🧠 X-Ray Inspection in SMT PCB Assembly – Overview

X-ray inspection (AXI – Automated X-ray Inspection) is a non-destructive testing method used to inspect the internal structures of solder joints and components on a Printed Circuit Board (PCB) that are not visible using traditional optical inspection. It is especially crucial for Ball Grid Array (BGA), CSP, QFN, and other bottom-terminated components, where solder joints are hidden beneath the package.

X-ray systems use radiographic imaging to reveal:

• Voids in solder joints

• Insufficient solder

• Cold solder joints

• Misaligned or missing balls in BGA

• Short circuits and opens under components

• Internal lead frame issues

🔟 10 Detailed Steps in the X-Ray Inspection Process

1. Transfer of PCB to X-Ray System

Description:
After reflow soldering, the PCB is transferred—either manually or via conveyor—to the X-ray inspection machine. For inline systems, this is part of the automated production flow.

2. Fixture or Stage Alignment

Description:
The PCB is positioned on the inspection stage, and the system aligns the board using fiducials or edge recognition. This alignment ensures the correct area is scanned during image capture.

3. Selection of Inspection Program

Description:
A predefined inspection program is selected based on the board type. The program includes:

• Component locations (from Gerber/CAD files)

• X-ray parameters (voltage, exposure time)

• Inspection criteria (e.g., acceptable void percentage)

4. Initial X-Ray Imaging and Setup Calibration

Description:
The system performs a test scan to:

• Calibrate image sharpness and exposure

• Set magnification and resolution

• Adjust X-ray source positioning
  This ensures clear imaging of solder joints and internal features.

5. Sequential Scanning of Hidden Solder Joints

Description:
The X-ray system captures images of hidden joints (e.g., BGAs, QFNs) component by component. It examines:

• Ball shape and size consistency

• Solder volume

• Connection between solder and pad
  This data is analyzed in real-time by the system.

6. Void Detection in Solder Joints

Description:
X-ray imaging reveals voids (air pockets) inside solder joints. The system:

• Calculates void percentage

• Flags joints that exceed the IPC allowable limits
  Voids can weaken the mechanical strength or affect thermal/electrical conductivity.

7. Short Circuit and Open Detection

Description:
The system identifies:

• Bridging under components (e.g., shorts between balls or pads)

• Opens where solder has not connected the pad and component lead/ball
  These are invisible in optical systems but clearly seen in X-ray images.

8. 3D Imaging and Tomography (Optional)

Description:
Advanced AXI systems use Computed Tomography (CT) to generate 3D images by taking multiple slices of the same point. This allows:

• Detailed view of layered components

• More precise fault localization

• Visualization of internal structures of BGAs

9. Defect Classification and Reporting

Description:
The software classifies detected defects as:

• Void

• Short

• Open

• Misalignment

• Solder insufficient
  Each defect is marked with location coordinates and image snapshots.

10. Operator Review and Disposition

Description:
Operators review flagged defects and determine whether to:

• Pass the board

• Send for rework

• Hold for further evaluation
  Defect data is stored for traceability and yield improvement analysis.

❓ 10 “How To” Questions for X-Ray Inspection in SMT Assembly

1. How to Set Up an X-Ray Inspection Program for a New PCB?

Description:

• Import Gerber or CAD data into the inspection software

• Define inspection zones (BGA, QFN, connectors)

• Set parameters (X-ray intensity, dwell time, void threshold)

• Save the configuration as a template for future use

2. How to Detect Voids in BGA Solder Balls Using X-Ray?

Description:

• Focus on spherical shape consistency of solder balls

• Use contrast to highlight low-density regions (voids)

• Measure void size as a % of total joint volume

• Compare against IPC-A-610 Class II/III standards

3. How to Interpret X-Ray Images for Shorts Under Components?

Description:

• Look for unusually large continuous regions of solder

• Identify bridging between adjacent pads or balls

• Use system software’s automatic bridge detection features

• Validate with multiple viewing angles if 3D/CT is available

4. How to Use 3D CT Imaging for Complex Fault Analysis?

Description:

• Switch to 3D tomography mode

• Capture multi-angle slices around the area of interest

• Reconstruct a volumetric image

• Analyze internal stacking, vias, or hidden defects in detail

5. How to Identify Opens or Insufficient Solder with X-Ray?

Description:

• Opens appear as gaps between component leads/balls and pads

• Insufficient solder results in smaller than expected joints

• Measure solder coverage and compare to nominal values

• Ensure complete wetting and attachment of joints

6. How to Calibrate X-Ray Equipment for Accurate Results?

Description:

• Use standard test boards or phantoms with known features

• Perform sharpness and alignment tests

• Set baseline for intensity and magnification

• Run routine calibration checks per ISO/IPC guidelines

7. How to Minimize Radiation Exposure in X-Ray Inspection?

Description:

• Use shielded enclosures and interlock systems

• Train operators on safe handling and emergency procedures

• Comply with radiation safety standards (e.g., ANSI N43, IEC 61010)

• Regularly monitor radiation levels around the machine

8. How to Reduce False Positives in X-Ray Defect Detection?

Description:

• Fine-tune detection algorithms and void thresholds

• Use historical defect patterns to improve classification

• Train system with a mix of good and defective boards

• Perform manual verification for borderline cases

9. How to Integrate X-Ray Data into MES or Quality Systems?

Description:

• Export inspection results via API, XML, or CSV formats

• Link defect logs to barcode or serial number tracking

• Generate yield reports, Pareto charts, and traceability logs

• Use feedback to adjust upstream processes (printing, placement, reflow)

10. How to Train Technicians to Read and Analyze X-Ray Results?

Description:

• Teach basics of X-ray physics and imaging

• Train on defect identification using actual images

• Conduct guided sessions on interpreting difficult cases

• Provide reference libraries of common and rare defects