solder paste inspection

**Solder paste inspection** is the **inline metrology process that measures deposited paste volume, area, and height before component placement** - it is a primary early-warning control for preventing downstream solder-joint defects. **What Is Solder paste inspection?** - **Definition**: SPI uses 2D or 3D optical systems to evaluate each printed pad against limits. - **Key Metrics**: Typical checks include volume percentage, height distribution, area coverage, and offset. - **Placement in Flow**: SPI runs after stencil printing and before pick-and-place operations. - **Control Role**: Results feed printer corrections and immediate containment for print excursions. **Why Solder paste inspection Matters** - **Defect Prevention**: Catches overprint and underprint before expensive downstream processing. - **Yield Improvement**: Strong SPI control reduces bridge, open, and tombstoning defects. - **Process Visibility**: Pad-level measurements reveal stencil wear and printer drift quickly. - **Cost Reduction**: Pre-reflow correction is far cheaper than post-reflow rework. - **Capability Tracking**: SPI trends provide direct evidence of print-process Cpk health. **How It Is Used in Practice** - **Threshold Design**: Set package-specific upper and lower limits for each critical pad class. - **Closed Loop**: Enable automatic printer offset and cleaning responses from SPI feedback. - **Data Governance**: Use historical SPI analytics to tune stencil and paste maintenance intervals. Solder paste inspection is **a core front-end quality gate in SMT assembly** - solder paste inspection is most effective when coupled to rapid corrective action, not used only as a reporting tool.