO 9000 and ISO 14001 management systems and comply with RoHS requirements.

One Data Layer Across Three Gates

The three stations are worth far more than the sum of their parts when their results are joined up:

• Defect attribution — a post-reflow bridge that also appeared as a pre-reflow paste bridge points at the printer, not the oven.
• Trend alarms — top-defect statistics per shift, per product and per feeder, with configurable alarm counts that can stop upstream equipment automatically.
• Full-board traceability — board images archived per serial number for audits and customer claims.
• MES/SPC docking — standard and customised data formats through a self-developed interface.

At a Glance

Conclusion

A single AOI at the end of the line tells you how many bad boards you made. Three measurement points tell you where, when and why — and let the line correct itself before the next panel is printed: measure the paste, measure the placement, measure the joint, and keep the three data sets in one place.