2026/03 Newsletter
As the industry moves deeper into 2.5D and 3D advanced packaging, many engineering teams are reaching the same conclusion: the most troublesome failures are no longer visible on the surface. Increasingly, the real risks are hidden inside the package.
As micro-bump pitches continue to shrink and Cu-Cu hybrid bonding migrates into the mainstream, TSVs, RDLs, and interposers are creating increasingly dense architectures. Under these conditions, optical inspection alone is no longer sufficient. It simply cannot see through multilayer, opaque structures. This is why AXI (Automated X-ray Inspection) has evolved from a discretionary "nice-to-have" into mission-critical infrastructure for advanced packaging lines.
The bottom line is clear: failure modes in advanced packaging are concentrating around invisible defects. These are not issues you can identify with surface checks. In many cases, only X-ray inspection can reveal internal bonding integrity without damaging the sample. For high-volume manufacturing, this is not just about adding another inspection tool—it is about shifting risk upstream and preventing yield loss earlier in the process flow.
Consider micro-bump and hybrid bonding applications. Common issues include voids, non-wet opens, misalignment, and partial bonding defects. In CoW (Chip-on-Wafer), WoW (Wafer-on-Wafer), or die-to-die bonding flows, if these defects go undetected, downstream scrap costs can be significant. If they escape into reliability testing or field operation, the consequences are far more severe.
TSVs follow a similar logic. Voids in TSV fill, seam defects, incomplete etch or fill, and cracks formed after thermal cycling may leave little or no visible evidence externally. Yet these internal flaws directly affect electrical performance and long-term reliability.
RDL and interposer structures introduce additional risks such as layer mis-registration, shorts and opens, line continuity concerns, and hidden delamination masked by overlying layers. In such cases, X-ray inspection becomes less of a supplementary check and more of a necessary safeguard.
That said, AXI is not without challenges. The most persistent issue for engineering and manufacturing teams is its fundamental trade-off: higher resolution typically comes at the cost of lower throughput. To resolve micro-bumps, Cu-Cu bonds, or TSV seams clearly, AXI systems require smaller X-ray focal spots, higher magnification, and longer exposure times. When CT or laminography is involved, even more projections are needed. In practice, the "Golden Quadrilateral" of resolution, throughput, field of view, and penetration depth remains a zero-sum game. A compromise is inevitable.
This constraint is precisely why many mature production lines adopt a hybrid inspection strategy. Inline AXI systems with moderate resolution are used for rapid screening to filter out gross defects and obvious anomalies. Offline, high-resolution AXI or CT systems are then applied for root cause analysis, qualification, or targeted inspection of high-risk lots. This approach balances capacity with risk control and aligns better with production realities.
Another important trend is the growing role of AI in AXI. What was once viewed as a marketing feature is now becoming a practical solution to manufacturing pain points. Automated void quantification, pattern recognition for bonding anomalies, reduction of false calls, and decreased reliance on highly experienced inspectors all translate directly into improved production efficiency and consistency. As packaging complexity continues to rise, the combination of AXI and AI is becoming increasingly common.
In summary, 2.5D and 3D advanced packaging push defects deeper into multilayer structures, beyond the reach of optical inspection. As a result, AXI is evolving from an optional capability into foundational infrastructure. However, the core tension between resolution and throughput remains, making hybrid inline screening and offline deep analysis the dominant strategy in high-volume manufacturing. With AI enhancing classification and stability, AXI is becoming better positioned to keep pace with the next generation of packaging technology.
