biased hast
**Biased HAST (bHAST)** is a **moisture reliability test performed with electrical bias that evaluates the electrochemical corrosion resistance of semiconductor packages under accelerated moisture and voltage stress** — applying operating voltage to the device during 130°C, 85% RH, >2 atm exposure to accelerate metal corrosion, dendritic growth, and electrochemical migration between biased conductors, testing whether the package can prevent moisture-driven electrical failures over its intended service life.
**What Is bHAST?**
- **Definition**: A HAST test performed with electrical bias (typically operating voltage or maximum rated voltage) applied to the device — the combination of moisture, temperature, pressure, and electric field accelerates electrochemical failure mechanisms including metal corrosion, ion migration, dendritic growth, and surface leakage current.
- **Electrochemical Focus**: The applied voltage creates an electric field between conductors — this field drives dissolved metal ions (Cu²⁺, Ag⁺, Al³⁺) through the moisture film from anode (+) to cathode (-), where they plate out as metallic dendrites that can bridge conductors and cause short circuits.
- **Corrosion Acceleration**: Bias accelerates anodic dissolution of metals — aluminum bond pads, copper traces, and silver-containing solder can all corrode under biased moisture conditions, with the corrosion rate proportional to the applied voltage and moisture concentration.
- **Standard**: bHAST follows JESD22-A110 with bias — typically 96 hours at 130°C/85% RH with operating voltage applied, monitoring leakage current and parametric shifts at readout intervals.
**Why bHAST Matters**
- **Corrosion Qualification**: bHAST is the primary test for validating that a package's passivation, mold compound, and metallization can resist electrochemical corrosion — failure indicates that moisture can reach biased conductors and cause corrosion in the field.
- **Dendritic Growth Detection**: bHAST accelerates dendritic growth between closely-spaced conductors — critical for fine-pitch packages where conductor spacing is < 20 μm and the risk of moisture-bridging short circuits is highest.
- **Leakage Current Monitoring**: bHAST monitors leakage current during the test — increasing leakage indicates moisture penetration and surface contamination, providing early warning before catastrophic failure.
- **THB Equivalent**: 96 hours of bHAST at 130°C is equivalent to 1000 hours of standard THB at 85°C — providing the same electrochemical stress in 10× less time.
**bHAST Failure Mechanisms**
| Mechanism | Description | Detection | Root Cause |
|-----------|------------|-----------|-----------|
| Aluminum Corrosion | Al bond pads dissolve under bias + moisture | Open circuit, resistance increase | Passivation cracks, moisture ingress |
| Dendritic Growth | Metal dendrites bridge conductors | Short circuit, leakage increase | Fine pitch, ionic contamination |
| Electrochemical Migration | Metal ions migrate under electric field | Leakage current increase | Surface contamination, moisture |
| Surface Leakage | Conductive moisture film on die surface | Parametric drift | Inadequate passivation |
| Copper Corrosion | Cu traces corrode at anode | Open circuit | Moisture + halide contamination |
**bHAST is the accelerated electrochemical reliability test that validates package corrosion resistance** — combining moisture, temperature, pressure, and electrical bias to rapidly assess whether semiconductor packages can prevent the metal corrosion, dendritic growth, and electrochemical migration that cause field failures in humid environments.