1.6 SPI Metrics & Closed Loop

SPI turns printing from an art into a controlled process by making paste volume measurable, trackable, and correctable in real time. With transfer efficiency as the anchor metric, it links aperture design, stencil performance, and printer stability into a single feedback loop. Automated limits, SPC charts, and closed-loop adjustments catch drift before it becomes scrap, while Pareto analysis points back to stencil fixes for lasting improvement. When trusted and maintained, SPI makes solder paste deposition as predictable as any other engineered process step.

1.6.1 SPI in one minute (what it’s for)

SPI (solderSolder pastePaste inspection)Inspection) is your 3D radar for printing quality. It doesn't just catch defects; it measures and tracks every single paste deposit, turning your stencil print into a quantifiable data stream. When correctly configured, SPI moves you from reacting to defects (scrap) to speedometerproactively correcting process drift for(yield printing.protection). ItThe goal is to use the data to stabilize the process and feed actionable metrics back to design, making printing the most stable part of your line.

1.6.1 The Core Metrics (And the Number That Matters Most)

SPI measures volume,three height,physical dimensions—Volume, Height, and area Area—at every pad, comparesthen results to limits, and—when you let it—tunescalculates the printerprocess (cleaning, pressure, speed, alignment) before small drifts become big defects.stability.

Metric	What It Tells You	Defect Risk	Control 1.6.2 The three numbers (and one that ties them together)Action
Volume (% of target)Target)	The total amount of solder available for the joint.	Tombstoning, Opens, Shorts.	Primary lever for squeegee pressure —and thecleaning main quality knob.cadence.
Height (µm)	The peak height of the paste deposit.	Scooping —(too catcheslow) scooped/shortor printsExcessive andPaste “paste(too onhigh).	Governed mask.”by stencil thickness, but sensitive to squeegee pressure (Chapter 1.5).
Area (% of pad)Pad)	The footprint of the deposit.	Bridging —(too greatwide) forand bridgingSmearing.	Indicator of gasket seal quality and edgeseparation smear.speed.
Transfer Efficiency (TE)	= (Measured Volume ÷/ Theoretical Volume)Volume.	Direct ×indicator 100%. Theoreticalof Volume =stencil aperturerelease area × stencil thicknessquality.	ProIf tip:low, watchcheck TEpaste stability by feature familyage/type (chipsCh vs1.1) QFNor edgesnano-coating vs(Ch BGA)1.3). It’s the cleanest way to see process drift.

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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.

1.6.32 Setting Limits by featureFeature family (start here, tighten with data)Family

UseNot twoall tierspads ofhave limits:the same tolerance. A large connector pad can handle Yellow±25% (warnvariation, & log) and Red (action). Tune per product afterbut a few0.4 lots.mm BGA will bridge instantly at ±15%. You must set tighter limits for the high-risk features.

Feature familyFamily	TEVolume targetTarget (TE)	Yellow bandBand (Warning)	Red band	HeightBand floor	What trips “Bridge?”(Action)
Chips 0201–0603(0201s, 0402s)	100% ±5%	±15%	±25%	≥ 60–70% of stencil	Area > pad by +10–15% or contiguous blobs
Fine-pitchPitch gull-wingGull Wing (≤ 0.5 mm)	95–95% –105%	±10%	±20%	≥ 60–70%	Any area growth into gap on 2+ neighbors
QFN/DFN edge pads	90–110%	±10%	±20%	≥ 60–70%	Same as above
QFN thermalThermal padPad (total)Total)	50–50% – 65% coverageCoverage	45–45% – 70%	< 45% or > 70%	—	N/A (use total coverage)
BGA/CSPBGA balls/ WLCSP Balls	90–90% – 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.

1.6.3 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 50–60%underside is contaminated.	Immediate Wet + Vac Clean.N/A (useOverride neighborprint rules)count).	Slow separation speed by 10% on the next print to improve paste release.
LargeVolume powerLow padsYellow /(± shields15% 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 (Ch 1.5).
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	80–100%Local board support failure or alignment drift.	Local Alignment Correction (X/Y/Theta) on the printer.	±15%Check and adjust	support pins±25%	≥ 60–70%	Areaunder “lips”the beyondfailing land outlinequadrant.

Volume

Why the fuss on QFN thermal? Too much paste ⇒ float/tilt; too little ⇒ voids/thermal trouble.

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1.6.4 SPC without pain (how to chart and react)

• Chart type: most lines use Individuals (I-MR) per panel or subgroups by quadrant.
• Warm-up: ignore the first panel after a wipe or stencil change for control limits; then include it for history.
• Western-Electric-lite rules:
• • Any Red point = stop the line and act (below).
  • 2 of 3 consecutive in Yellow on the same feature family = tighten cleaning or blade settings.
  • Trend of 6 (Walking up/downdown)

    Paste =rheology checkchanging temperature/pastedue ageto age/temp.

    Shorten Cleaning Interval to every 3 prints.

    Scoop and gasket.

What you plot on dashboards

• % First-Print Passreplace the working paste bead with a fresh one (noCh reprint,1.2).
  no

  1.6.4 wipe)

• Reprint RateData and Wipes per Board
• TE Cpk by familyTraceability (chips/QFN/BGA)
• DefectProtecting ParetoYour (low volume, high volume, bridge, smear)
  

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1.6.5 Closed-loop corrections (the simple decision tree)Business)

Let SPI push small buttons automatically; reserve big ones for humans.

1. Bridging alerts at a local region
    → Add understencil clean now (wet+vac if already did dry) → slow separation 10–20% on next prints in that region.
2. Low TE on fine features across panel
    → Ease blade pressure slightly (−0.1 to −0.2 kg) → slow blade speed one step → confirm bead size (keep it narrow).
3. High TE / smear
    → Increase angle or speed slightly → ensure pressuredata is not excessivejust (nofor graythe filmline; behindit’s blade).
4. Randoma lowscritical inpart oneof quadrant
    →the Localproduct alignmentquality correctionrecord.
   • Traceability Link: onEvery thatinspected quadrantboard →must checkhave its boardSPI supportdata pinsfile under that area.
   • Volume drift with time
      → Shorten cleaning interval; if still drifting, refresh bead; swaplinked to aits freshunique jarserial (7.2).

Only change one knob at a timenumber and record deltas in the jobMES. recipe.If SPIa shouldproduct tagreturns whichfrom auto-tweakthe wasfield appliedwith a cold joint or short, you can instantly pull the 3D print data for that specific board and whetherpad itlocation fixedfor 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 (Chapter 1.4). Feed this data back to the engineering team to spin a thinner or Electroformed stencil with a better aperture shape for the next panel.revision.

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1.6.6Final ReprintChecklist: &Locking wipethe policiesSPI (make them predictable)Loop

• Auto-reprintMetrics Defined: aSPI region if Volume Red or Bridgesetup is flaggedrunning andagainst theC_pk boardtargets, hasn’tnot moved.just ±% limits.
• EscalateLimits Set: Volume limits are customized for high-risk features (BGA/QFN) to wipebe iftighter twothan reprintspassive fail or if Bridging Red hits on adjacent apertures.chips.
• PanelClosed rejectLoop: onlyAutomatic whencorrection three regions fail in Red after a wipe—otherwise you’ll kill throughput.
  

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1.6.7 Trust but verifypolicies (measurementClean/Pressure/Alignment) health)

are

• Daily 3D calibration on the reference plate; weekly GR&R spot-checks on a golden panel (three ops × three repeats).
• Disable “smart smoothing” that hides real variation; better to see the wobbleenabled and fix it.
• Keep illumination profiles per mask color/finish—AOI lighting lessons apply to SPI as well.
  

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1.6.8 Turning charts into better stencils (close the loop to design)

When a single aperture geometry dominates the SPI Pareto, fix the shape—not the printer:

• Chips tombstoning → move to home-plate/inverted (7.4.2).
• QFN thermal out of band → adjust window coverage or add chimneys (7.4.3).
• BGA variation → confirm powder size and stencil reduction (7.1, 7.4.4).
   Document the tweak, spin the stencil, and watch the next lot’s TE Cpk climb.
  

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1.6.9 First-Article & recipe lock

• Run one FA panel, review SPI heat-maps, tweak one variable, reprint, then freeze angle/pressure/speed, separation, and cleaning cadencedocumented in the job file.
• CaptureWipe/Reprint golden SPI screenshotsPolicy: ofClear tightrules featuresfor when to clean, when to auto-reprint, and storethe withthreshold for stopping the lotline recordare so nights/weekends know what “good” looks like.
  

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1.6.10 Release checklist (stick this on the printer)

• TE/height/area limits set by family (table above) with Yellow/Red bands.posted.
• Closed-loopCalibration: enabled:Daily cleaning,3D pressure,calibration speed,and localweekly alignment.Gauge Repeatability & Reproducibility (GR&R) checks are performed to ensure the measurement is accurate.
• Reprint/wipe policyAudit: inThe Golden SPI data/screenshot from the recipeFirst (countsArticle andis escalation).archived to define what
• Calibration & GR&R"good" schedulelooks posted; golden panel available.
• Dashboards up: First-Print Pass, Wipes/Board, TE Cpk, Pareto by family.
• Design feedback path ready (tag risky apertureslike for reviewthe in 7.4).operators.
  

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Conclusion: Define clear SPI metrics, enforce closed-loop corrections, and feed findings into stencil design revisions. This keeps paste prints stable across shifts, reduces firefighting, and steadily drives yield upward.