The core of determining dominant stress by combining failure mode analysis (FMEA) with environmental comparison and material matching lies in "starting with the failure phenomenon, using environmental conditions as the background, and using material properties as the bridge," cross-validating these three aspects to accurately pinpoint the key stress factors leading to product degradation.
1. Failure Mode Analysis: Deducing "Causes" from "Results"
Failure modes are the most direct clues for determining dominant stress; different failure modes correspond to specific stress types.
Cracks and fractures occur:
Fatigue fracture (fatigue striations on the fracture surface) → pointing to alternating loads or vibration
Stress corrosion cracking (dendritic cracks) → tensile stress + corrosive medium working together
Thermal stress cracking (originating from a region with a large temperature gradient) → thermal cycling or rapid temperature change
Performance degradation without significant deformation:
Decrease in insulation resistance, increase in leakage current → electrochemical migration caused by damp heat + electrical bias (H3TRB)
Adhesive failure, delamination → mismatch in thermal expansion coefficients leading to interfacial stress
Surface degradation:
Powdering, discoloration, embrittlement → dominated by UV irradiation or thermo-oxidative aging
Rust, pitting corrosion → electrochemical corrosion under salt spray and humid environments
Key actions: Collect actual failed samples, perform fracture surface SEM analysis and corrosion product detection to confirm the microstructure and failure mechanism.
2. Environmental Comparison: Identifying "External Driving Forces"
Quantitatively compare the stress factors in the product's environment, identifying the 1-2 factors with the greatest intensity and longest duration.
|
Environment Type |
Dominant External Stress |
Typical Manifestations |
|
Tropical/Coastal |
High temperature and humidity + salt spray |
Hydrolysis, corrosion, electromigration |
|
Desert/Plateau |
Large diurnal temperature range (ΔT > 50℃) |
Thermal fatigue, weld cracking |
|
Industrial Area |
SO₂, NOₓ, ozone |
Chemical corrosion, rubber cracking |
|
Outdoor Exposure |
UV radiation + temperature cycling |
Polymer degradation, coating chalking |
|
Enclosed Space |
Constant temperature and low ventilation + volatile matter accumulation |
Slow oxidation, material precipitation |
Judgment Logic: If the product is used in an 85℃/85%RH environment, the damp heat stress is much higher than in a normal temperature dry environment; its impact should be assessed first.
3. Material Matching: Verifying "Intrinsic Sensitivity"
Different materials are highly sensitive to specific stresses; their response mechanisms need to be determined by considering material characteristics. Aluminum alloys and stainless steel: Prone to stress corrosion cracking under tensile stress and chloride ion environments.
Epoxy resin encapsulation materials: Highly hygroscopic, prone to expansion in humid and hot environments, leading to interfacial delamination.
Carbon fiber composites: Under high temperature and high strain rate, the matrix is prone to decomposition, creating weak points at the fiber/matrix interface.
Rubber seals: Prone to cracking in ozone environments, especially under static tensile conditions.
Key parameters: Pay attention to the material's coefficient of thermal expansion (CTE), moisture absorption rate, tensile strength, weather resistance rating, etc., and match the environmental stress.
4. Combining the Three Factors: Constructing a Closed-Loop Judgment
Cross-analysis of the above three aspects of information to form a closed-loop verification:
Observe the failure phenomenon → Preliminary judgment of possible stress types
Analyze the usage environment → Screen out the most likely external stresses under this environment
Check material properties → Verify whether the material is sensitive to this type of stress
Comprehensive judgment → If all three point to the same stress, it can be confirmed as the dominant factor
Example: A short circuit occurred on the PCB of an electronic control unit after use in Hainan
Failure mode: Dendrite growth, leakage current → Points to damp heat + bias voltage
Environment: High temperature and high humidity year-round (>35℃, >85%RH) → Strong damp heat stress
Material: FR-4 substrate is easily hygroscopic → Sensitive to damp heat
→ Conclusion: The dominant stress is damp heat coupled electrical stress, and HAST testing (JESD22-A101) should be used. Simulation Verification
Summary: Failure mode is the "symptom," environment is the "cause," and material is the "constitution." Only through collaborative analysis of all three can misjudgments be avoided, ensuring that accelerated aging tests truly reflect the product's reliability performance in actual use.
