📘 Pre-Reflow AOI Inspection – Overview

Pre-Reflow AOI is a crucial inspection step conducted after component placement but before reflow soldering. Its primary objective is to detect defects early in the process — such as misplaced components, missing parts, or polarity errors — while corrections are still possible. This step significantly reduces post-reflow rework costs and increases overall yield.

Pre-Reflow AOI uses high-resolution cameras and pattern recognition software to inspect:

• Component presence, position, orientation

• Part markings and polarity

• Solder paste coverage (optional)

• Height (if 3D AOI is used)

🛠️ 10 Detailed Steps in the Pre-Reflow AOI Process

1. PCB Transfer from Pick-and-Place Machine to AOI Station

Description:
The placed PCB is conveyed directly to the AOI system before entering the reflow oven. Inline systems automate this; offline AOI requires manual loading.

2. AOI Program/Recipe Loading

Description:
The AOI software loads a specific program (recipe) based on the PCB part number, which includes:

• Component reference designators

• Expected position, size, shape, and orientation

• Polarity information and no-go zones

3. Fiducial Recognition and Board Alignment

Description:
The AOI machine identifies fiducial marks to align the PCB’s image precisely. This ensures consistent inspection regardless of slight board misplacements.

4. Top-Side Imaging and Scanning

Description:
High-resolution cameras (2D or 3D) scan the top side of the board using:

• Structured lighting

• Multiple angles (in 3D systems)

• Stitched images for large boards

5. Component Presence/Absence Check

Description:
Each expected component is checked to ensure it exists on the board. Missing components are flagged for inspection or rework.

6. Component Orientation and Polarity Check

Description:
Polarized components (diodes, electrolytic capacitors, ICs) are verified for correct orientation. Incorrect polarity may cause functional failure post-reflow.

7. Component Placement Accuracy Verification

Description:
Checks if components are placed within acceptable tolerance zones (X-Y shift or rotation). Misaligned parts can lead to tombstoning or soldering defects.

8. Lead and Pad Alignment Check

Description:
For components with visible leads, AOI verifies if leads sit correctly over the pads — critical for ensuring solderability in reflow.

9. Defect Detection and Classification

Description:
Defects are detected and categorized such as:

• Missing

• Skewed

• Tombstoned (early indication)

• Upside-down (rotated 180°)

• Foreign material/debris
  Each is classified with severity and root cause hint.

10. Result Reporting and Disposition

Description:
Inspection results are:

• Displayed on-screen with defect images

• Logged for traceability

• Sent to MES or SPC systems
  Operators may rework flagged boards or approve them for reflow.

❓ 10 “How To” Questions for Pre-Reflow AOI – Detailed Explanations

1. How to Choose Between 2D and 3D AOI for Pre-Reflow?

Description:

• 2D AOI is faster and sufficient for presence/orientation checks

• 3D AOI adds Z-axis (height) data, beneficial for checking tombstoning or lead lift
  Choose 3D if you’re dealing with fine-pitch, high-density, or complex assemblies.

2. How to Create and Validate AOI Programs Efficiently?

Description:
Use:

• CAD data + BOM to auto-generate reference images

• Fiducial alignment for accurate overlay

• Golden board learning (optional)
  Validate with a known good board and optimize threshold settings to reduce false calls.

3. How to Reduce False Positives in AOI Inspection?

Description:
Tweak:

• Lighting settings

• Tolerance zones

• Gray level thresholds
  Also, exclude known harmless variations (e.g., cosmetic issues) from being flagged.

4. How to Detect Polarity Errors Before Reflow?

Description:
AOI checks component markings (like stripes or dots) and compares them with the expected orientation from the program. Train the AOI to identify visual polarity indicators correctly.

5. How to Integrate AOI with MES or Traceability Systems?

Description:
Modern AOI machines offer:

• Barcode scanning

• Real-time defect logging

• Operator authentication

• Pass/fail data uploads to MES
  This allows you to trace each board’s inspection history.

6. How to Inspect Small Passives Like 0201 or 01005 Packages?

Description:
Requires:

• High magnification lens

• Stable lighting to reduce glare/shadows

• Clean boards (dust looks like a defect at that size!)
  3D AOI is preferred for these miniature components.

7. How to Train Operators for AOI Defect Verification?

Description:
Training should include:

• Understanding defect codes

• Navigating AOI user interface

• Interpreting defect images

• Decision-making rules: rework, accept, or reject
  Use practice boards and defect libraries for learning.

8. How to Handle AOI-Flagged Defects for Rework?

Description:
Define a rework loop:

• Flag the board

• Annotate defect location

• Send to rework station

• Re-inspect post-repair
  All corrections should be logged and verified before proceeding to reflow.

9. How to Use SPC Data from AOI for Process Improvement?

Description:
Analyze:

• Defect trends per feeder or machine

• Frequent rotation/polarity errors by part

• Operator-based error distribution
  Use this data to refine placement processes or operator training.

10. How to Calibrate and Maintain AOI Equipment?

Description:
Routine tasks include:

• Camera focus and alignment

• Lighting system check

• Lens cleaning

• Software updates

• Annual factory calibration (recommended by OEM)
  Log each calibration event for audit trails.