How to Use X-Ray Inspection for Reliable Multilayer PCB Quality Control

X-ray inspection is a critical tool for identifying hidden defects in multilayer PCBs, enabling visibility into internal structures that traditional inspection methods cannot reach. As electronics shrink and designs grow more complex, x-ray inspection has become indispensable for ensuring quality in PCB assemblies across industries.

Key Takeaways

• X-ray inspection reveals hidden flaws in multilayer PCBs—such as solder voids, cracks, and misalignments—that surface-level checks miss, enhancing overall reliability.
• Combining 2D and 3D x-ray inspection enables fast, non-destructive detection of more defects, reducing rework costs and saving time.
• Pairing x-ray inspection with automated optical inspection (AOI) delivers comprehensive quality control, addressing both internal and surface-level issues in PCBs.

X-Ray Inspection in Multilayer PCBs

Why X-Ray for PCB Assemblies

Multilayer PCBs require rigorous quality checks to ensure performance, and x-ray inspection provides non-destructive visibility into internal layers and connections. As components miniaturize, critical joints (e.g., under BGAs or within vias) are hidden from view, making x-ray the only reliable way to verify their integrity.

LTPCBA utilizes advanced x-ray systems, including automated x-ray inspection (AXI) and computed tomography (CT) x-ray, to inspect PCBs:

• AXI quickly scans high volumes, detecting issues like insufficient solder or component misalignment.
• CT x-ray generates 3D images, revealing layer-to-layer traces and tiny flaws as small as 10 microns.

X-ray inspection ensures compliance with strict industry standards such as IPC-A-600 (PCB quality) and IPC-A-610 (assembly standards). Unlike visual checks or electrical tests, x-ray penetrates dense components and multilayer structures, making it essential for validating hidden connections.

Detecting Hidden Defects

To prevent field failures, multilayer PCBs must be free of internal flaws that surface inspections miss. X-ray inspection identifies:

• Solder voids under BGAs and QFNs
• Cold solder joints (visually intact but electrically faulty)
• Blocked or incomplete vias
• Interlayer cracks
• Layer/trace misalignments
• Hidden solder bridges

Traditional methods like in-circuit testing (ICT) or flying probe tests only verify functionality, not physical defects. X-ray, by contrast, visualizes every layer and joint from multiple angles, enabling detection of shorts, opens, and misalignments. Automated analysis accelerates this process, ensuring consistency.

LTPCBA integrates x-ray inspection at every production stage, using metrics like the Circuit Trace Fidelity Index and Layer Alignment Consistency Score to quantify quality. Early defect detection reduces costly rework and ensures compliance with stringent standards.

X-Ray Inspection Process and Benefits

Inspection Steps

X-ray inspection is non-destructive, allowing thorough checks without damaging PCBs. The process follows these steps:

1. Setup and Calibration

Secure the PCB in the x-ray system and configure settings (e.g., 80–120 kV for multilayer boards) based on thickness and complexity.

• Image Capture

Scan the PCB to generate images:

• Use 2D x-ray for simple designs.
  • Use 3D x-ray/CT for complex multilayer structures.
  • AXI handles high-volume scanning efficiently.
• Analysis

Review images to identify defects (e.g., voids, misalignments). Automated software flags issues, with zoom capabilities for tiny flaws.

• Reporting

Document findings with images, defect codes, and notes. Real-time data tracking supports process improvement and traceability.

Tip: Combine 2D and 3D x-ray to maximize detection of hidden defects in multilayer PCBs.

Types of Hidden Defects Found

X-ray inspection excels at uncovering internal flaws, particularly in dense or miniaturized designs:

Defect Type	Description
Solder Voids	Gaps in solder under BGAs/QFNs, weakening joints and causing overheating.
Cold Solder Joints	Poorly bonded joints that appear intact but fail electrically.
Misaligned Components	BGA balls or traces shifted from their design positions.
Blocked Vias	Unfilled or partially filled vias disrupting electrical connectivity.
Interlayer Cracks	Fractures between layers, risking signal loss or short circuits.
Hidden Solder Bridges	Unintended solder connections under components, causing shorts.

X-ray is especially critical for BGA and CSP packages, where solder joints are fully concealed. While AOI checks surface defects, only x-ray addresses internal issues.

LTPCBA Quality Assurance

LTPCBA prioritizes x-ray inspection alongside other quality controls to deliver reliable PCBs. The company holds certifications including ISO, IATF, UL, and IPC, ensuring compliance with global standards. Key quality metrics include:

Metric/Aspect	Value/Description	Impact on Quality/Reliability
SMT Line Pass Rate	99.5%	High production consistency.
Functional Test Failure Rate	0.0003%	Extremely low defect incidence.
First Pass Yield (FPY)	95% target	Reduces rework, accelerates delivery.
Pick-and-Place Accuracy	99.95%	Precise component positioning.
Inspection Technologies	AOI, X-ray, ICT, Functional Testing	99% defect detection rate via multi-layered checks.
Quality Control Methods	SPC, AQL, DPPM, Burn-in Testing	Ensures consistent reliability.

LTPCBA also undergoes third-party audits (e.g., ISO 13485:2016, IATF 16949:2016) and uses advanced x-ray systems to validate every PCB, ensuring it meets customer specifications.

Comparing X-Ray and AOI for PCB Assemblies

Strengths and Limitations

Aspect	Automated Optical Inspection (AOI)	X-Ray Inspection
Inspection Scope	Surface flaws (missing parts, solder bridges).	Internal/hidden flaws (voids, cracks, via issues).
Speed	Very fast, ideal for high-volume production.	Slower, especially for 3D/CT scans.
Cost	Lower initial investment.	Higher upfront cost.
Defect Detection	Limited to visible surface issues.	Detects internal flaws in dense/multilayer designs.
Best For	Simple boards, high-volume runs.	Complex/multilayer PCBs, BGA/QFN assemblies.

Best Use Cases

• AOI is optimal for rapid surface checks (e.g., missing components, lifted leads) in high-volume, simple PCBs.
• X-ray is critical for inspecting complex, multilayer boards, especially those with BGAs or hidden joints.

For comprehensive quality control, combine both methods. LTPCBA uses AOI for surface checks and x-ray for internal validation, ensuring no defects—surface or hidden—escape detection.

Impact of Hidden Defects on Reliability

Unaddressed hidden defects can compromise PCB performance:

Defect Category	Impact on Reliability
Solder Voids	Weakened joints, increased thermal resistance.
Opens & Cracks	Intermittent or complete circuit failure.
Shorts & Bridging	Short circuits, potential component damage.
Component Misalignment	Signal integrity issues, reduced functionality.
Insufficient Solder	Unreliable connections, prone to failure under stress.

Early detection via x-ray reduces rework costs, improves yield, and enhances product longevity. LTPCBA’s integration of advanced x-ray tools ensures consistent, reliable PCBs for critical applications.

FAQ

How does x-ray inspection detect hidden PCB defects?

X-ray penetrates PCB layers and dense components, visualizing internal structures to identify voids, cracks, misalignments, and blocked vias that are invisible to surface checks.

Is x-ray inspection suitable for all PCB types?

It works for most PCBs but is especially valuable for multilayer, high-density designs and assemblies with hidden joints (e.g., BGAs, QFNs).

What sets LTPCBA’s x-ray inspection apart?

LTPCBA uses advanced AXI and CT x-ray systems, paired with strict quality protocols and certifications, to deliver fast, accurate inspections and reliable PCBs.

See Also:

• Improving PCBA Quality Control Through Advanced X-Ray Inspection
• Ensuring Perfect BGA Assembly Using Effective Quality Control
• SPI Visual Inspection Importance After SMT Solder Paste Printing
• Conducting Thorough Incoming Inspections For SMC And SMD Parts
• Essential Methods To Enhance PCBA Reliability Over Time