PRISM Forum - Message Replies


Topic: PRISM Questions and Answers
Topic Posted by: SRC (src_forum@alionscience.com )
Organization: SRC
Date Posted: Wed Jan 12 8:33:33 US/Eastern 2000
Topic Description: Welcome to the PRISM forum! Please feel free to post your questions and comments about the PRISM assessment software here.

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Original Message:

Posted by: George Pyryt (gpyryt@draper.com )
Organization:Draper Lab
Date posted: Fri Jun 9 15:58:04 US/Eastern 2000
Subject: Non-op electrical overstress factor
Message:
During the calculation of failure rates, Prism includes a serperate term for electrical overstress failure rate for all models. If I am trying to calculate the failure rate during storage over a long period of time, I do not think the electrical overstress should be that large of a factor. Is there a suggested factor or approach that I could use to reduce the part failure rate by when evaluating storage reliability. The failure rate Prism gives for a given resistor for example can be 2 orders of magnitude larger than if using the Mil-Std-217 data reduced by a factor of .01 for storage. The Prism failure rate I used included your previous suggestion of using 0 degree temperature rise for each component and 0% duty time.


Reply:

David Dylis Subject: Non-op electrical overstress/ESD factor
Reply Posted by: (ddylis@alionscience.com )
Organization: Reliability Analysis Center (RAC)
Date Posted: Wed Jun 14 7:29:53 US/Eastern 2000
Message:

To calculate the failure rate during storage of a resistor, the analysis of each

component must incorporate a 0 degree temperature rise and 0% duty cycle.  Dormant failure

 rates were calculated for varying types of resistors using PRISM, RADC-TR-85-91 (Impact of

 Nonoperating Periods on Equipment Reliability) and MIL-HDBK-217F Notice 2 (Parts Count

 method) modified by an operating to dormant adjustment factor from the Reliability

 Toolkit: Commercial Practices Edition.  Differences in predicted failure rates between

 each method is highlighted below:

 

 All Failures are in Failures/10^6 hours

 

 Fixed, Carbon Composition (RC/RCR)

 PRISM Failure Rate 0.002073

 RADC-TR-85-91:  0.0001512

 MIL-HDBK-217F Notice 2 (Adjusted):  0.00132

 MIL-HDBK-217F Notice 2 (Not Adjusted):  0.0066

 

 Fixed, Film (RN)

 PRISM Failure Rate 0.001097

 RADC-TR-85-91:  0.00024

 MIL-HDBK-217F Notice 2 (Adjusted):  0.00222

 MIL-HDBK-217F Notice 2 (Not Adjusted):  0.0111

 

 Fixed, Network, Film (RZ)

 PRISM Failure Rate 0.002494

 RADC-TR-85-91:  0.001032

 MIL-HDBK-217F Notice 2 (Adjusted):  0.00096

 MIL-HDBK-217F Notice 2 (Not Adjusted):  0.0048

 

 Fixed, Wirewound. Power (RW)

 PRISM Failure Rate 0.001627

 RADC-TR-85-91:  0.001368

 MIL-HDBK-217F Notice 2 (Adjusted):  0.0039

 MIL-HDBK-217F Notice 2 (Not Adjusted):  0.0195

 

 Fixed, Thermistor (RTH)

 PRISM Failure Rate 0.006747

 RADC-TR-85-91:  0.00648

 MIL-HDBK-217F Notice 2 (Adjusted):  0.00084

 MIL-HDBK-217F Notice 2 (Not Adjusted):  0.0042

 

 Variable, Wirewound (RT)

 PRISM Failure Rate 0.012097

 RADC-TR-85-91:  0.002376

 MIL-HDBK-217F Notice 2 (Adjusted):  0.00144

 MIL-HDBK-217F Notice 2 (Not Adjusted):  0.0072

 

 Note:  The PRISM results in this analysis do not take into account the PRISM System Level

 Multipliers which could reduce (or increase) the PRISM calculated failure rate when

 component are integrated into an assembly/system based on system processes used.

 

 The following assumptions were made during these calculations:

 Environment:  Ground Benign (PRISM Defaults for Ground Stationary, Indoors)

 Duty Cycle: 0% in PRISM

 Temperature Rise:  0C in PRISM

 MIL-HDBK-217 Adjustment factor:  0.2 from active to passive ground benign  (Source: Reliability

 Toolkit: Commercial Practices Edition, Reliability Analysis Center (RAC), pg. 178).

 Note that factors vary between 0.03 and 1.0 based on environment of interest for resistors.

 

 As seen by this analysis predicted dormant failure rates vary amongst part types and

 method of prediction.  PRISM is based on the RACís most recent data and modeling

 development efforts while models developed in RADC-TR-85-91 and MIL-HDBK-217F Notice 2

 were developed in 1985 and 1995 respectively.  In fact, MIL-HDBK-217 was not developed

 to predict dormant failure rates and an adjustment factor was developed and published in

 the Reliability Toolkit as a means to develop a dormant failure rate estimate due to

 lack of tools to properly estimate dormant failure rates.

 

 PRISM has been developed to address the shortcomings of current reliability prediction

 techniques with one of these shortcomings being the ability to predict dormant

 failure rates.  Models in PRISM have been developed to allow the user to predict

 failure rates of a system based on its actual usage including dormancy.   The

 electrical overstress factor in the models addresses the probability of EOS/ESD

 causing component failure.  During the development of PRISM component models, it

 was assumed that the EOS failure rate is independent of the duty cycle. The reason

 for this assumption was that EOS is an externally applied stress and not a function of

 power being applied. Parts in storage may still be affected by this factor due

 to handling, etc

 

 

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