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1.6 SPI Metrics, Cp/Cpk, and Closed-Loop Rules

Solder Paste Inspection (SPI) is not just a "reject gate" to catch bad boards; it is a process control tool that tells you exactly how your printer is behaving in real-time. If you only use SPI to sort Pass/Fail, you are wasting 90% of its value.

The goal of this chapter is to convert SPI data into immediate engineering actions. We focus here on line control—interpreting the live dashboard to stabilize the printing process before defects occur.

The Vital Metrics: Volume is King

While SPI machines measure many parameters, only three drive process decisions. Ignore the noise and focus on these.

Metric

Definition

The Engineering Consequence

Target Window

Volume (%)

The total amount of paste deposited relative to the stencil aperture volume.

The single most critical metric. Volume correlates directly to joint reliability.

Low Volume = Opens/Weak Joints.

High Volume = Shorts/Solder Balls.

90% – 115% (Standard)

80% – 120% (Limit)

Offset (X/Y)

How far the deposit is shifted from the pad center.

Indicates alignment drift or board stretch. If offset exceeds 25% of the pad width, the risk of bridging or tombstoning spikes.

± 0 µm (Target)

< 30 µm (Warning)

Height (µm)

The peak height of the deposit.

Used to detect "dog ears" (peaks) or "scooping" (dishing). Important for fine-pitch consistency.

Stencil Thickness ± 10%

Pro-Tip: Do not obsess over Area %. A print can have perfect Area (coverage) but be totally insufficient in Volume (too thin). Always control by Volume.

Interpreting Cpk on the SMT Line

You do not need to be a Six Sigma Black Belt to use Cpk on the shop floor. Treat it as your Stability Score.

  • Cp (Potential Capability): "Is the machine precise enough?"
    • If Cp is low (wide spread), your process is shaky. Check squeegee pressure, paste viscosity, or board support.
  • Cpk (Real Capability): "Is the machine centered and precise?"
    • If Cp is high but Cpk is low, your process is stable but off-target. You likely just need a stencil alignment or offset adjustment.

The Rule of 1.33:

  • Cpk > 1.66: Process is Bulletproof. Do not touch it.
  • 1.33 < Cpk < 1.66: Process is Healthy. Monitor trends.
  • Cpk < 1.33: Process is Marginal. Risk of defects is high. Stop and adjustment required.
  • Cpk < 1.0: Process is Broken. You are producing scrap. Immediate Line Stop.

Closed-Loop Rules: Printer ↔ SPI Feedback

Modern SPI machines can talk to the printer to auto-correct drift. Enable this feature, but set strict limits to prevent the machine from "chasing its tail."

1. The Auto-Cleaning Trigger

Do not clean based on a fixed counter (e.g., "Every 3 boards"). Clean based on Transfer Efficiency.

  • Rule: If average Volume drops by > 5% over the last 3 boards → Trigger Wipe (Wet-Vac-Dry).
  • Why: This cleans the stencil exactly when the apertures start to clog, maximizing cycle time without risking quality.

2. The Alignment Auto-Correction

  • Rule: If X or Y offset drifts > 15 µm consistently for 3 boards → Send Offset Correction to Printer.
  • Limit: Cap the max correction at 50 µm. If the drift is larger, it indicates a board clamping failure or a stretched stencil, which software cannot fix. Alarm and stop.

Root Cause Mapping: From SPI Data to Fix

When the SPI screams, use this logic to find the physical knob to turn.

Scenario A: Volume is Consistently LOW (< 80%)

  • Diagnosis: Apertures are clogging or not filling.
  • Action 1: Check Paste Roll. Is it < 15mm diameter? Add paste.
  • Action 2: Check Squeegee Speed. If too fast (> 80mm/s), paste slides. Slow down.
  • Action 3: Check Separation Speed. If too fast, paste stays in aperture. Slow down.

Scenario B: Volume is Consistently HIGH (> 125%)

  • Diagnosis: Paste is leaking under the stencil (Gasket failure).
  • Action 1: Check Board Support. Is the board bowing down?
  • Action 2: Check Clamping. Is the stencil tight against the PCB?
  • Action 3: Check Under-Stencil Cleaning. Is the stencil smeared?

Scenario C: Volume is High on Center, Low on Edges

  • Diagnosis: Squeegee Blade Deformation.
  • Action: The blade is bowing under pressure. Reduce Squeegee Pressure or replace the blade assembly.

Final Checklist

Parameter

Setting / Rule

Volume Lower Limit (LSL)

75% - 80% (Depending on pitch)

Volume Upper Limit (USL)

125% - 130%

Offset Limit

< 25% of pad width

Bridge Threshold

150 µm (Detects smear between pads)

Critical Stop Rule

3 Consecutive Failures on the same reference designator.

Closed Loop Cleaning

ON (Trigger on Volume trend).

Closed Loop Alignment

ON (Limit max correction to 50 µm).


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