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?

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Subject: Temperature Dependency
Reply Posted by: Ralph Listl ( )
Organization: EADS
Date Posted: Tue Feb 28 7:40:15 US/Eastern 2006
Thank you very much for your prompt response. I agree to your statement of having the temperature as a parameter that has influence on the inherent failure rate of an item as well as to the process of manufacturing of the item. Temperature has from my point of view no influence on the desgin and development process. The modell that is used by PRISM makes it possible to change the temperature in the environment area. if you do this and change for example the OP. temperature from 85C to 70 C, trickle down through the system, you receive a different failure rate for the system and a different proces grade factor. This is as far as I understand reliability predictions not correct. The OP. temperature is the temperature used during operation of the item and is used for calculation of the inherent failure rate. Within PRISM it is also used for the environmental factor in the process grade factor. This must not be mixed. For the process grade factor other temperature settings must be used. How is the temperature really used within PRISM during calculation of the environmental factor ? I hope this explanation makes it more clear, where I see the difficulties in the PRISM model.

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