# SPI Machine Guide: Solder Paste Inspection Explained

An SPI machine inspects solder paste after printing and before placement. A 3D system typically measures deposit height, area, volume and offset. Its value is early process control: defects can be found before components and reflow make correction more expensive.

Measurements and Meaning

JUKI describes SPI as measuring area, height, quantity and misalignment with multi-direction lighting and 3D sensing. See JUKI process overview.

Limits Need Process Evidence

Pad geometry, aperture, paste and package risk affect what is acceptable. Establish a stable print, correlate measurements with post-reflow results, define critical features, review false calls and escapes, and control program changes.

SPI can send corrections or cleaning actions to a printer and offset information to placement on supported lines. JUKI provides examples at automation solutions. Safeguards are required so a bad reference does not cause repeated wrong correction.

Selection Checklist

Compare measurable feature size, board range, repeatability, coverage, cycle time, programming, review workflow, traceability, data export and printer/mounter interfaces. Challenge the system with representative boards and known defects.

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Does SPI replace AOI?

No. SPI evaluates paste before placement; AOI evaluates visible component and solder features later.

Is volume the only useful metric?

No. Height, area, offset, shape and trends help explain causes and risks.