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Limitations of MTBF vs. Reliability Physics
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o Mean Time Between Failures (MTBF) calculations for electronics are based on
empirical failure rate formulas. Failure rates are determined for each device
and summed to determine a unit failure rate.
o Telcordia SR-332
o MIL-HDBK-217
o Reliability physics focuses on understanding the cause and effect of physical
processes and mechanisms that cause degradation and failure of materials
and components.
o Reliability physics considers the actual physical loads and geometries at the locations of
potential failure.
o When potential failure mechanisms are better understood, specific design issues can be
identified and adjusted.
INTRODUCTION
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o Telcordia SR-332, Issue 4:
o 𝜆𝐵𝐵 = 𝜆𝐺𝜋𝑄𝜋𝑆𝜋𝑇 𝑓𝑎𝑖𝑙𝑢𝑟𝑒𝑠 𝑝𝑒𝑟 109ℎ𝑜𝑢𝑟𝑠
o MIL-HDBK-217F, Notice 2:
o 𝜆𝑝 = 𝜆𝑏𝜋𝑄𝜋𝐸𝜋𝑇𝜋… 𝑓𝑎𝑖𝑙𝑢𝑟𝑒𝑠 𝑝𝑒𝑟 106ℎ𝑜𝑢𝑟𝑠
o 𝜆𝐺 and 𝜆𝑏 are base device failure rates.o Ceramic capacitor, silicon general purpose diode, 32 bit
microprocessor, etc.
o The 𝜋 values are multiplying factors based on quality, temperature, environment, and electrical properties.
o Failure rates of all devices of a system are summed to determine total unit failure rate. MTBF is then the inverse of this value.
o Failure rate is assumed to be steady-state and random, focusing only on the steady-state portion of the bathtub curve.
o It does not investigate how damage accumulates during this lifetime and why failures occur.
o Studies have shown failure rate calculations to vary widely between handbooks and from field data by both over and underpredicting failures.
o The MTBF number is not failure free, it is a time to 63% failure.
MTBF METHODOLOGY BACKGROUND
Failure Rate
(Failures/109 Hours)
MTBF
(Hours)
MTBF
(Years)
𝜆𝐵𝐵1 + 𝜆𝐵𝐵2 +⋯
= 510
1,960,784 223.8
Image Taken from Telcordia SR-332, Issue 4
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o Reliability physics focuses in more detail on the mechanisms that cause failure
during a device’s operational lifetime.
o In DfR’s experience, failures that occur during a device’s operation lifetime are
almost never truly random in nature.
o Common assembly failures like plated through hole fatigue due to thermal cycling, solder
fatigue due to thermal cycling or vibration, mechanical shock failures due to drop or other
events, defect related issues, accelerated capacitor wearout, etc. are caused by specific
stress conditions and geometries.
o Reliability physics analyses are concerned with investigating how and why
these failures occur and how they can be mitigated early in the design
process.
RELIABILITY PHYSICS METHODOLOGY BACKGROUND
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o MTBF and ADA of an example assembly.
o Telcordia SR-332 MTBF calculation
o Natural frequency, random vibration, mechanical shock, and thermal risk assessment.
o Mounted in a car door in the orientation shown
EXAMPLE ANALYSIS
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o MTBF analysis:
o Component characteristics for all devices in the bill of materials (datasheet values),
including component quality
o Operating environment
o Simulation-Based Reliability Physics Analysis:
o Component characteristics for all devices in the bill of materials
o Operating environment
o Optionally, PCBA layout, mounting conditions, etc. as the design matures
TYPICAL INPUTS – BOARD LEVEL RELIABILITY ANALYSIS
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o MTBF does not directly account for board geometry.
o Option to include lab/field data in failure rate calculations.
o Physics of failure (Sherlock) incorporates geometries to determine how
component layout and board dimensions affect potential failure modes.
BOARD GEOMETRY
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BOARD GEOMETRY – RELIABILITY PHYSICS
o Natural frequencies and mode shapes:
NF 1:
114.41 Hz
NF 2:
197.95 Hz
NF 3:
403.65 HzNF 4:
533.78 Hz
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MECHANICAL SHOCK AND VIBRATION – MTBF
o MTBF accounts for mechanical stresses at a qualitative level through the incorporation of the environment factor.o Ground, Fixed, Controlled - 𝐺𝐵o Ground, Fixed, Uncontrolled (Limited) - 𝐺𝐿o Ground, Fixed, Uncontrolled (Moderate) - 𝐺𝐹o Ground, Mobile - 𝐺𝑀o Airborne, Commercial - 𝐴𝐶o Space-based, commercial - 𝑆𝐶
o The environments label vibration/shock and temperature cycling stresses from “low” to “extreme.”
o For illustrative purposes, MTBF calculations are shown for a 1,000 gate CMOS digital integrated circuit at 40˚C across the different defined environments.
Environment Device Failure Rate (Failures/109 Hours)
𝐺𝐵 2.77
𝐺𝐿 3.324
𝐺𝐹 4.155
𝐺𝑀 5.54
𝐴𝐶 8.31
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o Simulation allows for incorporation
of specific profiles.
o For example, a shopping cart strikes the
vehicle door 3 times per year.
MECHANICAL SHOCK – RELIABILITY PHYSICS
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o Example driving profile over the
lifetime of the vehicle
RANDOM VIBRATION – RELIABILITY PHYSICS
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o MTBF empirically accounts for steady-state ambient temperature with temperature factor.o 𝜋𝑇 is determined by an Arrhenius relationship that assumes failure rate increases with increasing
temperature (an assumption that is not necessarily true).
o Like shock and vibration, temperature cycling is only qualitatively accounted for with environment factor in the Telcordia prediction.
o MIL-HDBK-217 does account for thermal cycling by generating an “interconnection” failure rate for SMT PCBs.
o Failure rate/MTBF are typically calculated separately at individual temperatures and weighted accordingly.
o Failure rates over temperature range for ground, mobile condition:
TEMPERATURE EFFECTS - MTBF
Temperature
(˚C)
Time at
Temperature (%)
Failure Rate
(Failures/109 Hours)
20 25 3.324
35 50 4.432
50 25 9.418
Combined - 5.402
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o Physics-based accelerated device wearout (electrolytic capacitors and semiconductor devices)o Similar to MTBF methods in that the main outcome is a steady-state failure rate.
o In contrast to MTBF methods, reliability physics methods can provide predictions regarding specific degradation mechanism in more detail, such as hot carrier injection, time dependent dielectric breakdown, negative bias temperature instability, and electro migration.
o Thermal cycling driven solder fatigueo Can be accounted for at the board level or in detail at the component level, if necessary
o Included in some failure rate calculations, but not others.
o Thermal mechanical effectso Can account for stresses caused by board expansion during temperature change
TEMPERATURE EFFECTS – RELIABILITY PHYSICS
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o MTBF
o Component failure rates and total assembly failure rates, MTBF numbers, and probability
of failure over lifetime - potentially very inaccurate
o Reliability Physics Analysis
o PCBA strain maps and maximum strain values for all components under specific loading
conditions, and the corresponding failure risks associated with such strains.
o Probability of failure for individual components that corresponds to both individual
loading conditions and combined loading conditions.
o Probability of failure for the entire assembly that corresponds to both individual loading
conditions and combined loading conditions.
FINAL OUTPUTS
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MTBFo Simple inputs/simple numerical output
o Provides a reliability estimate that is likely unrealistic.
o Does not account for the mechanisms that actually cause fatigue – how do things actually break?
o An MTBF number that proved acceptable for a past design does not guarantee success on a current design
o Higher risk of missing potentially important design features and their effects on reliability
o Even an MTBF analysis that predicts a large assembly failure rate does not help with mitigation strategies
o Can exclude design characteristics that are potentially important to fully understanding the reliability of the assembly
DISCUSSION
Reliability Physicso More inputs/Vastly more detailed outputs
o Uses specific geometries and loads to glean much more information about the design and its potential failure risks
o More potentially important design/environment features are captured
o The output data is directly actionable – it can be used to both predict failures and determine the most effective mitigation strategies
o It is also iterative, and consecutive simulations can be used to optimize failure mitigation strategies like mount point adjustment, component staking, or layout alterations
o Can be refined to include more detail throughout the design process
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o Questions?
o Tyler Ferris
THANK YOU!