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

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.