design for reliabiliy

DESIGN FOR RELIABILITY

PRESENTED BYP.SUYA PREM ANAND

200821528ME - (CIM)

04/18/23 DESIGN FOR RELIABILITY 1

INTRODUCTION

• The reliability of the device used can be controlled to a large extent in the manufacturing phases , mainly by quality control methods.

• The circuit designer generally has little control over the design reliability of the device.

• Since the designer concentrated on the advent of (LSI) and (VLSI) for their progress, the factors influencing the reliability was not given importance.

• Without team approach , the designer correct design could be unreliable.

• Electronic system unreliability is due to defective components.


• Defects in devices are caused by mechanical loading and thermal stresses exceeding the critical value of the devices.

• Electronics system failures can be caused by mechanism other than load exceeding the strength.For ex :

Parameter drift in component Short circuit due to solder Tolerance mismatches


DESIGN FOR RELIABILITY

• Design for Reliability (DFR) is an integrated process of reliability, design and operations departments by applying techniques to review and improve product design, test and monitor reliability in manufacturing and field so as to assure we have capable system architecture, reliable components and adequate fault avoidance and fault tolerance schemes.

• Reliability is the ability of a product to function under given conditions and for a specified period of time withoutexceeding acceptable failure levels.


Specify reliability goals

Allocate Goals

Implements design methods

Failure analysis (FMEA/FMECA)

Are goals achieved?

System Safety Analysis (FTA)

Are goals achieved?

Ready for production

No

Yes

No

Yes

Reliability design is an iterative process that begins with the specification of reliability goals consistent with cost and performance objectives. It considers life-cycle costs of the system and system effectiveness


Conceptual preliminary design

Detailed design and development and prototyping

Production and Manufacturing

Product Use and Support

Specification Design methods Acceptance testing Preventive and predictive maintenance

Allocation Failure analysis Quality control Modification

Design methods Growth testing Burn-in and screen testing

Parts replacements

Safety analysis

Reliability activities and product life cycle


GOALS AND REQUIREMENTS

• The first step to better reliability is to know the reliability goals and requirements.

• Whether you are an equipment manufacturer or a user.

1. Equipment manufacturer (supplier)• Understand the exact reliability requirements of your

customer.• Be aware of the reliability level of your competitor product.• Know what reliability level is required in the market place


Considering the above inputs, set the reliability goals for each equipment at the beginning 2.Customer

• Customer responsibility is to make sure that equipment suppliers know your exact requirements.


ITEMS EXAMPLE

Time Factor , The age at which equipment should attain the reliability level

4 month after installation

Operational conditionsTemperature

Range 70 – 75 deg F

Humidity Range Range 40 – 45 %

GOAL ALLOCATION

• Once the goal were known to the manufacturer .• The manufacturer should break the equipment and system

level goals into bite – size goals for subsystem, modules and components

• It is relatively easy to achieve their respective product goals.• The process of breaking down the system level goal into sub –

level is called budgeting or allocation.


DESIGN OF METHODS

A product fails prematurely because of the inadequate design features, manufacturing part defects, abnormal stresses introduced due to packaging or distribution, operator and maintenance error, or external conditions that exceed the design parameters.

• Various activities and parameter that are involved in design of products:

1. Material selection. It involve consideration of following parameters• Tensile strength• Hardness


• Fatigue life• Creep2. Derating. It is use of a component under stress significantly

below its rated value3. Stress-strength analysis. The traditional approach is to design

safety margins, or safety factors in to the equipment/ component. Failure is likely to occur if safety factor is less than 1 or safety margin is negative

– Safety factor: The safety factor is the ratio of the capacity of the system to the load placed on the system

– Safety margin: The safety margin is the difference between the system capacity and load


4. Design Simplification. Anything can be done to reduce the complexity of the design, as a part is not required, eliminate it from the design . Reduce the number of parts through the combining functions

5. Redundancy. Using more than one part to accomplishing a given function, so that all parts must fail before causing a system failure.Redundancy allow a system to operate even though some parts have failed , thus increasing the reliability.


DESIGN FOR RELIABILITY TOOLS

• Failure mode effect analysis (FMEA) or failure mode effect, and criticality analysis (FMECA) is formalized design process with an objective to improve the inherent reliability.

• Modeling technique determine the analytical value of the

reliability level.• The Failure Mode and Effect analysis technique determine the

system level effect of parts failures.


FMEA comprises of following steps: 1. System definition:

This step is to identify those system components that will be subject to failure. A functional and physical description of the system provides the definition and boundaries for performing analysis.

2. Identification of failure modes

Failure modes will be identified by hardware or function approach. Failures modes are observable manners in which a component fails. For example: valve open, circuit short, pipe or valve rupture, power loss, etc.


Failure mode Category Cause Failure Mechanism

Capacitor short Electrical High voltage Derating

Failure of metal Chemical Humid and salty atmosphere

Corrosion

Connector fracture Mechanical Excessive vibration Fatigue

3.Determination of causes There are certain specific causes for each and every failure mode

to occur . A failure mode may have more than one cause.

Example includes


Failure mechanism Failure mode Failure effect

Corrosion Failure of tank wall

Tank rupture

Manufacturing defect in casing

Leaking battery Failure to flashlight to light

Frication and excessive wear

Drive belt break Shutdown of production line

Prolonged low temperature

Brittle seals Leakage in hydraulic system

4.Effect assessmentThe impact of each failure on the operation or status of the system is assessed. Effects may range from complete system failure to partial degradation to no impact on performance


5.Classification of severity

A severity classification is assigned to each failure mode to be used for prioritization of corrective actions. Generally severity is classified in four classes.

• Category I: Catastrophic. Significant system failure occurs that can result in injury, loss of life, or major damage.

• Category II: Critical. Complete loss of system occurs, performance unacceptable.

• Category III: Marginal. System is degraded with partial loss in performance.

• Category IV: Negligible. Minor failure occurs with no effect on acceptable system performance


6. Estimation of probability of occurrence Probability of occurrence of each failure mode is estimated

generally using handbook or existing databases. Some of the standard handbook on FMEA classifies qualitatively frequency of occurrence in five major levels

7. Determination of corrective action This is very dependent on the problem. Those failure modes

having high criticality index and severity classification should receive the most attention. Design activities should be oriented toward removing the cause of failure, decreasing the probability of occurrence, and reducing the severity of failure.


RELIABILITY OF ELECTRONIC COMPONENTS

• Increasing the level of integration and reducing the volume requirement , the power consumption also decreased.

• It also leads to the reduction in cost per function and improvement in reliability.

• But there is a difficult task of controlling the junction temperature in densely packed systems.

• The quality control of IC manufacture has improved recently due to competition and this has contributed to the development of reliability.


ELECTRONIC COMPONENTS


MICROELECTRONIC PACKAGING

• There are two method of packaging IC chips hermetic packaging and Plastic encapsulated IC.

• In hermetic packaging the die is attached to the package base, usually by soldering.

• Wire bond are connected between the die connector pads and the leadout conductors and the package is then sealed with a lid. Ex ( ceramics or metal )

• PEIC are cheaper than hermetic IC and therefore tend to be used in domestic purpose, commercial and industrial equipment.

• But it is not suitable for high temperature operation ( above 85 degc )


• This type of packaging cannot be used in the military equipment.

• They also suffer a life dependent(wearout) failure mode due to moisture ingress by the absorption through encapsulation material.

• since the overall volume get decreased, the power dissipation per unit volume increases will lead to difficult thermal management problems.

• This would seriously affect the reliability of the component.


BALL GRID ARRAY DUAL IN LINE PACKAGING


MICROELECTRONIC COMPONENT ATTACHMENT

• Microelectronic component in dual-in-line packages can either be soldered on the PCB or plugged into IC sockets which are soldered in place.

• Plugging IC into sockets provides three major advantages from the test and maintenance point of view.

1. Failed components can easily be replaced, with less danger of damaging the PCB or other components.

2. Testing and diagnosis is usually made much easier and more effective if complex devices such as microprocessors are not in place.

3. It is much easier to change components which are subject to modifications such as programmable memories.


• some draw back1. Heat transfer is degraded, so it might mot be possible to

derate junction temperature adequately.2. There might be electrical problems in high vibration, shock or

contamination environments.3. There is a risk of damage to the IC and the socket due to

handling.• IC sockets are used on many repairable systems, for especially

mass produced systems, such that the cost must be kept low.• This type cannot be used for high reliability structures such as

spacecraft and mobile military equipment.


PIN GRID ARRAY


MICROELECTRONICS DEVICE FAILURE MODE

• For hermetically sealed semiconductor devices, there are no inherent wear out failure mode.

• There are no failure mechanism which depend upon operating or non operating time, within a manufactured device.

• The only way through which it can fail is overloaded beyond its design ratings consider such as ( Temperature, voltage ).

• There is a defect already , which causes immediate or progressive weakening.

• Therefore the reliability is very dependent upon quality control of the manufacturing processes and effectiveness of the screening techniques used to remove defective devices.



Thermal/mechanical stress: can cause many types of failure including internal breaks.

Broken wire in inductor imaged using micro focus x-rays

Contamination, migration and wear: contamination can be introduced during manufacture or during use, migration can occur under electrical.

Worn electrical contact

MICROELECTRONIC DEVICE SPECIFICATIONS

• To control the quality and reliability of microelectronic devices for military purposes, the US military specification was developed such as (M – 38510 )

• This describes general control and separate sections (slash sheets) give detailed specification of particular devices type.

• Similarly other type of standard specifications are IEC – International Electro technical commission

BS 9400 – British standard institution • Components produced to this specifications are referred to as

approved component.• Latest devices type available on the market do not have such

specifications. ( They are having an approach called capability approval )


SOLDER JOINTS• The reliability of electronic assemblies depends on the reliability

of their individual elements and the reliability of the mechanical thermal, and electrical interfaces (or attachments) between these elements.

• The characteristics of these three elements—component, substrate, and solder joint together with the usage conditions.

• The design life and the acceptable failure probability for the electronic assembly determine the reliability of the surface mount solder attachment.


GRAIN STRUCTURE

• The grain structure of solder is inherently unstable. • The grains will grow in size over time as the grain structure

reduces the internal energy of a fine-grained structure. • This grain growth process is enhanced by elevated

temperatures as well as strain energy input during cyclic loading.

• The grain growth process is thus an indication of the accumulating fatigue damage.

• At the grain boundaries contaminants like lead oxides are concentrated; as the grains grow these contaminants are further concentrated at the grain boundaries, weakening these boundaries.


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