Electronic Packaging Electronic Packaging and Manufacturing Reliability mech14.weebly.com
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Expected Product Life
5 10 15 20
Years
Pro
ducts
Cell phone
Computers
Automobiles
Military and Aerospace
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What is Reliability?
Ability of a product to maintain its performance
over time
Performance should undergo minimal degradation
from Beginning of Life (BoL) to End of Life
(EoL)
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Consequences of poor reliability
CUSTOMER VENDOR
Loss of Product Warranty Claims
Loss of Product Capability Production Downtime
Production Downtime Test and repair cost
Spare parts and Maintenance Damage to reputation
Lost opportunities Loss of future business
Lecture notes: Prof. Chris Bailey, U of Greenwich
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Performance vs. Reliability
Performance: Does it work?
Reliability: How long will it work for?
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Reliability
Reliability
defined as the probability that a component or system will
continue to perform its intended function under stated
operating conditions over a specified period of time.
Reliability engineering:
design, production and operation of things to retain their
quality over time.
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Reliability Metrics
# of total parts = No
# of parts that have failed over time t = Nf (t)
# of parts surviving after time t = Ns(t) = No - Nf(t)
Probability of failure = F(t) = Nf (t) /No
Reliability = R(t) = 1 - F(t) [= Ns (t)/No]
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Reliability Metrics (cont.)
F (
t)
Time
The slope of this curve gives
the Instantaneous failure Rate
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Failure Rates Infant Mortality:
Engineering did not test products or systems or devices sufficiently, or
manufacturing made some defective products.
Decreases with time after early failures are removed by burn-in or other stress
screening methods.
Useful life:
Characterised by a constant failure rate
Operating life for product aims to remain in this region.
Reliability with a constant failure rate can be predicted by the exponential
distribution.
Wear-out stage:
failure rate increases as the products begin to wear out because of age or lack of
maintenance.
When the failure rate becomes high, repair, replacement of parts etc., should be
done.
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Reliability Metrics (cont.) Mean Time Between Failures (MTBF)
Average time between failures
Repairable systems
For non-repairable systems, we use MTTF (Mean Time To Failure)
UP
DOWNBetween
failures
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Electronic package failures
Source:
http://blog.optimumdesign.com
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Failure Modes and Mechanisms
Failure Mode
Observed failure due to the mechanism taking place
Solder Cracks; Die Attach Delamination; Wirebond liftoff, etc
Failure Mechanism
The physical mechanism that resulted in a particular failure mode
Fatigue, Corrosion, Creep
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Solder Joint Fatigue – thermal
expansion mismatch
Lecture notes: Prof. Chris Bailey, U of Greenwich
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Reliability or Failure Predictions
Empirical
based on field data
Physics of Failure
based on physical
understanding
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Testing for Reliability
Accelerated Degradation Testing
Thermal Cycling and thermal shock
Bake
HAST (high temp + humidity)
Mechanical vibrations and Drop tests
Voltage extremes and power cycling
High humidity and high pressure
Combinations of the above
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Strength vs. Loading
Stress Generators – Thermal, Mechanical, Electrical,
Chemical
Overstress WearoutNormal
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Acceleration Factor
Ratio between the times necessary to obtain a stated proportion
of failures for two different sets of stress conditions involving
the same failure modes and/or mechanisms.
AF = tN/tA
where
AF is acceleration Factor in Years/cycles
tN is life of failure mode under field use conditions
tA is life of a failure mode under accelerated degradation test conditions
# of cyclesP
erfo
rman
ce
Field use
Degradation test
Failure
limit
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Types of accelerated degradation
trends
Time/ # of cycles
Y
Time/ # of cycles
Y
Time/ # of cycles
Y
• Y – performance metric• Thermal resistance
• Crack length
• Flexural strength
• …
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Example – Arrhenius model
Thermal resistance (qjc) under Bake test
time
q jc
150 C
125
C
90 C
k = 8.617 x 10-5 eV/K
Ea – Activation energy (eV)
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Other models
Power Law
Peck’s Model – two accelerating variables (ex: T and RH)
Numerous other models in literature
S – stress condition (RH, T, V …)
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What is Physics of Failure (PoF)?
PoF is a methodology determining based on:
Understanding root-cause of failure
With knowledge of materials, hardware configuration AND history of
life-cycle stresses.
Based on these analyses, the life cycle can be PROACTIVELY
managed to minimize failures.
PoF is just good engineering……
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Fatigue
Material undergoes fatigue when it is repeatedly cycled under a certain
load.
Cracks may form under this repetitive loading eventually leading to failure
Strain
Str
ess
Yield
Stres
s
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Fatigue in Solder joints
Material undergoes fatigue when it is repeatedly cycled
under a certain load.
Cracks may form under this repetitive loading eventually leading
to failure
High Cycle Fatigue
Operates in Elastic Region (Von Mises stress < Yield Stress)
Low Cycle Fatigue
Plastic deformation (Von Mises stress > Yield Stress)
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Solder Joint Fatigue
Lecture notes: Prof. Chris Bailey, U of Greenwich
L: Half length of chip
h: Height of solder joint
Δ⍺: Difference in CTE (Chip - PCB)
ΔT : Difference in temperature
g : Shear strain at edge solder
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Fatigue in Solder joints
Source: GIAN course by Prof. A. Dasgupta, 2018
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Solder joint fatigue (cont.)
Thermal cycling causes large slow cycles of inelastic creep strains, generated
by thermal expansion mismatch, resulting in low cycle fatigue failures
Vibration causes many rapid cycles of smaller strain amplitude, generated by
PWB flexure, resulting in high-cycle fatigue
HIGH CYCLE FATIGUE
Vibration Induced
LOW CYCLE FATIGUE
Thermal Expansion Mismatch
Source: GIAN course by
Prof. A. Dasgupta, 2018
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Fatigue Life models
High Cycle Fatigue – Basquin Model (function of stress)
Low Cycle Fatigue – Coffin Manson Model (function of
plastic strain)
Nf – cycles to failure
Svm – Stress intensity factor (chane in von-Mises stress during loading)
a, b – material constants
Nf – cycles to failure
g – plastic strain
c1, c2 – material constants
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Popcorning
Moisture – a major threat in many electronic components
If stored in normal atmosphere, the components could absorb moisture
During reflow, fast rise in temperature causes absorbed water to vaporize
immense pressure build up within the package.
cracks appear and package can burst open
This bursting open of the package is designated as the popcorn effect.
Maximum storage time in normal atmosphere is given by the MSL (Moisture
sensitivity level)
https://www.youtube.com/watch?v=7JyHZ6vUUx0
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Tombstoning
Defect in surface mount packages
Caused by unbalanced wetting of
solder during component attach
one pad completes its wetting
process before the other, which
results in one side of a component
solidifying while the other is still in
process
wet pad pulls up the other pin still in
the process of wetting
entire component gets tilted on its
side, looking like a tombstone.
https://electronics.stackexchange.com/q
uestions/17710/should-i-worry-about-
the-risk-of-tombstoning
https://www.youtube.com/watch?v=MaaOmI5gO08
https://www.youtube.com/watch?v=scvfJmSFpMw
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Other failure modes - Components
Acknowledgments: Prof. A. Dasgupta, CALCE, U Maryland
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Other failure modes - PWBs
Acknowledgments: Prof. A. Dasgupta, CALCE, U Maryland
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