🔍 Solder Paste Inspection (SPI) – Detailed Process Overview

Solder Paste Inspection (SPI) is a crucial quality control step in the Surface Mount Technology (SMT) assembly process. It ensures the correct deposition of solder paste on PCB pads after the stencil printing process but before component placement. SPI systems use 2D or 3D imaging to measure the solder paste’s height, volume, area, shape, and alignment. Early detection of solder paste defects significantly reduces rework and ensures reliable solder joints in the final product.

By verifying the quality of solder paste application, SPI helps in:

• Preventing soldering defects (bridging, tombstoning, open joints)

• Increasing first-pass yield

• Reducing production downtime and cost

• Providing process control feedback to improve stencil printing

📋 10 Detailed Steps in the Solder Paste Inspection (SPI) Process

1. Loading the PCB into the SPI Machine

Description:
The PCB is automatically or manually loaded onto the SPI conveyor system. The board must be correctly oriented and positioned to match the inspection program.

2. Fiducial Recognition and Board Alignment

Description:
The SPI machine detects fiducial marks or PCB edges to align the inspection field with the exact location of the pads and deposited solder paste.

3. Program Selection for the PCB Model

Description:
Operators select the correct inspection program based on PCB ID or barcode. This program contains paste pad locations, acceptable measurement tolerances, and test criteria.

4. 3D Scanning of the Solder Paste Deposits

Description:
The SPI system scans the PCB using structured light (Moire fringe or laser triangulation) to create a 3D profile of the solder paste deposits, measuring volume, height, and shape.

5. Solder Paste Volume Measurement

Description:
The software calculates the total paste volume for each pad and compares it to the expected value. Volumetric accuracy is key to preventing soldering defects.

6. Height and Area Verification

Description:
The system checks the height of the paste to ensure uniformity and detects collapsed or insufficient prints. Area coverage is also verified for complete pad coverage.

7. Offset and Shift Detection

Description:
SPI checks if the solder paste is misaligned from the pad center. Excessive shift may lead to solder bridging, tombstoning, or unreliable joints.

8. Defect Classification and Flagging

Description:
If measurements fall outside of set tolerances, the SPI software flags defects such as:

• Insufficient paste

• Excess paste

• Bridging

• Offset prints

• Shape anomalies (slumping, smearing)

9. Feedback to Printer or MES System

Description:
Some advanced SPI machines provide closed-loop feedback to the stencil printer to auto-correct alignment or pressure errors. Data can also be sent to MES systems for traceability and analytics.

10. Pass/Fail Decision and Board Sorting

Description:
Boards are categorized as pass or fail. Failed boards are diverted for manual inspection or rework. High-accuracy sorting minimizes the chance of defects proceeding to placement and reflow.

❓ 10 “How To” Questions for Solder Paste Inspection (SPI)

1. How to Select the Right SPI System for Your SMT Line?

Detailed Description:
Choose based on:

• 2D vs. 3D inspection needs

• Camera resolution and speed

• Conveyor compatibility

• Integration with printer and MES

• SPC (Statistical Process Control) capability

High-mix lines may need faster program switching and barcode support.

2. How to Create an SPI Inspection Program for a New PCB?

Detailed Description:
Steps include:

• Import PCB data (Gerber, CAD, or pick-and-place)

• Define pad locations and solder paste regions

• Set tolerance limits for volume, height, and area

• Test and validate the program with sample boards

3. How to Calibrate and Maintain SPI Machines?

Detailed Description:
Daily or weekly calibration includes:

• Checking camera focus and lens cleanliness

• Verifying Z-axis accuracy using calibration coupons

• Running self-diagnostic routines

• Software updates and firmware checks

4. How to Interpret SPI Measurement Data Accurately?

Detailed Description:
Understand:

• Volume % deviation from target (e.g., ±25%)

• Height consistency across similar pads

• Shape abnormalities indicating print smearing or slumping

• Offset direction (X/Y)

Use heat maps and graphs for visual data representation.

5. How to Handle SPI False Positives and False Negatives?

Detailed Description:
Optimize:

• Lighting and contrast settings

• Thresholds and tolerance margins

• Masking irrelevant areas

• Program tuning for unusual pad geometries

Prevent misclassification that can slow production.

6. How to Use SPI Data for Process Improvement?

Detailed Description:
Use collected data to:

• Identify trends in print deviation

• Correlate defects with squeegee wear or stencil blockages

• Adjust print parameters (speed, pressure)

• Schedule preventive maintenance

7. How to Set Tolerances for SPI Inspection Criteria?

Detailed Description:
Based on IPC guidelines and empirical data:

• Volume: typically ±25–30%

• Height: varies by stencil thickness

• Offset: less than 50% of pad width
  Different components (e.g., BGAs, 01005s) may require tighter specs.

8. How to Integrate SPI into a Closed-Loop System with the Printer?

Detailed Description:
Enable feedback from SPI to stencil printer to:

• Auto-correct print misalignments

• Adjust pressure or snap-off in real-time

• Trigger stencil cleaning cycles
  Requires software communication and compatible equipment.

9. How to Train Operators for SPI Machine Usage?

Detailed Description:
Training must include:

• Understanding 3D inspection principles

• Loading/unloading boards safely

• Program selection and validation

• Defect classification and response

• Interpreting SPI reports for process feedback

10. How to Troubleshoot Common SPI Machine Errors?

Detailed Description:
Issues include:

• Camera calibration errors → Recalibrate or clean lenses

• Program mismatch → Verify barcode or model selection

• False bridging → Adjust lighting or thresholds

• Software crashes → Update firmware or reinstall

Log errors for traceability and continuous improvement.