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					Reliability
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.
               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.
           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.
                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.
            Reliability vs Cost
Three separate cost factors related to the reliability
  of an item throughout its life –

            Design & Development
            Production
            Maintenance & Repair
Cost-Reliability Functions
             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.
     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”.
               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 <1.
         Closer to 1 = greater reliability.
                           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)
       Factors Affecting
          Reliability


 Installation &             Design &
Environmental              Manufacture
                      Pre-Production Design
   Temperature         Control of Production
     Humidity          Working Tolerances
     Vibration          Material Quality
 Chemical Attack        Component Quality
                        Component Stress
 Interconnections
Factors Affecting Reliability


        Installation
             &
       Environmental
                   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.
    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.
                               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.

Vibration effects on electronic components has been minimised by the
  process of miniaturisation.
       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.
                  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.
Factors Affecting Reliability


          Design
            &
        Manufacture
              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
                  Operating Stresses
    Weighting Factors for Electronic Components

    Component                Operating Condition           Weighting Factor
    Resistors                 0.1 of max. rating                1.0
    Transistors
    Diodes        }           0.5 of max. rating
                                     max. rating
                                                                1.5
                                                                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) ]
Example
An electronic system uses :
    20 silicon transistors @ 0.1 x max rating    20 carbon resistors @ 0.5 x max rating
    10 silicon transistors @ 0.5 x max rating    50 ceramic capacitors @ 0.1 x max rating
    10 diodes @ 0.1 x max rating                 20 electrolytic capacitors @ 0.5 x max rating
    100 carbon resistors @ 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
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.
            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
            Arrhenius Equation



    = Rate of the chemical reaction.
    = A constant.
e   = Activation energy in electron volts (eV) that is
                associated with the chemical reaction.
K = Boltzman’s constant.
T = Absolute temperature.
           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
 Case Study


MOSFET Hi-Rel
 Processing
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
  High Temperature Gate Bias
           HTGB

                                  • Burn-in temperature 150°C.

                                  • 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.
High Temperature Reverse Bias
           HTRB

                                 • Burn-in temperature 150°C.

                                 • 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.
       Other Hi-Rel Processing
Salt Atmosphere – subjects the devices to a highly corrosive
atmosphere of salt and moisture at the elevated temperature of 35°C 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 -55°C to +150°C 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.
          Other Hi-Rel Processing
                (continued)
Temperature Cycle (air-to-air) – defined number of temperature
cycles from -55°C to +150°C 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 121°C @ 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.
  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

				
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