SMT CENTER

Introduction

SPI is often misunderstood as a defect inspection machine.
In reality, SPI is the earliest and most powerful process control tool in SMT manufacturing.

When SPI is optimized correctly, many downstream AOI defects never occur.

This page explains how to design SPI optimization as a process-stability system, not a rejection gate.

Content Body

1. The True Role of SPI

SPI should:

• Monitor solder paste volume, height, and area

• Detect early printing drift

• Provide quantitative process data

SPI should NOT:

• Reject boards for cosmetic variation

• Duplicate AOI responsibility

2. Common SPI Optimization Mistakes

• Over-tight volume thresholds

• Treating SPI as pass/fail inspection

• Ignoring Cp / Cpk trends

These mistakes push problems downstream instead of fixing them.

3. SPI Parameter Optimization Strategy

• Use statistical limits, not visual judgment

• Optimize per pad type and component size

• Focus on trend deviation, not single outliers

4. SPI as the Foundation of AOI Optimization

Stable SPI data allows:

• AOI tolerance relaxation

• False call reduction

• Higher inspection throughput

SPI stability = AOI freedom.

5. KPIs for SPI Optimization

• Cp / Cpk

• Trend deviation rate

• Print-related defect escape rate

Suggested Linking

→ AOI Optimization
→ SPI Data-Driven AOI Adjustment
→ Inspection Optimization Pillar