Progressive defect is a defect that grows or worsens over time — starting small enough to pass initial tests but expanding under operational stress until eventual failure, requiring time-dependent reliability testing to detect and prevent field failures.
What Is a Progressive Defect?
- Definition: Defect that increases in severity during device operation.
- Initial State: Sub-critical size at manufacturing.
- Growth: Expands under electrical, thermal, or mechanical stress.
- Failure: Eventually reaches critical size causing malfunction.
Why Progressive Defects Matter
- Delayed Failures: Pass manufacturing test, fail after weeks/months of use.
- Reliability Risk: Major contributor to infant mortality and early-life failures.
- Detection Challenge: Require accelerated testing to reveal.
- Cost: Field failures are 10-100× more expensive than factory catches.
Common Types
Electromigration: Metal atoms migrate under current, voids grow until open circuit.
Stress Migration: Mechanical stress causes void nucleation and growth.
Corrosion: Chemical attack progressively degrades materials.
Crack Propagation: Mechanical cracks extend under thermal cycling.
Dielectric Breakdown: Oxide degradation progresses until catastrophic failure.
Hillock Growth: Metal extrusions grow until they cause shorts.
Growth Mechanisms
Electromigration: Current density drives atomic diffusion, voids grow at cathode.
Thermal Cycling: Coefficient of thermal expansion (CTE) mismatch causes stress accumulation.
Voltage Stress: Electric field accelerates charge trapping and oxide degradation.
Humidity: Moisture enables corrosion and ion migration.
Detection Methods
Accelerated Life Testing: Elevated stress to speed up defect growth.
Burn-in: Extended operation at high temperature and voltage.
Thermal Cycling: Repeated heating/cooling to stress interconnects.
HTOL (High Temperature Operating Life): Long-term stress at elevated temperature.
Inline Monitoring: Track parameter drift over time.
Modeling Growth
``python
def model_void_growth(initial_size, current_density, temperature, time):
"""
Model electromigration void growth using Black's equation.
"""
# Black's equation parameters
A = 1e-3 # Constant
n = 2 # Current density exponent
Ea = 0.7 # Activation energy (eV)
k = 8.617e-5 # Boltzmann constant
# Temperature in Kelvin
T = temperature + 273.15
# Growth rate
growth_rate = A (current_density n) math.exp(-Ea / (k * T))
# Final void size
final_size = initial_size + growth_rate * time
return final_size
# Example
initial_void = 10 # nm
final_void = model_void_growth(
initial_size=10,
current_density=2e6, # A/cm²
temperature=125, # °C
time=1000 # hours
)
print(f"Void growth: {initial_void}nm → {final_void:.1f}nm")
``
Screening Strategies
Extended Burn-in: Longer duration to allow defects to grow and fail.
Elevated Stress: Higher temperature/voltage to accelerate growth.
Multi-Stage Testing: Progressive stress levels to catch different defect types.
Parametric Monitoring: Track resistance, leakage, speed over time.
Progressive vs Other Defects
Critical: Immediate failure, caught in test.
Latent: Dormant, sudden failure later.
Progressive: Gradual growth, predictable failure.
Intermittent: Comes and goes, hard to catch.
Reliability Prediction
Weibull Analysis: Model time-to-failure distribution.
Arrhenius Acceleration: Predict field lifetime from accelerated test.
Physics of Failure: Model based on failure mechanisms.
Trend Analysis: Extrapolate parameter drift to predict failure time.
Best Practices
- Accelerated Testing: Use elevated stress to reveal progressive defects.
- Parametric Trending: Monitor parameter drift during burn-in.
- Process Control: Minimize initial defect size through tight process control.
- Design Margins: Ensure structures can tolerate some defect growth.
- Field Monitoring: Track early returns to identify progressive failure modes.
Typical Timescales
- Electromigration: 1000-10000 hours to failure.
- TDDB: 100-1000 hours under stress.
- Thermal Cycling: 500-5000 cycles to crack propagation.
- Corrosion: Months to years depending on environment.
Progressive defects are reliability time bombs — starting small but growing inexorably until failure, making accelerated testing and robust screening essential to prevent field failures and maintain product reliability.