Download fundamentals of design for reliability

FUNDAMENTALS OF
DESIGN FOR RELIABILITY
Chapter Objectives

Introduce the need for design for reliability

List the main causes of reliability failures

How do failures relate to their mechanisms

Describe each failure

Propose design guidelines against the failure
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Introduction
Electronic Product:
• Performance
• Cost
• Size
• Reliability
Electrical:
• Performance
• Size
Manufacturing:
• Cost
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• Reliability
Reliability

Often not designed up-front.

Tested during the product qualification or after
the product is manufactured.

Expensive and time-consuming approach.

Design for RELIABILITY as well !!!
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5.1 What is Design for Reliability

Product performs the functions – reliable product

“Long-term” reliability (i.e. Automobile, Personal Computer)

Economically not viable to test “long-term” reliable products for
several years before they are sold out.

To ensure over an extended period of time, two approaches can be
taken:
Design the systems packaging up-front for reliability.
1.
2.
Conduct an accelerated test on the systems packaging for reliability
after the system is designed, fabricated & assembled.
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1. Design the systems packaging up-front for reliability

Predetermine various potential failure
mechanisms

Create and select materials and processes –
minimize/eliminate the chances for the failures

“up-front” design

Design for reliability
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2. Conduct an accelerated test on the systems
packaging for reliability after the system is
designed, fabricated & assembled

After a system is built and assembled, system
accelerated to test conditions.

Temperature

Testing for reliability – Chapter 22
,humidity
,voltage ,pressure
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Comparison and usage

Industrial practice uses Testing for Reliability

If {problems = TRUE}
Then (IC & system-level packages):
RE[designed, fabricated, assembled, tested]

Expensive and time consuming

Design for Reliability = Solution
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5.2 Microsystems Failures and Failure Mechanisms

High-level symptoms (i.e. computer, TV)

Underlying cause (i.e. chip, corrosion, moisture,
electrostatic discharge) – PRODUCT NOT RELIABLE

Design for Reliability understands, identifies, and
prevents such failures
Mechanisms – stress exceeds the
strength or capacity of the component and causes the
system failure. (single event)
 Overstress
Mechanisms – gradual and occurs even at
lower stress level. (repeated event)
 Wearout
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Failure mechanisms is microelectronic system packages
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5.3 Fundamentals of Design for Reliability

Important to understand the failure (why, where, how
long, application, etc.)

Two methods for design against failure:
1.
2.
By reducing the stress that cause the failure.
By increasing the strength of the component.

Either one can be achieved by:




Selecting materials
Changing the package geometry
Changing the dimensions
Protection
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5.4.1 What are Thermomechanically-induced Failures ?
- Caused by stresses and strains
generated within electrical package
due to thermal loading.
- Due to CTE (coefficient of thermal
expansion), thermally-induced
stresses are generated in various
parts of system.
- Figure - Illustration of thermo
mechanical deformation in
solder joints
- αb BOARD
- αc COMPONENT
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
Tmax
chip carrier αc(Tmax – T0) per unit length
board
αb(Tmax – T0) per unit length
- Difference between the two expansions = net shearing displacement:
L(αb - αc)(Tmax – T0)
where L – distance (of the solder joint) from the neutral point (DNP)

Tmin
chip carrier αc (Tmin – T0) per unit length
board
αb(Tmin – T0) per unit length
- Net shearing displacement:
L(αb - αc)(Tmin – T0)
- Difference in the displacement at Tmax and T min:
Δ = L(αb - αc)(Tmax – T0)
- Shear strain:
γ = Δ / h = (L / h)(αb - αc)(Tmax – Tmin)
where h – height of solder joint
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5.4.2 What is Fatigue?

Fatigue is the most common mechanism of failure and responsible
for 90% of all structural and electrical failures.

Occurs in metals, polymers, and ceramics.

Metal paper clip example
 Bend in both directions
 Repeat the process
 Breaks at lower load
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5.4.3 Definitions Relating to Fatigue Fracture

Two approaches in determining the number of cycles to fatigue
failure:
1.
High-cycle fatigue – based on stress reversals to determine the
number of cycles to fatigue failure.
2.
Low-cycle fatigue – based on strain reversals and is used for
situations where the material has plastic deformation.
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Typical Fatigue Load Cycle

Stress vs. time, max & min, ΔS, Sa

Fatigue cycle – successive maxima/minima in load or stress
The number of fatigue cycles to failure designed by Nf
The number of fatigue cycles per second – cyclic frequency
The average of the max and min stress – mean stress, Smean



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5.4.4 Predictive Fatigue Models

Used fatigue models for solder joints fall into following
categories:
(1)
Coffin-Manson-type fatigue model
Strain-energy-based fatigue model
Fracture-mechanics-based fatigue model
Continuum damage mechanics-based model
(2)
(3)
(4)
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Coffin-Manson Low-Cycle fatigue model

Predict low-cycle fatigue life, Nf, of metallic materials in terms of the
plastic strain range:
Where m and C are constants and is 1/2 of the plastic strain
accumulated over one fatigue cycle.

Solder joint fatigue applications, the fatigue can be expressed with
respect to inelastic shear strain range:
Where Nf - cycles to failure (fatigue life)
- fatigue ductility coefficient
c - fatigue ductility exponent
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Solomon’s Model

Determined low-cycle fatigue expressions for Pb-Sn (Lead-Tin)
solder joints for temperatures at [-50, 35, 125, 150] degree C.

Average values: θ = 1.14 and α = 0.51
In the table are given constants for θ and α

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Engelmaier’s Model

Based on Coffin-Manson model

The frequency-modified low-cycle fatigue model
Where
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Design Guidelines to Reduce Early Fatigue Failure

The strain increases with the CTE mismatch between the chip
carrier and the substrate. Use CTE close to the effective CTE of the
chip carrier.

The strain increases with distance from the neutral point. Design
distance from the neutral point as small as possible.

The strain in the solder interconnects increases with temperature.
Design thermal paths such that the heat is easily dissipated, so that
high thermal gradients do not exist.
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5.4.6 Design Against Brittle Fracture

Brittle fracture is an overstress failure mechanism that occurs rapidly
with little or no warning when the induced stress in the component
exceeds the fraction strength of the material.

Occurs in brittle materials (ceramics, glasses and silicon).

Applied stress and work could break the atomic bonds.
Where
is the fracture strength and E is the modulus of elasticity of
the material.

Flaw Modeled as an Edge Crack
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5.4.7 Design Guidelines to Reduce Brittle
Fracture

Designs with materials and processing conditions that
would produce the least stress in brittle materials should
be created.

The brittle material should be polished to remove surface
flaws to enhance reliability.
5.4.8 Design Against Creep-Induced Failure

What is Creep?

A time-dependent deformation process under load.

Thermally-activated process: the rate of deformation
for a given stress level increases significantly with
temperature.

Deformation depends on both
1. The applied load.
2. The duration through which the load is applied.
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5.4.8 Design Against Creep-Induced Failure

Creep can occur at any stress level.

Creep is most important at elevated
temperatures.

Homologous temperature:
 The
ratio of the operating temperature to the melting
point of the material in absolute scale.
 If homologous temperature is above 0.5, creep will be
a problem.
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Creep Example
Creep fatigue failure in a lead/tin solder circuit board connection
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5.4.8 Design Against Creep-Induced Failure

Creep Strain Curve

Arrhenius creep equation:

Creep Strain Rate = A(σn)e-(Q/RT)
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Design Guidelines to Reduce CreepInduced Failure.

Use materials with high melting point if the
application calls for harsh temperature
conditions.

Reduction of mechanical stress will reduce
creep deformation.

Creep is a time controlled phenomenon.
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5.4.9 Design Against Delamination-Induced
Failure

What is delamination?
 The
debonding or the separation of adjacent material
layers which were bonded before.

Two Categories
 Embedded:
delamination occurs in the interior of the
package.
 Free Edge: delamination occurs at an edge of the
package.
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Delamination Example
Delamination in the circuit board assembly
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5.4.9 Design Against DelaminationInduced Failure

Causes of Delamination
 Processing

Issues
Inadequate surface preparation, presence of
contaminants, moisture, inadequate baking,
inadequate material dispensing.
 High
Interfacial Stresses
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Design Guidelines to Reduce Delamination
Failure

Careful selection of processing conditions.

Reduce the mismatch in engineering properties between
adjacent materials.

Improve adhesion properties between different material
layers.

The geometry of the package should minimize sharp
corners.
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5.4.10 Design Against Plastic Deformation

What is Plastic Deformation?
 When
the applied mechanical stress exceeds the
elastic limit or yield point of a material.
 It is permanent.

Excessive deformation and continued
accumulation of plastic strain due to cyclic
loading will eventually lead to cracking of the
component and make it unusable.
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Design Guidelines Against Plastic
Deformation

Limit the design stresses in the packaging
structure below the yield strength of the
materials used. If possible, use materials that
have high yield strength.

Design and control the local plastic deformation
at regions of stress concentrations.
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5.5 Electrically Induced Failures

What are Electrically Induced Failures?
 Failures
 Three
caused as a result of electrical overstress.
Types

Electrostatic Discharge

Gate Oxide Breakdown

Electromigration
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5.5.2 Design Against Electrostatic Discharge

What is ESD?
 The
transfer of electrostatic charge between bodies at
different potentials caused by direct contact or
induced by an electrostatic field.
 Two


Types of Failure
Immediate Failure
Delayed Failure
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Guidelines against ESD

Workstations can be provided with measures like conductive
tablemats, wristbands, and conductive flooring.

Air ionizers neutralize static charges on nonconductive materials
used in manufacture.

All test and soldering equipment should be provided with ground
potential and should be checked periodically.

Antistatic foams can be used for protecting ESD sensitive devices
for storage and transportation.

Monitoring devices such as field meters can be used to measure
and control static charge on materials.
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5.5.3 Design Against Gate Oxide
Breakdown

What is Oxide Breakdown?
 An
electrical short between
the metallization and the
semiconductor disabling the
functionality of a MOSFET.
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5.5.3 Design Against Gate Oxide
Breakdown

Causes of Oxide Breakdown
 Process
induced defects or particles.
 Accidental discharge of voltage.

The risk of dielectric breakdown generally
increases with the area of the oxide layer, since
a larger area means the presence of more
defects and greater exposure to contaminants.
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5.5.4 Design Against Electromigration

What is Electromigration?
 Atom
flux induced in metal traces by high
current densities.
 Metal atoms (such as solders) experience a
mechanical force and get dislodged from their
position.
 This results in the formation of metal voids in
the conductor, which eventually result in
electrical opens.
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Electromigration Example
Before
After
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Design Guidelines Against Electromigration

Electromigration has been mostly noticed in
aluminum and silver metallization. Copper traces
are more resistant.

Use shorter traces. Tradeoff is more routing
layers and greater complexity during fabrication.

Tightly enforce current density design rules
based on electromigration data.
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5.6 Chemically Induced Failures

What are Chemically Induced Failures?
 Chemical
process such as electrochemical
reactions can result in cracking of vias, traces,
or interconnects leading to electrical failures.
 Two Types
Corrosion
 Intermetallic Diffusion

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5.6.2 Design Against Corrosion-Induced
Failure

What is Chemical Corrosion?
 The
chemical or electrochemical
reaction between a material,
usually a metal, and its
environment that produces a
deterioration of the material and
its properties.
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Design Guidelines to Reduce Corrosion

Metals with a high oxidation potential tend to
corrode faster.

Use hermetic packages to prevent moisture
absorption.

Ensure there are no trapped moisture or
contaminants during the processing an
assembly of the packages.
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5.6.3 Design Against Intermetallic Diffusion

What is Intermetallic Diffusion?
 During
wirebonding and solder reflow, the
joining process generates intermetallic layers
which are byproducts of the joining process.
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Design Guidelines Against Intermetallic
Diffusion
Limit the process temperatures and control
the time exposed to high temperatures
during the joining process.
 Control the temperature range and cycles
of exposure at the high temperature
period.
 Application of nickel/gold coating on the
bare copper pad surfaces.

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