temperature-humidity-bias failure analysis
**Temperature-Humidity-Bias Failure Analysis (THB FA)** is the **systematic investigation of semiconductor package failures that occur during or after THB/HAST reliability testing** — using optical microscopy, SEM/EDS, cross-sectioning, and chemical analysis to identify the specific corrosion products, migration paths, and failure locations that caused electrical failure, enabling root cause determination and corrective action to improve package moisture reliability.
**What Is THB Failure Analysis?**
- **Definition**: The post-test examination of semiconductor packages that failed THB or HAST testing — combining non-destructive techniques (X-ray, C-SAM) with destructive analysis (decapsulation, cross-sectioning, SEM/EDS) to identify the physical and chemical evidence of moisture-induced failure mechanisms.
- **Corrosion Product Identification**: THB FA identifies the specific corrosion products present — green/black deposits indicate copper corrosion (Cu₂O, CuCl₂), white deposits indicate aluminum corrosion (Al(OH)₃, AlCl₃), and metallic dendrites indicate electrochemical migration.
- **Migration Path Tracing**: For dendritic growth failures, FA traces the dendrite path from cathode to anode — identifying the moisture ingress route, the contamination source that provided mobile ions, and the conductor spacing that allowed bridging.
- **Root Cause Chain**: THB FA establishes the complete failure chain: moisture ingress path → contamination source → electrochemical mechanism → failure location → electrical symptom — enabling targeted corrective action.
**Why THB FA Matters**
- **Corrective Action**: Without FA, a THB failure provides no guidance for improvement — FA identifies whether the failure is due to passivation cracks, mold compound delamination, ionic contamination, or inadequate conductor spacing, each requiring different corrective actions.
- **Process Improvement**: FA often reveals manufacturing process issues — residual flux contamination, incomplete plasma cleaning, passivation pinholes, or mold compound voids that allowed moisture to reach the die surface.
- **Material Qualification**: FA results guide material selection — identifying which mold compounds, underfills, or passivation layers provide adequate moisture protection and which allow premature corrosion.
- **Design Rules**: FA findings feed back into design rules — establishing minimum conductor spacing, passivation thickness, and guard ring requirements to prevent moisture-induced failures in future designs.
**THB FA Techniques**
| Technique | What It Reveals | When Used |
|-----------|----------------|----------|
| Optical Microscopy | Surface corrosion, discoloration | First look after decap |
| SEM (Scanning Electron Microscope) | Dendrite morphology, corrosion detail | High-magnification imaging |
| EDS (Energy Dispersive Spectroscopy) | Chemical composition of deposits | Identify corrosion products |
| Cross-Section + SEM | Internal failure location, delamination | Subsurface analysis |
| C-SAM (Acoustic Microscopy) | Delamination mapping (non-destructive) | Pre-decap screening |
| X-ray | Wire bond integrity, internal voids | Non-destructive overview |
| Ion Chromatography | Ionic contamination species and levels | Contamination source ID |
**Common THB FA Findings**
- **Aluminum Bond Pad Corrosion**: Green/white deposits on bond pads — caused by moisture + chloride ions penetrating through passivation cracks or mold compound delamination.
- **Copper Trace Corrosion**: Dark discoloration and thinning of copper traces — anodic dissolution under bias in the presence of moisture and halide contamination.
- **Silver Dendrites**: Metallic tree-like growths bridging conductors — silver migrates fastest of common metals, requiring careful control of silver-containing materials near biased conductors.
- **Delamination-Enabled Corrosion**: Corrosion concentrated at delaminated interfaces — moisture accumulates in delamination voids, creating localized corrosion cells.
**THB failure analysis is the diagnostic discipline that transforms reliability test failures into actionable improvements** — identifying the specific corrosion mechanisms, contamination sources, and moisture ingress paths that caused failure, enabling targeted corrective actions in package design, materials, and manufacturing processes to achieve robust moisture reliability.