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Electronic Packaging and

Manufacturing

Reliability

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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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Quality

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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 Rate: Bath-tub curve

Source: www.weibull.com

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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.)

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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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Summary

Reliability: a critical component in design of electronic

systems

Not just performance but performance over life time

Reliability prediction

Accelerated Degradation tests

Statistical treatment

Physics of Failure analysis and models

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