Reliability

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Reliability
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• Definition of Terms
• - Failure Modes.
• - The Price of Reliability.
• Reliability Functions
• - Cost Functions
• - The Bathtub Curve.
• - Useful Lifetime.
• Reliability Impact Factors
• - Environmental Factors.
• - Design Manufacture.
• - Accelerated Lifetime.
• Case Study
• -Hi-Rel Processing.

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Definitions

• Reliability - The ability of an item to perform
  its required function under defined conditions
  for a stated period of time.
• Failure The termination of the ability of an
  item to perform its required function.

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Degrees of Failure

• Failures may be SUDDEN (non-predictable) or
  GRADUAL (predictable). They may also be PARTIAL
  or COMPLETE.
• A Catastrophic failure is both sudden and
  complete.
• A Degradation failure is both gradual and partial.

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Causes of Failure

• Misuse failures attributable to the application
  of stresses beyond the stated capabilities of the
  item.
• Inherent Weakness failures attributable to
  weakness inherent in the item itself when
  subjected to stresses within the stated
  capabilities of the item.

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Reliability vs Cost

• Three separate cost factors related to the
  reliability of an item throughout its life
• Design Development
• Production
• Maintenance Repair

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Cost-Reliability Functions
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MTBF MTTF

• Mean Time Between Failures Applies to
  repairable items.
• Mean Time To Failure Applies to non-repairable
  items.
• Both of these terms indicate the average time an
  item is expected to function before failure.

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Failure Rate vs Time
Early Failures substandard components,
manufacturing faults. Random Failures this is
the useful lifetime of the item. Reliability is
predictable in this region. End-of-Life Failures
items reaching the end of their useful life.
Also called the wear-out period.
Because of the characteristic shape, this is
commonly known as the Bathtub Curve.
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Useful Lifetime
Reliability is predictable.
R reliability. t time for which equipment is
run. m MTBF
Note that R has no units. The prediction yields a
number lt1. Closer to 1 greater reliability.
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Examples
If an item of equipment has MTBF of 500hrs, then
the reliability for 100hrs operation is -
0.8187 (81.87 probability of survival)
and if the equipment is operated for 1000hrs, the
reliability will be -
0.1353 (13.53 probability of survival)
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Factors Affecting Reliability
Installation Environmental Temperature Humi
dity Vibration Chemical Attack Interconnections

• Design Manufacture
• Pre-Production Design
• Control of Production
• Working Tolerances
• Material Quality
• Component Quality
• Component Stress

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Factors Affecting Reliability
Installation Environmental
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Temperature

• Generally, Operation at higher temperatures
  degrades reliability performance. Internally
  generated heat must be removed by mechanisms such
  as cooling fins or forced-air.

In high ambient temperatures, the process of
removing excess heat becomes more
difficult. Equipment operating in low ambient
temperatures will need to be designed using
components which can tolerate this
environment.
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Humidity

• Moisture can cause oxidation and corrosion and
  reduce insulation effectiveness. Particularly
  vulnerable are solder joints and connectors.
• Equipment designed for use in areas of high
  humidity will use components and materials which
  are selected for their resistance to damage by
  moisture.
• Vulnerable components, such as circuit boards,
  can be protected by encapsulation e.g. in resin.
  Individual components may be hermetically sealed.

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Vibration

• Vehicles (cars, ships, aircraft etc) are
  particularly prone to vibration damage.
• Vulnerable equipment can use flexible mountings.

Components on a PCB can be made less susceptible
to vibration by the use of encapsulation. Vib
ration effects on electronic components has been
minimised by the process of miniaturisation.
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Chemical Corrosion

• Atmospheric pollutants and natural airborne
  chemicals (such as salt air atmosphere in coastal
  regions) can corrode metals (PCB tracks, solder
  joints, connector terminals etc) and even break
  down some plastics used for insulation.

Selection of appropriate materials is crucial.
Again, encapsulation can help to protect
vulnerable components, particularly circuit
boards.
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Interconnections

• Interconnections are liable to degradation by
  vibration, humidity and chemical factors. They
  are one of the most vulnerable components in an
  electronic system.

Connections internal to electronic modules,
such as inverters, can be reasonably well
protected by appropriate mounting and by
encapsulation. However, other interconnections,
eg between solar panels, will be subject to
mechanical stress and corrosion damage.
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Factors Affecting Reliability

• Design
• Manufacture

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Component Reliability

• Typical Failure Rates of Electronic Components
• Component Type Failure Rate (/1000h)
• Capacitors Ceramic 0.025
• Paper 0.05
• Tantalum 0.1
• Electrolytic 0.2
• Diodes Silicon 0.001
• Resistors Carbon 0.005
• Wirewound 0.03
• Film 0.1
• Transistors Discrete Silicon 0.01
• Connections Soldered 0.001
• Connectors Per Pin 0.005

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Operating Stresses

• Weighting Factors for Electronic Components
• Component Operating Condition Weighting Factor
• Resistors 0.1 of max. rating 1.0
• Transistors 0.5 of max. rating 1.5
• Diodes max. rating 2.0
• Capacitors 0.1 of wkg voltage 1.0
• 0.5 of wkg voltage 3.0
• max wkg voltage 6.0

System Failure Rate (Component Failure
Rate) x (Quantity) x (Weighting Factor)
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• Example
• An electronic system uses
• 20 silicon transistors _at_ 0.1 x max rating 20
  carbon resistors _at_ 0.5 x max rating
• 10 silicon transistors _at_ 0.5 x max rating 50
  ceramic capacitors _at_ 0.1 x max rating
• 10 diodes _at_ 0.1 x max rating 20 electrolytic
  capacitors _at_ 0.5 x max rating
• 100 carbon resistors _at_ 0.1 x max rating 500
  soldered connections

Overall failure rate is 14.85 per 1000 hours.
The MTBF can be found by dividing this number
into 100,000.
Hours
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Production Monitoring Quality Control
Continuous assessment of key quality monitors
during manufacture allows early identification of
process variation and prompt action to optimise
processes.
Quality control feedback loops may also be
implemented on incoming materials and components.
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Accelerated Life

• The bathtub curve predicts a high early failure
  rate.
• Elevated temperatures are used to accelerate
  component aging and ensure that products move
  from the Early Failure area and into the Useful
  Lifetime area.
• The technique is used to pre-screen early
  failures during manufacturing.

High temperatures accelerate all known chemical
reactions. Almost all failure mechanisms
associated with semiconductor devices are the
result of a chemical reaction
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Arrhenius Equation

• Rate of the chemical reaction.
• A constant.
• e Activation energy in electron volts (eV)
  that is
• associated with the chemical reaction.
• K Boltzmans constant.
• T Absolute temperature.

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Acceleration Factor
is the elevated temperature. is the temperature
for which the new reaction rate is
calculated. Is the reaction rate at the elevated
temperature. Is the reaction rate at
The constant, , is the same for both
temperatures and has been cancelled out of the
equation
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Case Study

• MOSFET Hi-Rel
• Processing

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Process Flows
Standard process flow (left) Hi-Rel process flow
(right)
Hi-Rel process flows includes many more process
monitors during production as well as accelerated
life testing and other quality conformance
testing designed to enhance product reliability
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High Temperature Gate BiasHTGB

• Burn-in temperature 150C.
• Gate terminal is biased during burn-in.
• Typical burn-in time 48hrs.
• Failure criteria - failure to meet data
  sheet specifications.

Purpose is to check the integrity of the gate
oxide. This test identifies failures caused by
weak or damaged oxide or if the oxide is
contaminated.
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High Temperature Reverse Bias HTRB

• Burn-in temperature 150C.
• Drain terminal is biased during burn-in.
• Typical burn-in time 168hrs.
• Failure criteria - failure to meet data
  sheet specifications.

Purpose is to check the integrity of the field
termination and the quality of the body-drain
junction. This test also identifies failures
caused by surface contamination.
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Other Hi-Rel Processing
Salt Atmosphere subjects the devices to a
highly corrosive atmosphere of salt and moisture
at the elevated temperature of 35C to simulate
long-term exposure to seacoast atmospheric
conditions. Failure Criteria excessive
corrosion of package, loss of marking legibility,
loss of hermeticity. Thermal Shock
(liquid-to-liquid) defined number of
temperature cycles from -55C to 150C with
5-minute exposure at each temperature, maximum 5
second transfer time between temperatures. Tests
for die attach integrity and package hermeticity.
Any cracks present in the silicon chip will be
propagated by this test, leading to
failure. Failure Criteria Failure to meet
datasheet specification, loss of hermeticity.
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Other Hi-Rel Processing (continued)
Temperature Cycle (air-to-air) defined number
of temperature cycles from -55C to 150C with
10-minute exposure at each temperature, with a 5
minute dwell at ambient during transfer. Similar
to Thermal Shock but often activates different
failure mechanisms due to longer exposure to
temperature extremes and more gradual temperature
change. Failure Criteria Failure to meet
datasheet specification, loss of
hermeticity. Pressure Pot device subjected to
121C _at_ 15 PSIG in an atmosphere of 100 RH. To
check the performance of the device in humid
environments. Identifies passivation defects,
poor package sealing and contamination level
during assembly. Failure Criteria Failure to
meet datasheet specification.
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Full test details and comprehensive procedures
are detailed in the MIL-STD methods, Especially
- MIL-STD 750 - Standard Test Methods For
Semiconductor Devices MIL-STD 883 - Test
Method Standards - Microcircuits