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Fundamental Design For Reliability
What is Design for Reliability, Microsystems
Failure & Failure Mechanisms, Fundamental
of Design for Reliability, ThermomechanicallyInduced Failure, Electrical Induced Failure
and Chemical Induced Failure.
DMT 243 – Chapter 5
M.Nuzaihan
Fundamental Design For Reliability
Every electronic product is designed to meet four criteria.
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Performance
Cost
Size
Reliability
Electrical designers typically design for performance and size.
Manufacturing engineers typically design for cost.
Reliability?
Reliability – is defined as the probability that a system will
function within acceptable limits for a given period of time.
DMT 243 – Chapter 5
M.Nuzaihan
Fundamental Design For Reliability
Reliability
•
When a product performs as designed, it is said to be
reliable; when it does not, it is unreliable.
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Long term reliability?
- personal computer = 5-7 years
- electronic component = over 30 years
- automotive controller = 10 -15 years
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To ensure that the electronics systems packaging will be
reliable over an extended period of time, two
approaches need to be followed:
1.
Design for Reliability
2.
Reliability testing
DMT 243 – Chapter 5
M.Nuzaihan
Fundamental Design For Reliability
Design for Reliability
Design the systems packaging up-front for reliability
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Predetermine various potential failure mechanisms that
could result in product failure.
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Create designs and select materials and process that
would minimize or eliminate the chances for the failures.
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Design for reliability aims to understands, identify and
prevent such underlying failures even before the
packages are built.
DMT 243 – Chapter 5
M.Nuzaihan
Fundamental Design For Reliability
Reliability Testing
Conduct an accelerated test on the systems packaging for
reliability
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After a system is built and assembled, the system is
subjected to accelerated test conditions.
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Test conditions
thermal cycling
temperature and humidity cycling
power cycling for short periods of time
(by applying higher temperature, higher humidity, higher
voltage, higher pressure and more to accelerate the
failures process)
DMT 243 – Chapter 5
M.Nuzaihan
Fundamental Design For Reliability
Reliability Metrology
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Refers to the measurement and mathematical modeling
of reliability and patterns of failure.
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It uses the mathematical tools of probability and
statistical distributions to effectively collect, classify and
process the test data to understand the patterns of
failure and to identify the potential of failure.
DMT 243 – Chapter 5
M.Nuzaihan
Fundamental Design For Reliability
Microsystems Failures and Failure Mechanisms
Failure mechanisms occur at the lowest hardware level, the
effect are often at system level.
Example:
- A computer may not boot up when powered up.
- TV may not show any picture when turned on.
High – level symptoms , cause can be cracking of a chip due to
thermally-induced stress or an electrical opening of an
interconnect due to corrosion or a shorting of a circuit due to
moisture or electrostatic discharge.
- Whatever cause or failure mechanisms, the result is that the
system is not reliable or usable.
DMT 243 – Chapter 5
M.Nuzaihan
Fundamental Design For Reliability
Microsystems Failures and Failure Mechanisms
DMT 243 – Chapter 5
M.Nuzaihan
Fundamental Design For Reliability
Microsystems Failures and Failure Mechanisms
As seen in Figure: failure mechanisms can be classified into two
mechanisms:
- overstress mechanisms
- wearout mechanisms
Overstress mechanisms:
- Is one in which the stress, in a single event, exceeds the
strength or the cavity of the component and causes system
failure.
Wearout mechanisms:
- Is gradual and occurs even at lower stress levels.
- repeated application of lower stress over an extended period of
time results in cumulative damage that makes the component
eventually fail/ makes the system fail.
DMT 243 – Chapter 5
M.Nuzaihan
Fundamental Design For Reliability
Microsystems Failures and Failure Mechanisms
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We need to understand the various failure mechanisms
(overstress or wear out) in order to design against them
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We can design against failure by
1. Reducing the stresses that cause failure
2. Increasing the strength of the component
•
This involves
1. Selecting alternate materials
2. Changing the package geometry and dimensions.
3. Introducing new protection or encapsulation
4. Combination of these methods
DMT 243 – Chapter 5
M.Nuzaihan
Fundamental Design For Reliability
Microsystems Failures and Failure Mechanisms
Definition
Strain is defined as the increase in length per unit length of a body
subjected to an applied stress.
Stress is defined as the applied force per unit area of cross-section
of a body.
DMT 243 – Chapter 5
M.Nuzaihan
Fundamental Design For Reliability
Microsystems Failures and Failure Mechanisms
Typical failure mechanisms in semiconductor packaging are
categorized into few levels –
• Die level
• Substrate/board level
• Interconnect (between die-to -substrate and substrate-toboard)
• Assembly/package level
DMT 243 – Chapter 5
M.Nuzaihan
Fundamental Design For Reliability
DMT 243 – Chapter 5
M.Nuzaihan
Fundamental Design For Reliability
Thermomechanically-Induced Failures
These failures result from stresses and strain generated
within electronic package by external or internal heating
thermal loading of the system due to:
1. Mismatch of the thermal coefficient of expansion of the
different materials
2. Thermal gradients in the system
3. Geometric constraints
DMT 243 – Chapter 5
M.Nuzaihan
Fundamental Design For Reliability
Thermomechanically-Induced Failures
DMT 243 – Chapter 5
M.Nuzaihan
Fundamental Design For Reliability
Thermomechanically-Induced Failures
Figure
beside
illustrate the shear
strain
point,
wherein the max
strain is at the
outside-edge
solder ball where
the distance from
neutral
points
(DNP)
is
maximum.
DMT 243 – Chapter 5
M.Nuzaihan
Fundamental Design For Reliability
The Various Thermomechanical Failure
Mechanisms
1. Fatigue Crack
2. Brittle facture
- is a fracture that occurs rapidly (overstress failure
mechanism), with little or no warning when the induced stress
exceeds the fracture strength of the material
3. Creep
4. Interfacial delamination
5. Plastic deformation
DMT 243 – Chapter 5
M.Nuzaihan
Fundamental Design For Reliability
The Various Thermomechanical Failure
Mechanisms
DMT 243 – Chapter 5
M.Nuzaihan
The Various Thermomechanical Failure
Mechanisms
Cu
PCBCu
140 m Crack
length
PCB
100 m Crack
length
Crack
Crack
Initiation
Cu
DMT 243 – Chapter 5
M.Nuzaihan
Fundamental Design For Reliability
Electrically-Induced Failures
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All failures in electronic products are electrical failures.
However we should carefully distinguish electrical failure that
are mechanically-induced, electrically-induced or chemicallyinduced and eventually exhibit themselves as electrical failures.
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Electrically induced-failures.
1. Electrostatic discharge
2.
3.
Gate Oxide breakdown
Electromigration
DMT 243 – Chapter 5
M.Nuzaihan
Fundamental Design For Reliability
Electrically-Induced Failures
1.
Electrostatic discharge (ESD) is the transfer of charge
between two bodies at different potentials by direct contact or
induced by an electrostatic (electromagnetic) field.
2.
An electrical short between the gate metallization and the
channel of a MOSFET destroys the operation of the device
and is called gate oxide breakdown.
3.
Electromigration is an atom flux induced in metal traces by
high current densities.
DMT 243 – Chapter 5
M.Nuzaihan
Fundamental Design For Reliability
Figure depicts an SEM image of metal trace before and after electromigration has
taken place. The ion flux generated due to the high current density has caused an
electrical open in the circuit.
DMT 243 – Chapter 5
M.Nuzaihan
Fundamental Design For Reliability
Chemically-Induced Failures
There are three chemically driven processes that can lead to
cracking/failure of the package
1. Electrochemical reactions (corrosion)
2. Diffusion of material
3. Dendritic growth (saliran bentuk ranting )
The reactions are often aggravated by increased temperature,
increased voltage and increased thermally
DMT 243 – Chapter 5
M.Nuzaihan
Fundamental Design For Reliability
Chemically-Induced Failures
There are three chemically driven processes that can lead to
cracking/failure of the package
1. Electrochemical reactions (corrosion)
2. Diffusion of material
3. Dendritic growth (saliran bentuk ranting )
The reactions are often aggravated by increased temperature,
increased voltage and increased thermally
DMT 243 – Chapter 5
M.Nuzaihan