Reliability Analysis and Testing of Medical Devices ... · PDF fileJohn Whitehurst Reliability...

John Whitehurst

Reliability Analysis and Testing

of Medical Devices

John Whitehurst, QA Engineer

Minnetronix Inc.

ASTR 2013, Oct. 9-11, San Diego, CA

Reliability Analysis and Testing of

Medical Devices

Minnetronix Case Studies

John Whitehurst


of Medical Devices

Background

Using reliability testing and analysis during design

and manufacturing

Case studies

ASTR 2013 Oct 9-11, San Diego, CA

Today’s Presentation

John Whitehurst


of Medical Devices

Minnetronix

Partner with medical device companies to design, test,

and manufacture electro-mechanical medical devices

High complexity class II and III devices

17 years experience

John Whitehurst

QA engineer for many years. Perform design

verification and reliability testing for many devices

including heart pump controllers, sinus debriders, etc.


John Whitehurst


of Medical Devices

Medical Devices & Reliability

Cannot tolerate field failures

• Patient safety

• Regulated environment

• Business financial ramifications


John Whitehurst


of Medical Devices

Learn About Reliability Early

Design Phase – Reliability assessment as part of

the design process

Production Phase - Stress screening of a device

as part of the manufacturing process


John Whitehurst


of Medical Devices ASTR 2013 Oct 9-11, San Diego, CA

Design Methods

Determine reliability goals

Reliability planning

• Mechanical stress analysis

• HALT at the component or

system level

• Electrical component analysis

• Life testing

John Whitehurst


of Medical Devices

Production Methods

Stress screening

Trend analysis


John Whitehurst


of Medical Devices

Case Study 1: Design Reliability - Single Board

Computer reliability assessment

Case Study 2: Design Reliability – Mobile Power

Unit reliability testing during DVT

Case Study 3: Production Reliability - Stress

screening of a device as part of the

manufacturing process


Case Studies

John Whitehurst


of Medical Devices

Medical Grade Single Board Computer

design initiative – component of a blood

analysis device

Determination of reliability goals

Creation of reliability plan

Testing to assess alpha reliability and

what design changes were necessary

for beta iteration


Case Study 1

John Whitehurst


of Medical Devices

SBC Reliability Goal: “A mean time between failure

(MTBF) of 5 years or approximately 20,000 hours

of continuous use will be achieved.”


Case Study 1

John Whitehurst


of Medical Devices

SBC Reliability Plan

Step 1: Apply mechanical analysis tool

(mechanical stress)

Step 2: Perform HALT testing

Step 3: Apply traditional MTBF component

analysis (electrical stress)


Case Study 1

John Whitehurst


of Medical Devices

Step 1 - Mechanical Analysis Tool

Used to provide feedback on early-stage design and

layout

Models board stack-up and components

Simulates based on models, life cycle conditions


Case Study 1

John Whitehurst


of Medical Devices

Simulation Input and Output

Inputs

CAD files that describe all layers, including drill file

BOM and Pick & Place files

Life cycle conditions

Outputs

Failure rates

Problem areas


Case Study 1

John Whitehurst


of Medical Devices

Performing the Analysis

Set life cycle conditions

Temperature cycles

Shocks

Vibrations – random and harmonic

Used customer expectations of field conditions to

generate the “daily stresses” on the board


Case Study 1

John Whitehurst


of Medical Devices

Running Simulations

Simulations gave results on:

Plated through hole fatigue

Risk of CAF formation (migration of copper filaments)

Solder joint lifetime (both thermal and vibration)

Interconnect failures

MTBF


Case Study 1

John Whitehurst


of Medical Devices

Reliability Report

Gives lifetime estimate

Assigns modular scores

Highlights “problem areas” on the board due to

thermal, vibration, and shock stresses

Identifies components with shortest estimated

lifetimes


Case Study 1

John Whitehurst


of Medical Devices

Life Prediction Curve


Case Study 1

John Whitehurst



Case Study 1

Vibration Analysis - Indicates components susceptible to

vibration

John Whitehurst


of Medical Devices

Step 2 – HALT

Subassembly Level HALT

Find design problems early

Different failure modes

Allows more stringent testing

Can validate efforts from ME Analysis

Requires robust test setup


Case Study 1

John Whitehurst


of Medical Devices

Circuit Board

HALT Testing


Case Study 1

John Whitehurst


of Medical Devices

Circuit Board HALT Testing

Specified limits prior to testing

• 5 to 45grms and -10 to 90C

Uncover design weaknesses

• Critical findings incorporated in Beta design

• Sample size of N=3

No pass/fail criteria

• Observe general error codes/flickering of display


Case Study 1

John Whitehurst


of Medical Devices


Robust test setup

VGA screens outside of chamber for UI analysis

Used HyperTerminal to gather data

External power supplies powered boards

Extensive labeling of cables


Case Study 1

John Whitehurst


of Medical Devices


Results:

Reached 45 grms, -5C and 90C

X-ray inspection of boards:

• BGA component on pad; no solder bridges

• No cracked solder joints

• Minimal signs of voiding at solder attachments

• Good electrical connections

• No visual issues with BGA components


Case Study 1

John Whitehurst


of Medical Devices

Step 3 - MTBF Analysis

Ops A La Carte

Sent BOM and Schematic

Defined operational limit of 45C

• Room temp plus 20C internal rise of case

Used HALT Calculator with testing results

Comprehensive electrical stress analysis


Case Study 1

John Whitehurst


of Medical Devices

Early design analysis assisted with layout and

placement of PCB board

HALT testing verified robustness of design

MTBF calculations used to predict/verify life of

design


Case Study 1 Conclusions

John Whitehurst


of Medical Devices









Case Studies

John Whitehurst


of Medical Devices

Mobile Power Unit - Component to a life support

device

Portable device for use in a home environment

Foreseeable rough use for the device

Demands high reliability

Life testing during DVT to learn about reliability and

design weakness before manufacturing.


Case Study 2

John Whitehurst


of Medical Devices

Mobile Power Unit - Design Reliability

Cable Flex Testing – 95/95

System Level Life Testing – 2 Years

Cycle Testing – 1460 Cycles


Case Study 2

John Whitehurst


of Medical Devices

Cable Flex Testing

Reliability Target: 95/95

Pull Force: 31 Lbs.

Sample Size (Single Failure):

95

(reduced to 20 samples X5)

Raw Cable, Strain Relief &

Housing


Case Study 2

John Whitehurst


of Medical Devices

System Level Life Testing

Reliability Target: 90/80 @ 2 years

Test Environment: 55C @ 846 Hrs

(Arrhenius model – 10.35 AF)

Sample Size (Single Failure): 30

Continuous Monitoring of outputs

(loaded)


Case Study 2

John Whitehurst


of Medical Devices

Cycle Testing – Critical Features

Battery, AC Power & Alarm Cycled

Reliability Target: 90/80 @ 2 years (1460 Cycles)

Simulated power cycling, alarm activation & VAD connect

/ disconnect

Sample Size (Single Failure): 30

Continuous Monitoring of outputs


Case Study 2

John Whitehurst


of Medical Devices

Battery capacity better than expected

Flex testing drove a design update to the shielding

Had to compensate for thermal rise from test units

during life test exposure



John Whitehurst


of Medical Devices









Case Studies

John Whitehurst


of Medical Devices

Class III life support device

Small enclosure, primarily electronics and display

Learning about reliability in manufacturing

• Manufacturing screen to remove early stage failures, infant

mortality, of device before entering the field


Case Study 3

John Whitehurst


of Medical Devices

Electronic failure mechanisms include infant

mortality as well as wear out.

Environmental stress screens (ESS) can be used to

reduce infant mortality failures


Case Study 3

John Whitehurst


of Medical Devices

What is ESS?

Use accelerated stress to detect latent faults

Vibration

Thermal Cycling

Power Cycling

Goal: find devices with latent faults prior to shipping


Case Study 3

John Whitehurst


of Medical Devices

Manufacturing Screen Plan

Decide approach

System level screen – thermal/power cycling

Board level screen – random vibration

Minimize risk of potential damage to device under test (DUT)

Design scalable test fixture

Automated data collection

Limited sample sizes initially (clinical trials)

Process analysis

Use data to verify and track failures

Case Study 3

John Whitehurst


of Medical Devices

Random Vibration


Case Study 3

Board Level Assembly

& TestRI

Top Level Assembly,

Test & Calibration

Thermal/Power

Cycling

Final Inspection Shipping

Random Vibration

• Board Level Screen – Random Vibration

• Boards unpowered during vibration

• Functional test post vibration exposure

• Heavy burden – external testing

• Yielded few faults

John Whitehurst


of Medical Devices

Thermal/Power Cycling


Case Study 3

System Level Screen – Thermal/Power

Cycling Units powered on/off during thermal cycling

Continuous monitoring

Lowest burden – in-house equipment

Cost effective screen

Board Level Assembly

& TestRI

Top Level Assembly,

Test & Calibration

Thermal/Power

Cycling

Final Inspection Shipping

Random Vibration

John Whitehurst


of Medical Devices

Thermal Cycling Profile


Case Study 3

-60

-40

-20

0

20

40

60

80

100

0 1000 2000 3000 4000 5000 6000

Thermal Cycling

Thermal transition rates: 5C/min

Thermal limits: +80C & -40C

10 Minute dwell times

12 – 14 Thermal Cycles

Powered off at temperature extremes

John Whitehurst


of Medical Devices

Thermal Cycling

Maintain a consistent screen

environment

Thermal modeling

Good (constant) air circulation

Mounting trays can restrict or

direct air flow

Augment air circulation

Robust screen environment


Case Study 3

John Whitehurst


of Medical Devices

System Qualification Screen Validation – Effectiveness / End of Life

Qualified For Intended Use

Test System Design Documentation

Identify intended use and audience

Requirements – System & software

Risk Management – Identify associated risks and control measures

Design documents (schematics, drawings, software, BOM)

Configuration Management

Test Procedures and Expected Results

Installation, Calibration and Maintenance detail

Record of results from qualification activities (Report).


Case Study 3

John Whitehurst


of Medical Devices

Process Monitoring

Mechanical Vibration – Board level screen failed to

detect latent faults.

Removed the screen

Thermal Cycling – System level screen catches

latent faults.

Improved this screen


Case Study 3

John Whitehurst


of Medical Devices

Screen Effectiveness / End

Of Life

Process monitoring

Number of thermal cycles

increased to 14

Analyzed life consumed –

Less than 1%


Case Study 3

0 2 4 6 8 10 12

John Whitehurst


of Medical Devices

Faults detected and removed prior to field release

Poor solder connections of…

• Large components

• BGA components

• RTC battery

• PCBA connectors

• Open VIA

Failed components (RTC, diodes, caps)

LCD failure (custom LCD)



John Whitehurst


of Medical Devices

Reliability as part of design

Uncover design weaknesses

• Done early in design

• Stress board outside operational limits

Reliability as part of production

Done during manufacturing process

• Reduce infant mortality

Uncover faults not caught during board level


In Summary…

John Whitehurst



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