Reliability Engineering for Medical Devices
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Transcript Reliability Engineering for Medical Devices
Reliability Engineering
Richard C. Fries, PE, CRE
Corporate Manager, Reliability
Engineering
Baxter Healthcare
Round 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
Failure Rate
Electrical Reliability
Time
Failure Rate
Mechanical Reliability
Time
Failure Rate
X-Axis
Theoretical Software
Reliability
Time
X-Axis
Failure Rate
X-Axis
Practical Software
Reliability
Time
X-Axis
Failure Rate
System Reliability
Time
Set the Reliability Goal
Based on similar equipment
Used as the basis for a reliability budget
Listed as Mean Time Between Failures
(MTBF) in hours or cycles
MTBF = the time at which 63% of the
units in the field will have failed
Minimum 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 replaced
Time terminated, no replacement
Failure terminated, failed parts replaced
Failure terminated, no replacement
Test until first failure
Test 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 level
Life 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
10
50
100
Number of Cyles
89,777,817
17,955,563
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 calculated
A 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 vibration
rapid temperature transitions
voltage margining
frequency 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 product
Quickly induce failures
By 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/humidity
Storage temperature/humidity
EMC
Surges/transients
Brown-outs
Electrocautery
Cell phones
ESD
Altitude
Environmental Testing
Autoclave
Shock
Vibration
Shipping
Tip testing
Threshold testing
Temperature Chamber
Walk-In Temperature
Chamber
Autoclave Testing
Customer Misuse
Excess weight on tabletop
Fluid spillage
Cross connection of wires
Pulling unit by non-pulling parts
Wrong order of pressing keys
“Knowing” how to operate the unit without
reading the manual
Making a Design Foolproof
The biggest mistake engineers make
when trying to make a design
completely foolproof
is 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 cycles
Can 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 month
Develop a reliability growth chart
Reliability Growth Example
Ventilator Reliability Growth
MTBF (Hours)
80000
60000
40000
20000
0
1997
1998
1999
Year of Report
2000
Reliability Growth Example
MTBF (hours)
Ventilator Reliability Growth
50000
0
1996
1997
1998
1999
Year of Report
2000
Reliability Growth Example
200000
Pre-June, 1997
Build
150000
100000
Post-June, 1997
Build
50000
0
19
97
19
98
19
99
20
00
20
01
20
02
20
03
MTBF (hours)
Estimate of Two Vaporizer Builds
Year of Build
The Reliability Group
You make it,
We’ll break it