M.Nuzaihan

DMT 243 – Chapter 5

Fundamental Design For Reliability

What is Design for Reliability, Microsystems Failure & Failure Mechanisms, Fundamental of Design for Reliability, Thermomechanically- Induced Failure, Electrical Induced Failure and Chemical Induced Failure.

M.Nuzaihan

DMT 243 – Chapter 5

Fundamental Design For ReliabilityEvery electronic product is designed to meet four criteria.

• 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.

M.Nuzaihan

DMT 243 – Chapter 5

Fundamental Design For Reliability

Reliability

• When a product performs as designed, it is said to be reliable; when it does not, it is unreliable.

• Long term reliability?- personal computer = 5-7 years- electronic component = over 30 years- automotive controller = 10 -15 years

• 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 Reliability2. Reliability testing

M.Nuzaihan

DMT 243 – Chapter 5

Fundamental Design For Reliability

Design for Reliability

Design the systems packaging up-front for reliability

- Predetermine various potential failure mechanisms that could result in product failure.

- Create designs and select materials and process that would minimize or eliminate the chances for the failures.

- Design for reliability aims to understands, identify and prevent such underlying failures even before the packages are built.

M.Nuzaihan

DMT 243 – Chapter 5

Fundamental Design For Reliability

Reliability TestingConduct an accelerated test on the systems packaging for reliability

- After a system is built and assembled, the system is subjected to accelerated test conditions.

- 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)

M.Nuzaihan

DMT 243 – Chapter 5

Fundamental Design For Reliability

Reliability Metrology

- Refers to the measurement and mathematical modeling of reliability and patterns of failure.

- 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.

M.Nuzaihan

DMT 243 – Chapter 5

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.

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DMT 243 – Chapter 5

Fundamental Design For ReliabilityMicrosystems Failures and Failure Mechanisms

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DMT 243 – Chapter 5

Fundamental Design For ReliabilityMicrosystems Failures and Failure MechanismsAs 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.

M.Nuzaihan

DMT 243 – Chapter 5

Fundamental Design For ReliabilityMicrosystems Failures and Failure Mechanisms

• We need to understand the various failure mechanisms (overstress or wear out) in order to design against them

• We can design against failure by1. Reducing the stresses that cause failure2. Increasing the strength of the component

• This involves1. Selecting alternate materials2. Changing the package geometry and dimensions.3. Introducing new protection or encapsulation4. Combination of these methods

M.Nuzaihan

DMT 243 – Chapter 5

Fundamental Design For ReliabilityMicrosystems Failures and Failure Mechanisms

DefinitionStrain 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.

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DMT 243 – Chapter 5

Fundamental Design For ReliabilityMicrosystems Failures and Failure Mechanisms

Typical failure mechanisms in semiconductor packaging arecategorized into few levels –• Die level • Substrate/board level• Interconnect (between die-to -substrate and substrate-to-

board)• Assembly/package level

M.Nuzaihan

DMT 243 – Chapter 5

Fundamental Design For Reliability

M.Nuzaihan

DMT 243 – Chapter 5

Fundamental Design For ReliabilityThermomechanically-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 system3. Geometric constraints

M.Nuzaihan

DMT 243 – Chapter 5

Fundamental Design For ReliabilityThermomechanically-Induced Failures

M.Nuzaihan

DMT 243 – Chapter 5

Fundamental Design For ReliabilityThermomechanically-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.

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DMT 243 – Chapter 5

Fundamental Design For ReliabilityThe Various Thermomechanical Failure

Mechanisms

1. Fatigue Crack2. 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. Creep4. Interfacial delamination5. Plastic deformation

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DMT 243 – Chapter 5

Fundamental Design For ReliabilityThe Various Thermomechanical Failure

Mechanisms

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DMT 243 – Chapter 5

140 m Crack length

100 m Crack length

Cu

PCB

Cu

PCB

Cu

Crack

Crack Initiation

The Various Thermomechanical Failure Mechanisms

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DMT 243 – Chapter 5

Fundamental Design For ReliabilityElectrically-Induced Failures

• All failures in electronic products are electrical failures. • However we should carefully distinguish electrical failure that

are mechanically-induced, electrically-induced or chemically-induced and eventually exhibit themselves as electrical failures.

• Electrically induced-failures.1. Electrostatic discharge 2. Gate Oxide breakdown3. Electromigration

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DMT 243 – Chapter 5

Fundamental Design For ReliabilityElectrically-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.

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DMT 243 – Chapter 5

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.

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DMT 243 – Chapter 5

Fundamental Design For ReliabilityChemically-Induced Failures

There are three chemically driven processes that can lead tocracking/failure of the package

1. Electrochemical reactions (corrosion)2. Diffusion of material3. Dendritic growth (saliran bentuk ranting )

The reactions are often aggravated by increased temperature, increased voltage and increased thermally

M.Nuzaihan

DMT 243 – Chapter 5

Fundamental Design For ReliabilityChemically-Induced Failures

There are three chemically driven processes that can lead tocracking/failure of the package

1. Electrochemical reactions (corrosion)2. Diffusion of material3. Dendritic growth (saliran bentuk ranting )

The reactions are often aggravated by increased temperature, increased voltage and increased thermally