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: John Podulka (poduljo@mail.northgrum.com )
Organization:Northrop Grumman ESSD
Date posted: Fri Jun 30 10:36:21 US/Eastern 2000
Subject: Drawing content
Message:
I am interested in finding out about what criteria is used to determine the quality factor on Semi's and Microcircuits. This information needs to go into our piece part drawings. Under 217, of course, we had the different classes of microcircuits, and semi's have the "TX" etc ratings.
Passives of course have the Exponential failure rates as well as the old Weibul letters.
What does Prism use, and how can I find out this information without buying the software package?
Thanks for your consideration
John Podulka Proj Eng Component Engineering


Reply:

David Dylis Subject: PRISM Process Grades
Reply Posted by:
Organization: Reliability Analysis Center (RAC)
Date Posted: Fri Jun 30 10:59:55 US/Eastern 2000
Message:

PRISM accounts for part quality in a different manner than does the current MIL-HDBK-217 prediction methodology. Based on user inputs, PRISM determines an assembly or system "inherent" failure rate. This failure rate, however, represents only the physical attributes of the components that comprise it, subject to the environmental conditions and operating profile characteristics associated with the application. The system or assembly failure rate that will actually be experienced in the field may be potentially better or worse than the inherent failure rate, depending on the processes used by the respective manufacturer(s) to ensure that:

  1. their design is reliable and robust,
  2. parts of acceptable quality are selected and controlled,
  3. manufacturing practices do not degrade reliability performance,
  4. management processes encourage good requirements definition and design practices,
  5. maintenance activities do not induce failures,
  6. cannot duplicate incidents are minimized,
  7. wearout and infant mortality issues are understood and addressed,
  8. reliability growth is emphasized throughout the design and development phases,

This process-related variability around the assembly or system inherent failure rate is implemented within PRISM via a methodology called Process Grade Factors. By answering a series of questions within a specific grade type, the user establishes a scoring profile whose total value is translated into a quantitative pi-factor multiplier for that type that impacts the predicted failure rate (positively or negatively). The process grade types currently defined within PRISM, which includes part quality, are:

  • Design
  • Manufacturing
  • Parts Quality
  • System Management
  • CND
  • Induced
  • Wearout
  • Growth
  • Infant Mortality

Each of these types must be scored, and the types combined into a system- or assembly-level process grade set, in order to have their influence reflected in an assembly or system reliability prediction. The specific questions relating to part quality and other process grade types are available and can be tested in the PRISM demo. This demo can be downloaded for free from the RAC PRISM web site.


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