2026/01 Newsletter
In the era of Advanced Packaging (e.g., CoWoS, Chiplet), heterogeneous integration introduces complex material interfaces. Consequently, thermal stress and warpage have made Delamination a primary yield killer.This brief explores the "Gold Standard" for detecting these defects and answers a common industry question: "Why can't our expensive AXI (X-Ray) systems see these failures?"
1. The Primary Workhorse: Scanning Acoustic Microscopy (SAM)
SAM is widely recognized as the industry standard for non-destructive delamination detection.
The Principle
SAM operates on the principle of Acoustic Impedance Mismatch. Ultrasonic waves reflect nearly 100% at solid-to-gas interfaces (e.g., Silicon to Air). Therefore, even extremely thin layers at the submicron level can still exhibit extremely high acoustic contrast under appropriate frequency conditions.
C-SAM vs. T-SAM: Which one to use?
While both fall under SAM, their applications in advanced packaging differ significantly:
- C-SAM (C-mode, Pulse-Echo Reflection) – The Industry Mainstream
- Mechanism: Transmits a pulse and receives the reflection.
- Advantage: Uses Time-of-Flight (ToF) to provide Z-depth localization. You can precisely determine if the delamination is at the Die Attach, Underfill, or Mold Compound interface.
- Application: Standard for IC packaging, Flip-chip, and BGA inspection.
- T-SAM (Through-Transmission) – Niche Application
- Mechanism: Transmitter on one side, receiver on the other; measures the penetrating wave.
- Limitation: Provides only a "shadow" image. It cannot determine defect depth and requires access to both sides of the sample.
- Application: Typically limited to raw materials or thick plates.
- C-SAM (C-mode, Pulse-Echo Reflection) – The Industry Mainstream
2. The Challenger: Can AXI Detect Delamination?
A frequent question in the fab is: "We have high-end AXI (Automated X-ray Inspection) on the line. Do we still need SAM?"
The Answer: Yes. AXI is generally ineffective for delamination detection.
- Why X-Ray Misses the Gap
- Physics: X-ray imaging relies on Density/Mass Absorption Contrast.
- The Nature of the Defect: Delamination is essentially a "thin air gap."
- The Result: For an X-ray beam passing perpendicularly through the sample, a micron-level air gap causes negligible change in mass absorption. Unless the delamination is severe enough to cause physical displacement (warpage or lift-off), it remains invisible in X-ray images.
Complementary Roles: SAM vs. AXI
Advanced packaging requires Multi-modal Inspection. One tool cannot do it all.
| Feature | SAM (Acoustic) | AXI (X-Ray) |
|---|---|---|
| Physical Mechanism | Acoustic Impedance | Density Absorption |
| Delamination | ⭐ Excellent (High sensitivity) | ❌ Poor (Blind to thin gaps) |
| Voiding | ✅ Good (Underfill voids) | ✅ Good (Solder voids) |
| Interconnects | ❌ Poor (Blocked by metal layers) | ⭐ Excellent (Open/Short/Bridge) |
| Best For | Interfaces, Adhesion, Cracks | TSVs, Micro-bumps, RDL Traces |
3. Recommended Detection Strategy (Best Practice)
To maximize yield and reliability, we recommend the following staged approach:
- In-line Screening (Production):
- C-SAM: Perform rapid tray-based scanning to screen out units with structural delamination.
- Electrical Test: Screen for electrical continuity failures.
- Process Development:
- SAM + AXI (CT): Cross-reference data. Use SAM to check structural integrity (e.g., "popcorn" defects) and X-ray CT to inspect internal TSV or Bump deformation.
- Failure Analysis (Root Cause):
- Step 1: Use Non-destructive C-SAM to pinpoint the X-Y-Z coordinates of the defect.
- Step 2: Perform Destructive Analysis (FIB or Cross-section polishing) at the targeted coordinates and validate using SEM.
4. Conclusion
As Moore's Law slows and packaging shifts from 2D to 3D, defect detection becomes exponentially more challenging. Effective Quality Assurance relies on understanding physical limitations—using X-Ray for structure and Ultrasound for interfaces. Adopting a Multi-modal Inspection strategy is the only way to accurately intercept potential failures without unnecessary cost.
Key Takeaways
- SAM is the King of Interfaces: For any issues involving adhesion or layer separation, ultrasound is the only reliable non-destructive method.
- Don't rely on X-Ray for gaps: X-Ray is an expert at seeing metal and structure, but it is effectively "blind" to air gaps.
- Precision matters: In advanced packaging, always use C-SAM capable of Phase Inversion analysis to distinguish between true delamination (air) and high-density inclusions.
