Physics of Failure
PoF is an alternative approach / methodology to reliability that is focused on failure mechanism, failure site & root cause analysis instead of the more conventional approach that looks at failure modes & effects alone. The PoF approach characterizes reliability through lifetime distributions (probability distribution of frequency of fails v/s time) instead of hazard rates (failure rate v/s time). PoF approaches involve the followings steps:
1. Study of the hardware configuration: geometry, design, materials, structure
2. Study of life cycle loads: operational loads (power, voltage, bias, duty cycle) & environmental loads (temperature, humidity, vibration, shock)
3. Stress analysis: Stress-strength distributions/interference, cumulative damage assessment & endurance interference, FMEA, hypothesize failure mechanisms, failure sites & associate failure models, root cause analysis, calculate RPN's to rank & prioritize failures.
4. Reliability assessment: Rel metrics characterization, life estimation, operating/design margin estimation.
5. Interpret & apply results: Design tradeoffs & optimization, ALT planning & development, PHM & HUMS planning.
PoF is an alternative approach / methodology to reliability that is focused on failure mechanism, failure site & root cause analysis instead of the more conventional approach that looks at failure modes & effects alone. The PoF approach characterizes reliability through lifetime distributions (probability distribution of frequency of fails v/s time) instead of hazard rates (failure rate v/s time). PoF approaches involve the followings steps:
1. Study of the hardware configuration: geometry, design, materials, structure
2. Study of life cycle loads: operational loads (power, voltage, bias, duty cycle) & environmental loads (temperature, humidity, vibration, shock)
3. Stress analysis: Stress-strength distributions/interference, cumulative damage assessment & endurance interference, FMEA, hypothesize failure mechanisms, failure sites & associate failure models, root cause analysis, calculate RPN's to rank & prioritize failures.
4. Reliability assessment: Rel metrics characterization, life estimation, operating/design margin estimation.
5. Interpret & apply results: Design tradeoffs & optimization, ALT planning & development, PHM & HUMS planning.
Use HW configs and life cycle loading to understand & prioritize failure mechanisms/root causes, to use a failure model (LSR) to develop a lifetime distribution (Weibull, exponential, lognormal, etc) to calculate reliability metrics.
Helps prioritize risks/failures, plan for risk mitigation, design tradeoffs/margins and estimate reliability metrics.
Defines reliability as probability of meeting a given lifetime/ service life at a given confidence level.
Reliability / Life Estimation
Run ALT & collect TTF data
Fit TTF data to life distribution (for eg, 2P Weibull), to generate eta (test) , beta (test = field, const for given failure mechanism)
Use eta as life characteristic (MTTF), and model eta (test) as function of (test) stress, using a LSR
Estimate LSR model parameters
Knowing LSR, and using stress = use case/ field condition, calculate eta (field)
Use eta (field) and beta (test=field) to model life distribution under use case / field condition, to predict useful life and other reliability metrics
Use POF to understand failure mechanism > Use LSR [Nf = f(stress)] > Lifetime distribution > Calculate rel metrics
ALT to get TTF > TTF distribution (Test) > Estimate LSR model parameters > Lifetime distribution > Calculate rel metrics
Extrapolating Accelerated Test TTF to Predict Use Level Lifetime
Collect TTF at multiple accelerated stress levels.
Fit TTF data to life-distribution (for eg. 2P Weibull) for each stress level, and estimate parameters - each distribution with different eta but same beta (for the same failure mechanism).
Life distribution characteristic (eta) is a function of stress & is equated to stress level using a given Life-Stress-Relationship (LSR) - for eg Eyering Model
Estimate model constants
Now, knowing model constants, estimate eta at various stress levels, including at use level. And knowing beta (for the given failure mechanism) use-level life distribution can be predicted.
Collect TTF at multiple accelerated stress levels.
Fit TTF data to life-distribution (for eg. 2P Weibull) for each stress level, and estimate parameters - each distribution with different eta but same beta (for the same failure mechanism).
Life distribution characteristic (eta) is a function of stress & is equated to stress level using a given Life-Stress-Relationship (LSR) - for eg Eyering Model
Estimate model constants
Now, knowing model constants, estimate eta at various stress levels, including at use level. And knowing beta (for the given failure mechanism) use-level life distribution can be predicted.
Environmental Stress Screens (ESS)
Product Qualification
BI/HTOL/LU/ESD(HBM/CDM)/THB or BHAST/PCT
BI/HTOL used to calculate ELFR & DPPM during useful life, that can be used to generate the FIT rate
BI/HTOL used to calculate ELFR & DPPM during useful life, that can be used to generate the FIT rate
Device level : ESD/LU, HTOL, EM
Pkg level: TCG/B/J, UHAST, BHAST, HTS, THB (85/85), Autoclave, Preconditioning, MR/Hammer
BLRT: TC, bend, drop/shock, vibration
Functional: Acoustic, Electromagnetic, Electrical compliance
BLRT
Strain gauges, electrically monitor resistance of daisy chained balls
Temperature cycling: -40 to 125C
Vibration: Axes, RMS acceleration/displacement/velocity, 3G, time per axis
Drop / Shock: Axes, no of pulses, 200G -1500G, ms of pulse
Monotonic or cyclic bend
Dye & Pry
Fails - Exact or Censored
Failure times are either EXACT or CENSORED
Right censored or suspended = Unit has survived when test is stopped
Interval censored = Unit has failed between "Last Inspection" and now
Left censored = Similar to Interval Censoring but "Last Inspection"/Start is 0
FIT Rates
h(t) = hazard rate = f(t)/R(t)
Hazard rate is instantaneous failure rate. But usually, average failure rate for a given time period is more useful, for eg. failure rate per hour.
FIT rate is defined as ppm failure per 1000 hours or number of fails per 10^9 device hours.
Multiply hourly fail-rate by 10^9 to generate FIT rate.
Hourly fail rate = No of rejects / (No of devices x No of hours x AF)
FIT rate = Hourly fail rate x 10^9
No of rejects is determined by Chi-square distribution = [(x^2)/2], commonly at 60% confidence (alpha) and dof: 2r + 2
Power cycling
Motherboard, mux board & fan controller board - design & fabrication, DAQ system, Labview software, package + socket + heat sink + wind tunnel/flow channel, system integration, system setup & debug, calibration & testing.
Failure Mechanisms
Coffin Manson model predicts lifetime for LCF failure for solder joints, assuming IPL model for non-thermal mechanical stress (from strain range per cycle).
Engelmaier improves on the CM model by showing that LCF failure lifetime is related to damage per cycle from plastic and viscoplastic deformation, that in turn is related to the temperature swing per cycle.
Norris Landzberg adds an Arrehenius/exponential terms to the Engelmaier model to show that LCF is also temperature dependent - and is a thermo-mechanical failure mechanism.
The N-L model adds an exponential / Arrhenius term to the Engelmaier model (that is based on the CM / IPL model) to predict Nf tor LCF failure for solder joint fatigue.
Failure Analysis Techniques: Resolution
STM, AFM, EELS, SIMS, TEM (Angstroms) < AES (2nm) < XPS (5nm) < SEM (10nm) < BSE/EBSD (30nm) < EDX/WDX/XRF (0.3u) < FTIR (3u)
Failure Analysis: Tools & Techniques
Microstructural analysis:
Topography: SEM (low voltage, inelastic collisions, higher resolution, low contrast) /BSE (high voltage, elastic collisions, lower resolution, high contrast)
Morphology: (lattice geometry, crystallographic structure) EBSD/TEM/AFM/STM
Material analysis:
Elemental: EDX/WDX/XRF
Chemical: (structural bonds, oxidation states) AES/XPS/EELS/SIMS/FTIR
STM, AFM, EELS, SIMS, TEM (Angstroms) < AES (2nm) < XPS (5nm) < SEM (10nm) < BSE/EBSD (30nm) < EDX/WDX/XRF (0.3u) < FTIR (3u)
Failure Analysis: Tools & Techniques
Microstructural analysis:
Topography: SEM (low voltage, inelastic collisions, higher resolution, low contrast) /BSE (high voltage, elastic collisions, lower resolution, high contrast)
Morphology: (lattice geometry, crystallographic structure) EBSD/TEM/AFM/STM
Material analysis:
Elemental: EDX/WDX/XRF
Chemical: (structural bonds, oxidation states) AES/XPS/EELS/SIMS/FTIR
Interaction between primary electrons & matter: SEM, TEM, BSE, EBSD, EDX & WDX
Interaction between primary X-Rays & matter: XPS, AES, XRF
SEM: Inelastic collisions, low energy, low contrast, higher resolution
BSE: Elastic collisions, high energy, high contrast, low resolution
Other techniques: Opticals, X-Rays, CSAM, Curve Trace, TDR, IR & thermal imaging, SQUID, LSM (LIVA/OBIC for opens & TIVA/OBIRCH for shorts), x-sections, P-laps & FIB cuts
Flow: C/T, TDR, Delid/optical, X-Ray, CSAM, XS, SEM/EDX, FIB, P-lap, SQUID, LSM
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