Reliability Engineering for Medical Devices

Richard C. FriesManager, Reliability EngineeringDatex-OhmedaMadison, Wisconsin

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

More General 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

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T im eX -Axis

X-A

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Practical Software ReliabilityF

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T im eX -Axis

X-A

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System ReliabilityF

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Set the Reliability Goal

Based on similar equipment

Used as the basis for a reliability budget

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

Component Testing

Cycle/life testing of individual components

Comparison of multiple vendors of components

Determine applicability for the intended use

HALT

Acronym for Highly Accelerated Life Teing

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

Usually performed during the design phase

HASS

Acronym for Highly Accelerated Stress Screen

Uses the highest possible stresses as determined by HALT testing

Performed on 100% of the units being manufactured

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

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 does not demonstrate that a product will function in its intended environment

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

Goals of HALT Testing

The goal is more effectively met by testing at the lowest possible subassembly, typically individual PC boards

Card cages are not usually used due to the dampening effect of the cage on vibration

Cages also can block air flow, thus reducing stresses

Environmental Testing

Operating temperature/humidityStorage temperature/humidityEMC

Surges/transients Brown-outs Cell phones

ESDAltitude

Environmental Testing

AutoclaveShockVibrationShipping Tip testingThreshold 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

High Performance Air Compressor

Prototype Front Panel for Ventilator

Plastic Structure

Plastic Structure

Plastic Structure

Manifold Port

Prototype Switch

Autoclave Testing

Critical Care Ventilator

Critical Care Ventilator

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

Failure Analysis

Failure: device does not operate according to its specification

Determine root cause of the failure

Suggest methods to address the failure

The Reliability Group

You make it,We’ll break it