4.5 SPC & Dashboards

Statistical Process Control (SPC) is the operational discipline that converts raw machine data into predictive intelligence. By continuously charting the vital signs of the SMT line, SPC enables early, small corrections to process drift before defects ever occur. Dashboards are the visual interface for SPC, providing real-time guidance to operators and engineers. The success of an SPC system is measured by its simplicity and the immediate, effective action it compels when a metric drifts.

4.5.1 SPC Fundamentals in Production

SPC involves monitoring key performance indicators (KPIs) to distinguish common cause variation (inherent, acceptable process noise) from special cause variation (a sudden, specific problem like a clogged nozzle or broken sensor).

• Process Control: SPC is about controlling the mean (µ) and the variation (σ) of critical parameters like Solder Paste Transfer Efficiency (TE).
• The Goal: The aim is to maintain processes in a state of statistical control, ensuring that the process is consistently capable (C_pk ≥ 1.33) of meeting specifications (Chapter 4.1).
• Visual Control: Simple charts must be prominently displayed so operators can read the process status at a glance—like a fuel gauge or a thermometer.

4.5.2 Key Process Metrics (By Station)

Effective SPC requires tracking the "few, vital" parameters that predict downstream failure, rather than monitoring everything.

Station	Key Metric	Indicator of...	Response Triggered
Printer (SPI)	TE Cpk by Feature Family	Predicts bridges, tombstones, HIP risk.	Immediate cleaning cycle or squeegee pressure adjustment.
	Wipes/Board Trend	Stencil/paste hygiene drift (Chapter 1.5).	Adjust cleaning interval; inspect stencil nano-coating.
PnP	Starvation Minutes/Hour	Feeder management discipline (Chapter 2.3).	Load spare feeder; adjust splice alarm threshold.
	Miss/Retry Rate by Feeder	Bad tape, worn nozzle, or vision instability.	Fix peel path; quarantine faulty component reel/feeder.
Reflow	TAL / Peak Mean	Solder wetting completeness and thermal margin.	Small belt speed or zone temperature nudge (Chapter 3.2).
	Cross-Board ∆T	Convection uniformity and thermal profile stability.	Adjust blower speed or soak zone setpoint (Chapter 3.1).
Inspection (AOI/AXI)	False Calls/Board (by RefDes)	Programming noise and manual verification costs (OpEx).	Programmer review and tuning of specific lighting/ROIs (Chapter 4.4).

4.5.3 Dashboards and Alarm Tiers

Effective dashboards use visual alerts to enforce a standardized response protocol. Every deviation must be tied to a documented action (the "if this, then that" playbook).

A) Visualization

• Control Charts: The I-MR (Individuals and Moving Range) chart is standard for tracking continuous data like SPI TE and Reflow TAL/Peak, showing the process mean (µ) and the variation (σ) over time.
• Traffic Lights: Green/Yellow/Red status indicators clearly communicate the health of each parameter.

B) Alarm Tiers

Alarms must be tiered to prevent nuisance alerts from overwhelming the operator, ensuring only critical issues halt production.

Tier	Condition	MES Response	Management Action
Green	All metrics within 3σ.	Continuous flow.	No action required.
Yellow (Warning)	2 of 3 consecutive data points outside 2σ or a run of 6 points trending up/down.	Log warning, send alert to supervisor, recommend first correction (e.g., "Wipe Stencil Now").	Supervisor enforces immediate corrective action.
Red (Stop)	Any data point exceeds the Specification Limit (USL/LSL) or a sustained violation of the Cpk floor.	Hard stop of the line, mandatory engineering sign-off to resume.	Open Non-Conformance Report (NCR) and conduct root cause analysis.

4.5.4 Closing the Loop (The Corrective Cycle)

The primary function of the SPC system is to enable the closed-loop feedback that drives continuous process improvement.

1. Detection: An alarm tier (Yellow or Red) is triggered by a metric drift (e.g., SPI TE µ drops 5 points).
2. Correction: The operator executes the predefined First Correction (e.g., increase squeegee speed slightly). This must be a simple, single-parameter fix.
3. Annotation: The action taken is automatically logged against the time/product on the control chart, creating a vertical line labeled "Process Change." This logging is critical for auditing the process.
4. Verification: The system monitors the next 3–5 panels to verify the correction successfully brought the mean back toward the center without increasing the variation.
5. Engineering Fix: If repetitive corrections fail, the issue is deemed Special Cause (e.g., worn stencil, faulty feeder) and requires Engineering/Quality approval to implement a permanent solution, which updates the Golden Recipe.

Final Checklist: SPC Implementation

Requirement	Control Point	Management Focus
Metrics	Select and track the fewest possible predictive metrics (TE Cpk, TAL/Peak, Miss Rate).	Prevents data overload; ensures focus on high-risk areas.
Action Plan	Every Yellow/Red alarm tier must be tied to a documented First Correction playbook.	Eliminates human guessing and wasted time during drift events.
Data Integrity	All machines must sync to a single time base; every touch and recipe change must be auto-logged and annotated on charts.	Ensures data is trustworthy for root cause analysis.
Visualization	Dashboards use simple control charts and traffic lights that clearly show the control limits.	Promotes immediate, effective operator response.