PRISM Forum - Message Replies
Topic: PRISM Questions and Answers
Topic Posted by: 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.
Posted by:Thomas Urbanczyk
(thomas.urbanczyk "at" systema-gmbh.de
Date posted: Fri Jan 20 5:14:35 US/Eastern 2006
Subject: IC,Memory/Microprocessor Model
By performing a MTBF FaultTree breakdown (PRISM was used for MTBF data input), I noticed the following issue regarding the IC,Memory/Microprocessor Model as utilized for main processors:
How is it possible, that the PRISM model of a highly complex microprocessor (e.g. Freescale PowerPC) will deliver a lower (baseline) item failure rate as the PRISM model of a lowcost And-Gate?
Both components has been modelled with the same parameters. I would expect the defect rate of the microprocessor significant higher.
Could you please explain this circumstance?
Could you furthermore make a proposal, how to represent the complexity of such highly complex ICs versus simple AND-Gate ICs with the given options in PRISM?
Subject: IC Memory/Microprocessor model
Reply Posted by: David Dylis
Organization: System Reliability Center (SRC)
Date Posted: Tue Jan 31 9:11:32 US/Eastern 2006
To alleviate this issue, it is recommended that the junction temperature calculation be reviewed for complex integrated circuits as the default provided in PRISM may not be appropriate for the components in question.
The IC "Model Parameters" folder tab contains options for Junction Temperature Calculation and Hermeticity. The default selections, for which an IC failure rate will be automatically calculated with no input from the user, are:
• The use of ambient temperature and a default temperature rise to calculate IC junction temperature
• Non-hermetic package
The user has the ability to select three other options:
• actual temperature rise
• junction-to-ambient thermal resistance and power dissipation
• IC case temperature, junction-to-case thermal resistance, and power dissipation
It is recommended that you choose one of these three additional options when determining the failure rates of the complex circuits used in your design.
Case Temperature 50°C
Power Dissipated 1W
Reply Posted by: Thomas Urbanczyk
Organization: Systema Engineering
Date Posted: Thu Feb 2 2:59:45 US/Eastern 2006
thank you for your reply.
To be more precise, I'll make an example:
I modelled a simple "IC,Digital,Gate,AND" with the PRISM model "Use Case Temperature + Theta-JC*P".
The utilized parameters were:
Then I modelled a highly complex microprocessor by utilizing the IC,Digital,Micro Computer PRISM Part Type (I noticed, that it doesn't make a difference if I choose the IC,Digital,MICROPROCESSOR,16 BIT PRISM Part Type). I used the same PRISM model with identical input parameters as utilized for the AND-Gate.
The computed baseline failure rate of the microprocessor was not significant higher, as I would have expected. It was far from it. The PRISM computed baseline failure rate for the highly complex microprocessor was even roundabout 1% lower than the baseline failure rate of the simple AND-Gate. This does obviously not meet my understanding of reliability of complex components.
On this basis it is not possible for us to perform a MTBF breakdown of a complete module, as the MTBF critical main component won't be for example a PowerPC processor but already one or two AND-Gates.
I hope this explanation makes the problem more clear.