PRISM Forum - Message Replies

Topic: PRISM Questions and Answers
Topic Posted by: SRC ( )
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: Ralph Listl ( )
Date posted: Mon Feb 27 5:27:44 US/Eastern 2006
Subject: Prism Reliability Model
The Prism Reliability Model uses an initial failure rate multiplied by a process grade factor. The calculation of the inital failure rate for all via RACrates model supported items (Capacitors, Resistors...) makes use of a temperature dependent factor in their models. Therefor all base failure rates are temperature dependent. The process grade factor does also include an environment factor that is temperature dependent. If a complete system has been set up and the temperature is changed it influences the base failure rates as well as the process grade factor. From my point of view a temperature dependency should either be applied on the base failure rate or for the process grade factor but not on both, because this is double influence. A temperatue dependence on item level is understandable, but a temperature dependence on a process grade is for my understanding not correct. Could you please explain why the Prism model is set up like this?

Previous reply

Subject: Temperatue Dependancy
Reply Posted by: David Dylis ( )
Date Posted: Tue Feb 28 9:52:18 US/Eastern 2006
PRISM differs from other prediction methodologies as it takes into account processes that affect the reliability of a system. Prior to PRISM these factors were typically ignored and only the failure rates of components were considered.

The achievement of system reliability relies on the following elements:
• Obtaining valid system requirements and managing interface dependencies
• Good quality parts must be chosen
• The parts must be designed into a system in a robust manner to ensure that they can meet performance requirements over the system’s design life
• The design must be validated through analysis and test
• The system must be manufactured without inducing damage or adding defects
• A management philosophy that supports the achievement of the above five elements

The PRISM design PI factor considers failures resulting from an inadequate design. Examples are tolerance stack-up, unanticipated logic conditions, a non-robust design for given environmental stresses, etc. Below is a sample of some questions from the PRISM design process grade that address temperature related issues:

Is design of experiments (DOE) used to ensure robustness of the FRU in the product under all operational and environmental variations?
Will mechanical stress analysis be performed on relevant components, materials and structures?
Will the chassis that this FRU is mounted in be thermally measured and analyzed and operating temperatures assured to be at a safe margin below device limits?

In addition, PRISM does not have a PI-Q factor at the component level as in MIL-HDBK-217 and other methodologies. Part quality is assessed at the system level …

I will send a copy of a report that describes the basic process on how the PRISM system level models were derived and the process that addresses the PI-E factor in PRISM.

If you have additional questions, feel free to contact me.

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