2.6 SPI Metrics and Closed Loop
SPI (Solder Paste 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 proactively correcting process drift (yield protection). The 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.
2.6.1 The Core Metrics (And the Number That Matters Most)
SPI measures three physical dimensions—Volume, Height, and Area—at every pad, then calculates the process stability.
Metric | What It Tells You | Defect Risk | Control Action |
Volume (% of Target) | The total amount of solder available for the joint. | Tombstoning, Opens, Shorts. | Primary lever for squeegee pressure and cleaning cadence. |
Height (µm) | The peak height of the paste deposit. | Scooping (too low) or Excessive Paste (too high). | Governed by stencil thickness, but sensitive to squeegee pressure. |
Area (% of Pad) | The footprint of the deposit. | Bridging (too wide) and Smearing. | 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: Cpk
Cpk (Process Capability Index) is the single number that measures how well your printing process stays centered within its tolerance limits.
– Cpk < 1.0: Process is unstable or producing defects. Scrap is guaranteed.
– Cpk = 1.33: Minimum acceptable for six sigma (Yellow Belt standard).
– Cpk ≥ 1.67: Golden Standard. The process is highly capable and stable.
Your goal is to keep Cpk for Volume on critical features above 1.33.
2.6.2 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.
2.6.3 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 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. |
2.6.4 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. |
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