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4.3 AXI Techniques

Automated X-ray Inspection (AXI) is a non-destructive testing method that uses X-ray technology to penetrate components and the Printed Circuit Board (PCB), generating images of internal structures and hidden solder joints. AXI is mandatory for products featuring Ball Grid Arrays (BGAs), Quad Flat No-leads (QFNs), and other area-array or bottom-terminated packages, where the critical connections are invisible to standard optical inspection (AOI). The investment in AXI is justified by the requirement for superior reliability in safety-critical industries (e.g., medical, automotive, aerospace).

4.3.1 AXI vs. AOI: The Hidden Defect Trade-Off

AXI is not a replacement for AOI; it is a complementary technology used to cover structural defects that AOI cannot detect.

Feature

Automated Optical Inspection (AOI)

Automated X-ray Inspection (AXI)

Inspection Medium

Visible Light and Cameras

X-rays (penetrates materials)

Defects Detected

Missing components, Polarity, Skew, Surface-level Bridging.

Voids, Hidden Bridges, Head-in-Pillow (HIP), Open/Cold Joints beneath components.

Coverage

Visible Joints only. Ineffective for BGAs, QFNs, and shielded areas.

Hidden Joints. Near 100% structural coverage of the board.

Speed/Cost

Fast (10 – 20 seconds/board). Lower CapEx.

Slower (30 – 60 seconds/board). High CapEx and OpEx (X-ray tube replacement, safety).

Best Use

High-volume, high-speed lines; standard consumer electronics.

High-reliability, low-volume/high-complexity boards (automotive, medical).

Strategic Mandate: The complexity of the board's BOM and the Cost of Failure determine the necessity of AXI. If a hidden solder void can cause a crash or patient harm, AXI is non-negotiable.

4.3.2 Defect Detection: Seeing the Unseen

AXI detects defects based on material density. Solder (containing heavy elements like Lead or Tin) absorbs more X-rays and appears dark, while voids, flux residue, and fiberglass appear lighter.

A) Critical Hidden Defects

  • Voids: Trapped gas or flux residue inside solder joints. Voids reduce the joint's mechanical strength and thermal conductivity, leading to localized overheating.
    • Control Limit: The acceptable voiding limit is typically ≤ 25% of the total joint area for standard BGAs, but high-reliability specifications often require ≤ 15% on thermal pads (QFN/power components).
  • Hidden Bridging/Shorts: Unintended solder connections between adjacent balls or leads under BGAs or QFNs.
  • Head-in-Pillow (HIP): Verification of complete solder ball collapse and full fusion of the component ball with the paste on the pad.

B) Technology Types

  • 2D AXI: Provides a single, top-down transmission image. Used primarily for initial defect detection (large voids, gross shorts). Limited effectiveness on double-sided boards as images from both sides overlap.
  • 3D AXI (Planar CT/Tomography): Uses oblique viewing angles and image reconstruction to generate virtual cross-sections of individual solder joints. This technology allows for clear isolation and measurement of defects on double-sided boards and provides the clearest view of BGA ball shape and HIP defects.

4.3.3 AXI Protocol and Process Control

AXI data must be used for process optimization, not just final inspection.

  1. Programming: AXI does not rely on a Golden Board (Chapter 2.5) because it sees through material. The programming uses the CAD data to define inspection regions. Experienced programmers are required to set accurate pass/fail limits for voiding and density.
  2. Process Feedback: AXI data provides direct feedback to the reflow process:
    • Excessive Voiding – Requires Reflow Profile tuning (longer soak/preheat to vent flux) or an atmosphere change to Nitrogen (Chapter 3.3).
    • Inconsistent Joint Collapse – Requires Time Above Liquidus (TAL) or Peak Temperature adjustment (Chapter 3.2).
  3. Safety and OpEx: Strict X-ray safety protocols are mandatory. High operational costs necessitate strategic use; AXI should be applied selectively on the high-risk components (BGAs, QFNs) rather than inspecting 100% of all components on the entire board to maintain line throughput.

Final Checklist: AXI Implementation

Requirement

Control Point

Quality/Cost Focus

Application

AXI used for 100% of hidden joints (BGAs, QFNs, etc.).

Ensures structural integrity of non-visible connections.

Technology

3D AXI used for double-sided boards and critical BGA inspection.

Provides clean cross-section images, eliminating overlap error.

Limits

Voiding limits are defined by specification (e.g., 25% IPC maximum) and locked in the AXI program.

Quantifies defect acceptability.

Process Loop

Data on excessive voiding must trigger an immediate review of the Reflow Profile and Paste Selection (Chapter 3).

Drives continuous process improvement upstream.

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