Reliability Engineering for Medical Devices Richard C. Fries Manager, Reliability Engineering Datex-Ohmeda Madison, Wisconsin
Dec 18, 2015
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
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
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
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