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

SPI (Solder Paste Inspection)Inspection (SPI) is your 3D radar for printing quality. It doesn'tnot just a "reject gate" to catch defects;bad boards; it measuresis anda tracksprocess everycontrol singletool pastethat deposit,tells turningyou exactly how your stencilprinter printis intobehaving ain quantifiablereal-time. dataIf stream.you Whenonly correctly configured,use SPI movesto sort Pass/Fail, you fromare reactingwasting to90% defectsof (scrap)its tovalue.

proactively correcting process drift

 (yield protection). The goal of this chapter is to useconvert SPI data into immediate engineering actions. We focus here on line control—interpreting the datalive dashboard to stabilize the printing process andbefore feeddefects actionable metrics back to design, making printing the most stable part of your line.occur.

The CoreVital MetricsMetrics: (AndVolume is King

While SPI machines measure many parameters, only three drive process decisions. Ignore the Number That Matters Most)

SPI measures three physical dimensions—Volume, Height,noise and Area—atfocus everyon pad, then calculates the process stability.these.

Metric	What It Tells YouDefinition	DefectThe RiskEngineering Consequence	ControlTarget ActionWindow
Volume (% of Target))	The total amount of solderpaste availabledeposited forrelative to the joint.stencil aperture volume.	Tombstoning,The Opens,single Shorts.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)	PrimaryHow leverfar forthe 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.	squeegee± pressure0 µm (Target) < and30 cleaningµm cadence(Warning).
Height (µm)	The peak height of the paste deposit.	ScoopingUsed to detect "dog ears" (too low)peaks) or Excessive Paste"scooping" (toodishing). high).Important for fine-pitch consistency.	Stencil Thickness ± 10%

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

thickness,

Interpreting butCpk sensitiveon 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).
AreaClosed (%Loop of Pad)Alignment	The footprint of the deposit.	BridgingON (tooLimit wide)max andcorrection Smearingto 50 µm).	Indicator of gasket seal quality and separation speed.
Transfer Efficiency (TE)	Measured Volume / Theoretical Volume.	Direct indicator of stencil release quality.	If low, check paste age/type or nano-coating.

The Manager's KPI: C_pk
C_pk (Process Capability Index) is the single number that measures how well your printing process stays centered within its tolerance limits.
– C_pk < 1.0: Process is unstable or producing defects. Scrap is guaranteed.
– C_pk = 1.33: Minimum acceptable for six sigma (Yellow Belt standard).
– C_pk ≥ 1.67: Golden Standard. The process is highly capable and stable.
Your goal is to keep C_pk for Volume on critical features above 1.33.

Setting Limits by Feature Family

Not all pads have the same tolerance. A large connector pad can handle ±25% variation, but a 0.4 mm BGA will bridge instantly at ±15%. You must set tighter limits for the high-risk features.

Feature Family	Volume Target (TE)	Yellow Band (Warning)	Red Band (Action)
Chips (0201s, 0402s)	100% ±5%	±15%	±25%
Fine-Pitch Gull Wing (≤ 0.5 mm)	95% –105%	±10%	±20%
QFN Thermal Pad (Total)	50% – 65% Coverage	45% – 70%	< 45% or > 70%
BGA / WLCSP Balls	90% – 110%	±10%	±20%

The Red Band – Immediate Stop. Any deposit hitting the Red band must immediately halt the line for a correction or board rework.

Closed-Loop Correction Protocol

The "closed loop" is the automatic communication between the SPI and the printer. It’s what moves quality from operator guesswork to system control.

SPI Alert	Printer Diagnosis	Automated Action	Secondary/Manual Action
Bridging Red (Area too high)	Stencil underside is contaminated.	Immediate Wet + Vac Clean. (Override print count).	Slow separation speed by 10% on the next print to improve paste release.
Volume Low Yellow (± 15% drift)	Blade pressure is too high or paste is starving.	Reduce Squeegee Pressure by 0.1 kg or one step.	Verify that the paste bead size is adequate.
Volume High Yellow (±15% drift)	Pressure is too low or paste bead is excessive.	Increase Squeegee Pressure by 0.1 kg or one step.	Increase squeegee speed slightly to thin the wipe.
Random Lows in One Quadrant	Local board support failure or alignment drift.	Local Alignment Correction (X/Y/Theta) on the printer.	Check and adjust support pins under the failing quadrant.
Volume Trend of 6 (Walking up/down)	Paste rheology changing due to age/temp.	Shorten Cleaning Interval to every 3 prints.	Scoop and replace the working paste bead with a fresh one.

Data and Traceability (Protecting Your Business)

SPI data is not just for the line; it’s a critical part of the product quality record.

• Traceability Link: Every inspected board must have its SPI data file linked to its unique serial number in the MES. If a product returns from the field with a cold joint or short, you can instantly pull the 3D print data for that specific board and pad location for forensic analysis.
• Pareto to Design: Periodically run a Pareto analysis on all Red failures. If 70% of all failures are due to low volume on 0.4 mm QFNs, the problem is not the printer—it’s the stencil design. Feed this data back to the engineering team to spin a thinner or Electroformed stencil with a better aperture shape for the next revision.

Final Checklist: Locking the SPI Loop

This table summarizes the mandatory requirements for an operational, closed-loop SPI system, ensuring that data is trustworthy and directly linked to process control actions.

Requirement	Control Point / Metric	Action Mandate
Capability Baseline	Volume Cpk must be defined and targeted at ≥ 1.33 for all critical features (BGA, fine-pitch).	The line stops if Cpk drops below 1.0.
Limit Enforcement	Yellow and Red limits are set by feature family (tighter for BGAs, looser for chips).	Limits must be locked in the Golden Recipe; no operator override.
Automated Loop	Closed-Loop Correction policies (Clean/Pressure/Alignment) are enabled.	System must perform the first correction automatically when a Yellow alarm is triggered.
Process Feedback	Pareto Analysis of Red failures must be run weekly.	Data on persistent failures must be fed back to Stencil Design for revision.
Trust and Audit	Daily 3D Calibration and weekly GR&R checks are completed and logged.	Golden SPI Data/Screenshot from the First Article is archived as the visual standard.
Traceability	SPI Data File must be linked to the PCB's unique serial number (SN) in the MES.	Allows forensic analysis of failed boards returned from the field.