Reliability Engineering

Richard C. Fries, PE, CRECorporate Manager, Reliability EngineeringBaxter HealthcareRound Lake, Illinois

Definition of Reliability

The probability, at a desired confidence level,

that a device will perform a specified function,

without failure,under stated conditions,

for a specified period of time

Customer’s Definition of Reliability

A reliable product:

One that does what the customer wants,

when the customer wants to do it

Reliability Basics

Reliability cannot be tested into a product

It must be designed and manufactured into it

Testing only indicates how much reliability is in the product

Purpose of the Reliability Group

Determine the weaknesses in a design

AND correct them

before the device goes to the field

Areas Covered by Reliability

Electrical

Mechanical

Software

System

Electrical ReliabilityF

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Mechanical ReliabilityF

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Theoretical Software Reliability

Fa

ilure

Ra

te

T im eX -Axis

X-A

xis

Practical Software ReliabilityF

ailu

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ate

T im eX -Axis

X-A

xis

System ReliabilityF

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Set the Reliability Goal

Based on similar equipmentUsed as the basis for a reliability

budgetListed as Mean Time Between Failures

(MTBF) in hours or cyclesMTBF = the time at which 63% of the

units in the field will have failedMinimum goal is ten years with a 98%

reliability

Parts Count Prediction

Uses MIL-HDBK-217

Indicates whether the design approximates the reliability goal

Indicates those areas of the design with high failure rates

Chemical Compatibility

Test plastics with typically used chemical agents (alcohol, anesthetic agents, cleaning agents)

Cleaning agents are the worst

Force Puller

Component Testing

Cycle/life testing of individual components

Comparison of multiple vendors of components

Determine applicability for the intended use

Philosophy of Testing

Test to have the units pass

Test with the addition of stresses to check the margins of functionality

Types of Tests

Time terminated, failed parts replacedTime terminated, no replacementFailure terminated, failed parts

replacedFailure terminated, no replacementTest until first failureTest until all samples fail

Determining Sample Size

Uses Chi-Square table

SS = Chi-square Value(MTBF goal)/2

Chi-square value includes confidence level and degrees of freedom = 2f+2

Component testing – 90% confidence levelLife testing – 95% confidence level

Sample Calculation

Want to test valves to be used for 2,000,000 cycles per year with a 10% failure rate after 10 years

Reliability = e(-t/MTBF)MTBF = -t/ln Reliability = -20,000,000/ln 0.90 = 389,914,514 cycles

Sample Calculation

MTBF = 389,914,514 cycles

Number of Samples Number of Cyles10 89,777,817

50 17,955,563 100 8,977,782

Component Test Setup

Component Test Setup

Component Test Setup

Calculating Sample MTBF

MTBF = (# of samples)(length of test) # of failures

Calculating MTBF Where No Failures Occur

A sample MTBF cannot be calculatedA lower one-sided confidence limit is

calculated and the MTBF stated to be greater than that number

One-sided limit = 2(#units)(test time) Chi square value for the confidence limit and 2 degrees of freedom

Sample Calculation for a No Failure Test

10 valves are tested for 10,000 cycles with no failures. Calculate using a 90% confidence level.

One-sided limit = 2(10)(10,000) 4.605 = 43,431 cycles MTBF > 43,431 cycles

HALT

Acronym for Highly Accelerated Life Testing

Used to find the weak links in the design and fabrication process

Usually performed during the design phase

HALT Testing

Possible stresses that can be applied:random vibrationrapid temperature transitionsvoltage marginingfrequency margining

The product is stressed far beyond its specifications

The test can be set up to find the destruct limits

HALT Chamber

Goal of HALT Testing

Overstress the productQuickly induce failuresBy applying the stresses in a

controlled, stepped fashion, while continuing monitoring for failures, the testing results in the exposure of the weakest points in the design

This test, if successful, will expose weak points in the design

Environmental Testing

Operating temperature/humidityStorage temperature/humidityEMC

Surges/transients Brown-outs Electrocautery Cell phones

ESDAltitude

Environmental Testing

AutoclaveShockVibrationShipping Tip testingThreshold testing

Temperature Chamber

Walk-In Temperature Chamber

Autoclave Testing

Customer Misuse

Excess weight on tabletopFluid spillageCross connection of wiresPulling unit by non-pulling partsWrong order of pressing keys“Knowing” how to operate the unit without

reading the manual

Making a Design Foolproof

The biggest mistake engineers makewhen trying to make a design

completely foolproofis underestimating the ingenuity

of complete fools

Failure Analysis

Failure: device does not operate according to its specification

Determine root cause of the failure

Suggest methods to address the failure

Prototype Front Panel

Plastic Structure

Plastic Structure

Autoclave Testing

Manifold Port

Prototype Port

Life Testing

Operate the device in its typical environment and application

Use appropriate on/off cyclesCan be used to verify the reliability

goal or a specific period of time, such as the warranty period

Tracking Reliability Growth in the Field

Collect manufacturing data on how many units were manufactured by month

Collect field failure data, by monthDevelop a reliability growth chart

Reliability Growth Example

Ventilator Reliability Growth

0

20000

40000

60000

80000

1997 1998 1999 2000

Year of Report

MT

BF

(H

ou

rs)

Reliability Growth Example

Ventilator Reliability Growth

0

50000

1996 1997 1998 1999 2000

Year of Report

MT

BF

(ho

urs

)

Reliability Growth Example

Estimate of Two Vaporizer Builds

0

50000

100000

150000

200000

Year of Build

MT

BF

(h

ou

rs)

Pre-June, 1997Build

Post-June, 1997Build

The Reliability Group

You make it,We’ll break it