Process Optimization

1. Understanding the Different Roles of SPI and AOI

Before optimization, it is critical to understand what SPI and AOI are best at detecting.

SPI excels at:

• Solder paste volume and height

• Insufficient or excessive paste

• Printing alignment issues

• Early process deviation detection

AOI excels at:

• Component presence and polarity

• Placement offset and rotation

• Solder joint shape and wetting

• Post-reflow assembly defects

Optimizing both systems to detect the same issues leads to redundancy and instability.

2. The Common Mistake: Overlapping Inspection Responsibility

A frequent optimization error is:

• SPI checking too many cosmetic details

• AOI over-analyzing solder defects already controlled by SPI

This overlap increases:

• Inspection time

• False calls

• Operator review workload

Effective optimization starts with clear inspection responsibility separation.

3. Define a Clear SPI–AOI Inspection Boundary

A joint strategy assigns responsibility deliberately:

Clear boundaries reduce conflict between inspection systems.

4. Use SPI as a Process Control Tool, Not a Defect Gate

SPI should focus on:

• Trend monitoring

• Early warning of printing drift

• Process stability indicators

Avoid using SPI as a hard pass/fail gate for cosmetic variation, which creates unnecessary NG boards.

5. Optimize AOI Based on SPI Feedback

SPI data can be used to:

• Dynamically adjust AOI tolerance

• Identify areas where AOI false calls can be relaxed

• Detect real process drift vs normal variation

AOI should react intelligently to SPI trends, not operate blindly.

6. Reduce AOI False Calls Through SPI-Aware Programming

If SPI confirms stable paste conditions:

• Relax AOI solder joint thresholds

• Reduce false calls caused by acceptable variation

• Focus AOI resources on placement-related defects

This coordination significantly improves AOI throughput.

7. Align SPI and AOI Cycle Time with Line Takt

Joint optimization must consider takt time:

• SPI too slow → early bottleneck

• AOI too strict → downstream blocking

Balancing both inspection speeds prevents accumulation and line stoppage.

8. Plan Recovery and Review Flow Together

SPI and AOI recovery logic should be aligned:

• Clear rules for when boards stop the line

• Defined escalation paths

• Offline review for ambiguous cases

A shared recovery strategy reduces confusion and downtime.

9. Inline vs Offline Strategy in Joint Optimization

Inline SPI + AOI works best when:

• Product mix is stable

• Inspection programs are mature

Offline or hybrid strategies are better when:

• False call rates are high

• Engineering validation is ongoing

• Line stability is at risk

Joint optimization includes choosing the right inspection mode.

10. KPIs to Monitor in SPI–AOI Joint Optimization

Key indicators include:

• SPI trend deviation rate

• AOI false call rate

• Combined inspection cycle time

• Line blocking incidents

• First-pass yield (FPY)

Tracking both systems together reveals real improvement.

Conclusion

SPI and AOI should not compete—they should collaborate.

A joint optimization strategy:

• Reduces false calls

• Improves inspection efficiency

• Stabilizes SMT line flow

• Increases overall yield and ROI

True inspection optimization happens at the system level, not machine by machine.

Linking Suggested reading:

• AOI False Call Optimization

• Inspection and Buffering Strategy

• SMT Line Blocking and Bottlenecks

• When Offline Solutions Are Better Than Inline

Thomao Engineering Insight

SPI controls the process.
AOI protects the result.
Optimization connects both.