FUNDAMENTALS OF DESIGN FOR RELIABILITY Marko Bundalo Adeel Baig.

FUNDAMENTALS OF DESIGN FOR RELIABILITY

Marko BundaloAdeel Baig

Microsystems Packaging

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


IntroductionElectronic Product:• Performance• Cost• Size• Reliability

Electrical:• Performance

• Size

Manufacturing:• Cost

• 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 !!!


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:

1. Design the systems packaging up-front for reliability.

2. Conduct an accelerated test on the systems packaging for reliability after the system is designed, fabricated & assembled.


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


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 ,humidity ,voltage ,pressure

Testing for reliability – Chapter 22


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


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

Overstress Mechanisms – stress exceeds the strength or capacity of the component and causes the system failure. (single event)

Wearout Mechanisms – gradual and occurs even at lower stress level. (repeated event)


Failure mechanisms is microelectronic system packages


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. By reducing the stress that cause the failure.2. By increasing the strength of the component.

Either one can be achieved by: Selecting materials Changing the package geometry Changing the dimensions Protection


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


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


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


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.


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

Typical Fatigue Load Cycle


5.4.4 Predictive Fatigue Models

Used fatigue models for solder joints fall into following categories:

(1) Coffin-Manson-type fatigue model

(2) Strain-energy-based fatigue model

(3) Fracture-mechanics-based fatigue model

(4) Continuum damage mechanics-based model


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


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


Engelmaier’s Model Based on Coffin-Manson model

The frequency-modified low-cycle fatigue model

Where


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.


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

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 both1. The applied load.2. The duration through which the load is applied.



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.


Creep Example

Creep fatigue failure in a lead/tin solder circuit board connection



Creep Strain Curve

Arrhenius creep equation: Creep Strain Rate = A(σn)e-(Q/RT)


Design Guidelines to Reduce Creep-Induced 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.


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.


Delamination Example

Delamination in the circuit board assembly


5.4.9 Design Against Delamination-Induced Failure

Causes of DelaminationProcessing Issues

Inadequate surface preparation, presence of contaminants, moisture, inadequate baking, inadequate material dispensing.

High Interfacial Stresses


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.


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.


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.


5.5 Electrically Induced Failures

What are Electrically Induced Failures? Failures caused as a result of electrical overstress.

Three Types Electrostatic Discharge

Gate Oxide Breakdown

Electromigration


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


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.


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.


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.


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.


Electromigration Example

Before After


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.


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


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.


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.


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.


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.


FUNDAMENTALS OF DESIGN FOR RELIABILITY Marko Bundalo Adeel Baig.

Documents

reliability product

cost reliability slide

reliability chapter

failure slide

reliability important

microsystems failures

reliable design

system failure