2.19 AXI Techniques

Automated X-ray Inspection (AXI) is a critical, 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 packages, where connections are invisible to optical inspection (AOI). The high CapEx of AXI is justified by the requirement for superior structural reliability in safety-critical industries.

2.19.1 AXI vs. AOI: The Cost of Coverage

AXI is a structural test that complements AOI, focusing on defects that lie beneath components, where optical access is impossible.

Strategy: AXI must be applied selectively to high-risk components (BGAs, QFNs) to maintain line throughput, while AOI handles the high-volume visual checks.

2.19.2 AXI Technology: 2D vs. 3D Laminography

AXI systems utilize the property that dense materials, like solder, absorb more X-ray energy than lighter materials (fiberglass, silicon, air). This creates contrast in the resulting image.

2D AXI Limitations

2D AXI provides a single, top-down transmission image. While useful for checking basic presence and large voids, its primary limitation is image overlap on double-sided boards. The solder joints from the top and bottom interfere, making accurate, quantitative analysis of internal joints ambiguous or impossible.

3D Laminography (Planar CT)

Laminography (also known as Planar Computed Tomography, or PCT) is the solution for eliminating image overlap. It is the core technology behind modern 3D AXI.

• Mechanism: The X-ray source and detector move in synchronized paths (oblique angles) relative to the board, capturing multiple images. Software then uses mathematical reconstruction to create virtual cross-sections or "slices" of the board.
• Function: Laminography allows the inspector to focus digitally on a specific layer (e.g., the component side joint plane) while blurring out the overlapping features from the opposite side.
• Application: This is mandatory for reliable inspection of double-sided BGAs and stacked packages, providing clear, quantifiable measurements of defects like voiding and HIP.

2.19.3 Defect Detection and Process Control

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

Critical Hidden Defects

Process Loop

AXI provides critical data for tuning the Reflow Profile:

• Excessive Voiding – Requires Reflow Profile tuning (longer soak/preheat to vent flux) or an atmosphere change to Nitrogen.
• Inconsistent Joint Collapse (HIP risk) – Requires adjustment of Time Above Liquidus (TAL) or Peak Temperature.

Final Checklist: AXI Implementation