Electronics Guide

Failure Rate Phases

The bathtub curve consists of three distinct phases:

• Infant mortality period: The initial phase characterized by a decreasing failure rate, where defective units with latent manufacturing defects, weak components, or workmanship errors fail early
• Useful life period: The middle phase with a relatively constant, low failure rate, representing random failures during normal operation
• Wear-out period: The final phase with an increasing failure rate as components age and materials degrade beyond their useful life

Environmental stress screening specifically targets the infant mortality period, aiming to precipitate failures before products ship to customers.

Infant Mortality Characteristics

Infant mortality failures exhibit distinct characteristics that differentiate them from other failure types:

• Defect-related: These failures result from inherent defects introduced during component manufacturing, assembly, or handling
• Time-dependent precipitation: The defects manifest as failures over time, with higher stress conditions accelerating the precipitation process
• Decreasing hazard rate: As defective units fail and are removed, the remaining population becomes stronger, causing the failure rate to decrease
• Screening effectiveness: Properly designed screens can remove the majority of infant mortality defects, dramatically improving field reliability

Bathtub Curve Analysis

Analyzing field failure data through the bathtub curve framework enables optimization of screening strategies:

• Failure mode identification: Categorizing failures as infant mortality, random, or wear-out based on timing and root cause
• Screen effectiveness assessment: Comparing pre-screen and post-screen failure rates to quantify screening benefits
• Duration optimization: Determining the point at which infant mortality transitions to useful life to set optimal screen duration
• Weibull analysis: Using Weibull distribution to characterize failure patterns and predict failure rates at different life stages
• Reliability growth: Tracking improvements in the bathtub curve as design and process improvements reduce defect populations

Burn-in Mechanisms

Several physical and chemical mechanisms cause defects to manifest during burn-in:

• Electromigration: Electric current density causes metal atom movement in conductors, accelerating failures in weak interconnects
• Hot carrier injection: High-energy carriers in transistors cause oxide damage that accumulates over time
• Time-dependent dielectric breakdown: Gate oxide defects progress to failure under electrical stress
• Thermal expansion mismatch: Differential expansion between materials stresses interfaces and connections
• Metal corrosion: Elevated temperature accelerates corrosion processes in contaminated areas
• Intermetallic growth: High temperature promotes intermetallic compound formation at solder interfaces

Static vs. Dynamic Burn-in

Burn-in can be performed with different levels of circuit activity:

• Static burn-in: Products are powered but operate in a quiescent state with minimal circuit activity, primarily stressing power supply circuits and leakage-related defects
• Dynamic burn-in: Products execute test patterns or operational software that exercises circuit functions, stressing signal paths, logic circuits, and memory elements
• Combined approaches: Some burn-in procedures alternate between static and dynamic modes to stress different failure mechanisms
• Effectiveness comparison: Dynamic burn-in generally provides higher defect precipitation rates but requires more complex test equipment and monitoring

Burn-in Equipment

Burn-in requires specialized equipment to maintain controlled conditions while powering multiple units:

• Burn-in chambers: Temperature-controlled enclosures that maintain uniform temperature distribution across all units under test
• Burn-in boards: Custom circuit boards that interface units under test with power supplies and test electronics
• Driver electronics: Systems that provide power and, for dynamic burn-in, stimulus signals to units under test
• Monitoring systems: Equipment that measures unit parameters during burn-in to detect failures in real time
• Data logging: Systems that record temperature, time, and unit status throughout the burn-in process
• Safety systems: Over-temperature protection, smoke detection, and fire suppression systems

Temperature Selection

Burn-in temperature significantly affects both screening effectiveness and process economics:

• Acceleration factor: Higher temperatures accelerate failure mechanisms according to Arrhenius relationship, reducing required burn-in time
• Component ratings: Temperature must remain within component maximum ratings to avoid damaging good units
• Junction temperature: For active devices, junction temperature (not ambient) determines stress level and must be calculated considering power dissipation
• Typical ranges: Common burn-in temperatures range from 85 degrees Celsius to 125 degrees Celsius, with higher temperatures for shorter durations
• Derating considerations: Some specifications require derating maximum temperature when burn-in is performed

Duration Optimization

Burn-in duration must be sufficient to precipitate defects without excessive cost:

• Infant mortality window: Duration should cover the period during which infant mortality failures occur at the burn-in stress level
• Economic analysis: Balancing burn-in costs against warranty and field failure costs to optimize duration
• Typical durations: Burn-in periods commonly range from 24 to 168 hours, depending on product reliability requirements
• Diminishing returns: Extending burn-in beyond the infant mortality period provides minimal additional benefit while increasing cost
• Product-specific optimization: Duration should be based on actual failure data and adjusted as product and process mature

Electrical Stress Conditions

Electrical stress parameters affect which failure mechanisms are activated:

• Voltage stress: Operating at or near maximum rated voltage increases stress on dielectrics and accelerates voltage-dependent failures
• Current stress: Higher operating currents accelerate electromigration and thermal-related failures
• Clock frequency: For dynamic burn-in, operating at elevated clock frequencies increases switching stress
• Power cycling: Periodic power cycling exercises power-up related failure modes and thermal cycling effects
• Test pattern selection: Patterns that maximize circuit activity and toggle rates provide more effective screening

Profile Validation

Burn-in profiles must be validated to ensure effectiveness:

• Failure rate monitoring: Tracking failures during burn-in to verify defect precipitation is occurring
• Post-burn-in testing: Comprehensive testing after burn-in to detect degradation or failures
• Field correlation: Comparing field failure rates for burned-in versus non-burned-in products
• Mechanism verification: Analyzing burn-in failures to confirm targeted failure mechanisms are being precipitated
• Sample evaluation: Periodic destructive analysis of burn-in units to detect any damage to good units

Temperature Cycling Mechanisms

Temperature cycling activates different failure mechanisms than steady-state burn-in:

• CTE mismatch stress: Materials with different coefficients of thermal expansion create stress at interfaces as temperature changes
• Solder fatigue: Repeated strain in solder joints causes crack initiation and propagation
• Wire bond stress: Differential expansion between bond wire and substrate stresses bond interfaces
• Die attach fatigue: Repeated stress in die attach material causes delamination and cracking
• Package cracking: Brittle package materials may crack under thermal stress
• PCB delamination: Repeated thermal stress can cause layer separation in multilayer boards

Cycling Parameters

Temperature cycling effectiveness depends on several key parameters:

• Temperature range: The difference between high and low temperatures determines stress magnitude, typically spanning from -40 to +85 degrees Celsius or wider
• Dwell time: Duration at temperature extremes allows thermal equilibration and maximum stress development, typically 10 to 30 minutes
• Transition rate: Rate of temperature change, typically 5 to 20 degrees Celsius per minute for screening applications
• Number of cycles: Total cycles required to precipitate defects, commonly 10 to 100 cycles for screening
• Powered vs. unpowered: Cycling with power applied adds electrical stress and enables real-time monitoring

Air-to-Air vs. Liquid-to-Liquid Cycling

Different thermal cycling methods offer trade-offs between stress severity and equipment complexity:

• Air-to-air cycling: Products transfer between hot and cold air chambers, providing moderate transition rates and good process control
• Liquid-to-liquid cycling: Products immerse in hot and cold inert liquid baths, providing very rapid transitions and severe thermal shock
• Single chamber cycling: Temperature changes within a single chamber, providing slower transitions but simpler equipment
• Transition rate impact: Faster transitions create more severe stress but may exceed some component capabilities
• Equipment considerations: Liquid systems require compatible fluids and handling procedures for electronic assemblies

Temperature Cycling Equipment

Specialized equipment supports temperature cycling operations:

• Thermal shock chambers: Dual-zone chambers with rapid transfer mechanisms for moving products between temperature extremes
• Environmental chambers: Single-zone chambers capable of temperature ramping for slower cycling
• Product carriers: Fixtures that support products and enable rapid thermal equilibration
• Temperature monitoring: Thermocouples or data loggers that verify actual product temperature
• Transfer mechanisms: Automated systems that move products between zones at programmed intervals

Vibration-Precipitated Defects

Random vibration activates mechanical failure mechanisms:

• Solder joint cracks: Vibration stress opens latent cracks in solder joints caused by manufacturing defects
• Loose hardware: Improperly torqued fasteners or loose components become intermittent under vibration
• Cold solder joints: Joints with poor metallurgical bonding separate under mechanical stress
• Wire and cable defects: Poorly crimped connections or damaged conductors fail under flexing
• Component attachment: Inadequately bonded components separate from substrates
• PCB cracks: Stress concentrations at mounting holes or component locations may propagate

Random Vibration Screening

Random vibration provides the most effective mechanical screening:

• Broad frequency spectrum: Random vibration simultaneously excites all resonant frequencies, stressing all mechanical elements
• Power spectral density: The vibration level is specified by PSD in g-squared per Hertz across the frequency range
• Frequency range: Typical screening covers 20 to 2000 Hertz, encompassing most structural resonances
• GRMS level: Overall vibration severity expressed as root-mean-square acceleration, typically 3 to 10 Grms for screening
• Duration: Screen duration commonly ranges from 5 to 30 minutes, based on defect precipitation rate

Vibration Equipment

Vibration screening requires specialized equipment and fixturing:

• Electrodynamic shakers: Shaker systems capable of producing controlled random vibration across the required frequency range
• Vibration controllers: Digital controllers that generate the specified random vibration spectrum and monitor response
• Fixturing: Custom fixtures that transmit vibration to products while maintaining proper orientation and support
• Accelerometers: Sensors that measure actual vibration levels on fixtures and products
• Power amplifiers: Amplifiers that drive shaker systems with sufficient power for required vibration levels

Fixture Design Considerations

Effective vibration screening requires careful fixture design:

• Resonance avoidance: Fixtures should have resonances above the test frequency range to prevent amplification and non-uniform stress
• Uniform transmission: All products on the fixture should experience similar vibration levels
• Secure mounting: Products must be held securely without inducing artificial stress concentrations
• Electrical interface: Provisions for powering and monitoring products during vibration if required
• Mass loading: Total mass on the shaker must remain within system capacity

Temperature and Vibration Combined

Simultaneous temperature and vibration stress is particularly effective:

• Synergistic effects: Thermal stress reduces material strength while vibration applies mechanical load, precipitating defects more rapidly
• CTE-related defects: Thermal expansion combined with vibration accelerates solder joint and interconnect failures
• Equipment requirements: Combined testing requires chambers that can provide both thermal control and vibration input simultaneously
• AGREE testing: Advisory Group on Reliability of Electronic Equipment developed early combined environment screening specifications
• Profile optimization: Combined stress levels may be lower than individual screening levels to avoid over-stress

Temperature Cycling with Vibration

Applying vibration during temperature cycling adds mechanical stress during thermal transitions:

• Transition stress: Vibration during temperature transitions when CTE mismatch stress is maximum provides enhanced screening
• Sequential vs. simultaneous: Some approaches apply vibration only during dwell periods rather than throughout the cycle
• Process efficiency: Combined cycling reduces total screening time compared to sequential application
• Equipment complexity: Requires chambers capable of both rapid temperature change and vibration

Multi-Axis Vibration

Applying vibration in multiple axes provides more thorough mechanical screening:

• Three-axis testing: Sequential or simultaneous vibration in X, Y, and Z axes stresses all mechanical elements regardless of orientation
• Six degrees of freedom: Some systems provide both translational and rotational vibration for complete mechanical excitation
• Reorientation approach: Products can be reoriented between vibration tests as an alternative to multi-axis equipment
• Cost considerations: Multi-axis systems are significantly more expensive than single-axis shakers

Humidity and Temperature

Combined temperature and humidity exposure activates moisture-related failure mechanisms:

• Corrosion acceleration: Elevated temperature and humidity accelerate corrosion processes
• Moisture absorption: High humidity exposure tests moisture sensitivity of components and materials
• Condensation testing: Temperature cycling through the dew point can precipitate moisture-related failures
• Popcorning: Moisture absorbed by plastic packages can cause damage during subsequent thermal exposure
• 85/85 testing: Standard highly accelerated stress test condition of 85 degrees Celsius and 85 percent relative humidity

HALT Objectives

HALT serves different purposes than production screening:

• Design margin discovery: Finding actual operating and destruct limits rather than merely verifying specification compliance
• Weakness identification: Discovering failure modes that would not appear in normal testing or field use
• Robust design: Providing information to improve design margins before production begins
• Root cause focus: Each failure is analyzed and design improved, iteratively strengthening the product
• Time compression: Rapidly exposing weaknesses that would take months or years to discover otherwise

HALT Stress Application

HALT applies stress in a progressive manner to find limits:

• Cold step stress: Temperature is reduced in steps until operating or destruct limit is reached
• Hot step stress: Temperature is increased in steps to find upper operating and destruct limits
• Rapid thermal transitions: Products experience thermal shock rates of 40 to 70 degrees Celsius per minute or higher
• Vibration step stress: Vibration level is increased in steps to find mechanical limits
• Combined stress: Temperature extremes and vibration are applied simultaneously at increasing levels
• Precipitation screen: Final combined stress screen designed to precipitate remaining latent defects

HALT Equipment

HALT requires specialized chambers with unique capabilities:

• Rapid temperature change: Liquid nitrogen cooling and powerful heaters enable extremely rapid temperature transitions
• Repetitive shock vibration: Pneumatic actuators provide six-degree-of-freedom vibration with broad frequency content
• Wide temperature range: Chambers typically span from -100 to +200 degrees Celsius
• High vibration levels: Systems capable of 50 Grms or higher broadband random vibration
• Product monitoring: Provisions for monitoring product function throughout testing

HALT Process and Analysis

Effective HALT requires systematic process execution:

• Baseline functional test: Complete functional verification before stress application begins
• Operating limit determination: Finding where product fails to operate but recovers when stress is reduced
• Destruct limit determination: Finding where product permanently fails and cannot recover
• Failure analysis: Detailed analysis of each failure to understand root cause and identify corrective actions
• Corrective action implementation: Design changes to address weaknesses found during HALT
• Verification testing: Repeated HALT after corrections to verify improvements

HASS Development from HALT

HASS profiles are derived from HALT results:

• Operating limit basis: HASS stress levels are set below operating limits found in HALT to avoid failing good units
• Safety margins: Typical HASS temperatures are 10 to 20 degrees Celsius below HALT operating limits
• Vibration levels: HASS vibration is typically 50 to 80 percent of HALT vibration operating limit
• Design maturity: HALT must be completed and design issues corrected before implementing HASS
• Process validation: HASS profiles are validated using seeded defect studies and proof-of-screen testing

HASS Profile Elements

Typical HASS profiles include several stress elements:

• Thermal shock: Rapid transitions between temperature extremes, typically in single-digit minutes per cycle
• Temperature dwell: Brief periods at temperature extremes to achieve thermal equilibration
• Vibration application: High-level random vibration applied during thermal transitions and dwells
• Power cycling: Periodic power cycling during temperature transitions
• Functional monitoring: Continuous or periodic functional verification during stress

Proof of Screen

HASS effectiveness must be validated through proof-of-screen testing:

• Seeded defects: Intentionally introduced defects verify the screen detects targeted failure modes
• Detection rate: Measuring the percentage of seeded defects detected by the screen
• False positive rate: Verifying good units are not failing during screening
• Screen strength: Evaluating whether the screen is too weak (escapes) or too strong (yield loss)
• Correlation studies: Comparing HASS results with field failure data

HASS vs. Traditional ESS

HASS offers advantages over traditional screening approaches:

• Shorter duration: HASS screens typically complete in 1 to 4 hours versus 24 to 168 hours for traditional burn-in
• Higher precipitation rate: Combined stress at aggressive levels precipitates defects more rapidly
• Broader defect coverage: Combined thermal and mechanical stress addresses more failure mechanisms
• Lower total stress: Shorter exposure to stress reduces cumulative damage to good units
• Equipment requirements: HASS requires specialized HALT/HASS chambers rather than standard burn-in equipment
• Development investment: HASS requires completed HALT and proof-of-screen validation

Arrhenius Model

The Arrhenius model describes temperature acceleration of chemical and physical processes:

• Exponential relationship: Failure rate increases exponentially with temperature according to the activation energy
• Activation energy: The energy barrier for the failure mechanism, typically 0.3 to 1.2 electron volts for electronics
• Acceleration factor: Ratio of failure rates at two temperatures, enabling time transformation between stress levels
• Temperature conversion: Absolute temperature in Kelvin must be used in calculations
• Mechanism dependence: Different failure mechanisms have different activation energies

Coffin-Manson Model

The Coffin-Manson model describes fatigue life under cyclic strain:

• Low-cycle fatigue: Relates cycles to failure to strain range in the plastic deformation regime
• Exponent value: Fatigue exponent typically ranges from 1.5 to 3 for solder materials
• Temperature range effect: Larger temperature swings cause more strain and fewer cycles to failure
• Application: Widely used for solder joint fatigue life prediction
• Combined models: Often combined with Arrhenius for temperature-dependent fatigue

MIL-HDBK-217 and Reliability Prediction

Standard methodologies provide failure rate prediction frameworks:

• Part stress analysis: Calculating failure rates based on component stress factors
• Environmental factors: Modifying base failure rates for operating environment severity
• Quality factors: Adjusting predictions based on component quality level
• Limitations: Predictions may not correlate well with field data for modern technologies
• Alternative methods: Physics-of-failure approaches provide mechanism-specific predictions

Weibull Analysis

Weibull distribution analysis characterizes failure populations:

• Shape parameter (beta): Values less than 1 indicate infant mortality, equal to 1 indicates random failures, greater than 1 indicates wear-out
• Scale parameter (eta): Characteristic life at which 63.2 percent of population has failed
• Probability plotting: Graphical analysis enables parameter estimation from failure data
• Confidence intervals: Statistical bounds on parameter estimates based on sample size
• Mixed distributions: Multiple failure modes can be separated and analyzed individually

Defect Detection Rate

Measuring the proportion of defective units identified by screening:

• Detection efficiency: Percentage of defective units that fail during screening
• Escape rate: Percentage of defective units that pass screening and escape to field
• Defect type coverage: Analyzing which defect types are detected versus which escape
• Measurement challenges: True defect population size is often unknown, requiring estimation methods
• Seeded defect studies: Introducing known defects to measure detection capability

Screening Strength

Quantifying stress severity and precipitation effectiveness:

• Stress level metrics: Temperature, vibration level, and duration parameters that define screen severity
• Equivalent time: Converting different stress profiles to equivalent time at a reference condition
• Acceleration factors: Ratio of failure rate under screen conditions to field conditions
• Precipitation fraction: Estimated fraction of latent defects precipitated by the screen
• Screening factor (SS): Commonly used metric expressing screen effectiveness as a multiplier

Yield Impact

Assessing the effect of screening on production yield:

• Screen-induced failures: Good units that fail due to screening stress rather than inherent defects
• Yield loss cost: Economic impact of failing good units during screening
• Optimization trade-off: Balancing defect detection against yield loss
• Over-screening indicators: Failure analysis revealing damage patterns inconsistent with defect precipitation
• Process stability: Yield loss increases indicate process or design issues requiring attention

Field Reliability Improvement

Measuring the ultimate impact of screening on field performance:

• Before/after comparison: Comparing field failure rates for screened versus unscreened populations
• Early life failures: Reduction in infant mortality failures after screening implementation
• Warranty cost reduction: Economic benefit from reduced warranty claims
• Customer satisfaction: Improvements in customer-facing quality metrics
• Return on investment: Comparing screening costs against field failure cost avoidance

Design for Reliability

Robust design reduces the defect population requiring screening:

• Derating: Operating components well below maximum ratings provides margin against variation
• Thermal design: Adequate cooling prevents thermally-induced degradation and failures
• Mechanical robustness: Design for shock and vibration resistance reduces mechanical failures
• Material selection: Compatible materials with appropriate properties for the application environment
• Design margin analysis: Systematic verification that design margins are adequate for manufacturing variation

Process Control

Manufacturing process control minimizes defect introduction:

• Statistical process control: Monitoring key parameters to maintain process stability
• Process capability: Ensuring processes are capable of meeting specifications consistently
• Preventive maintenance: Equipment maintenance prevents process drift and defect introduction
• Contamination control: Cleanroom and ESD practices prevent contamination-related defects
• Training and certification: Ensuring operators are qualified for critical processes

Component Quality

Component quality directly affects infant mortality rates:

• Supplier qualification: Rigorous evaluation of supplier capability and quality systems
• Incoming inspection: Verification testing to detect defective components before assembly
• Component burn-in: Some high-reliability applications require component-level burn-in
• Counterfeit prevention: Supply chain controls to prevent introduction of counterfeit components
• Specification enforcement: Ensuring components meet all specified parameters

Continuous Improvement

Systematic improvement reduces defect levels over time:

• Failure analysis: Root cause analysis of all screening and field failures
• Corrective action: Implementing design and process changes to eliminate defect sources
• Trend monitoring: Tracking defect rates over time to identify emerging issues
• Screen tailoring: Adjusting screen parameters as defect types and rates change
• Knowledge sharing: Applying lessons learned across products and processes

Facility and Equipment

Screening operations require appropriate infrastructure:

• Chamber capacity: Sufficient burn-in and environmental chamber capacity for production volume
• Power infrastructure: Adequate electrical power for burn-in loads and chamber operation
• Cooling systems: Heat removal capability for burn-in chambers and facilities
• Safety systems: Fire detection, suppression, and ventilation for burn-in areas
• Material handling: Efficient flow of products into and out of screening operations

Process Documentation

Formal documentation ensures consistent screening execution:

• Screening procedures: Detailed instructions for equipment operation and product handling
• Profile specifications: Documented stress parameters including tolerances and verification requirements
• Failure criteria: Clear definition of pass/fail criteria and disposition procedures
• Data recording: Requirements for documenting screening conditions and results
• Change control: Process for evaluating and approving changes to screening parameters

Quality System Integration

Screening must integrate with overall quality management:

• Traceability: Linking screening records to product serial numbers and lot codes
• Non-conformance handling: Procedures for managing units that fail screening
• Statistical monitoring: Tracking screening failure rates and yields over time
• Customer requirements: Ensuring screening meets contractual and regulatory requirements
• Audit readiness: Documentation supporting quality system audits

Cost Optimization

Screening economics require ongoing optimization:

• Capacity planning: Balancing equipment investment against production requirements
• Cycle time reduction: Minimizing screening duration while maintaining effectiveness
• Automation: Reducing labor costs through automated handling and monitoring
• Energy efficiency: Managing power consumption of chambers and support systems
• Screen tailoring: Adjusting screening as product and process mature to optimize cost-benefit

Military Standards

Military standards established many ESS practices:

• MIL-HDBK-344: Environmental Stress Screening of Electronic Equipment, providing ESS methodology guidance
• MIL-STD-2164: Environment Stress Screening Process for Electronic Equipment
• MIL-STD-810: Environmental Engineering Considerations and Laboratory Tests
• MIL-HDBK-217: Reliability Prediction of Electronic Equipment
• MIL-STD-883: Test Methods and Procedures for Microelectronics, including burn-in procedures

Industry Standards

Industry organizations provide additional guidance:

• JEDEC standards: Semiconductor industry standards including JESD22-A108 for temperature cycling and JESD22-A104 for thermal shock
• IPC standards: Electronics assembly standards addressing stress screening requirements
• SAE standards: Automotive industry standards for component and system qualification
• Telcordia standards: Telecommunications equipment reliability requirements
• IEC standards: International standards for environmental testing (IEC 60068 series)

Application-Specific Requirements

Different applications have varying screening requirements:

• Aerospace: AS9100 quality requirements and extensive screening for flight hardware
• Medical devices: FDA requirements and IEC 60601 compliance affecting screening practices
• Automotive: AEC-Q100 and AEC-Q200 qualification requirements for components
• Consumer electronics: Generally reduced screening requirements driven by cost constraints
• Industrial: Application-dependent requirements based on safety and reliability needs