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: S.Arun Kumar
Date posted: Wed Feb 9 12:48:39 US/Eastern 2005
Subject: PRISM Model for PCBs
Message:
When we want to compute the FR for PCB, we were not able to find a model for it in PRISM. We are Currently using MIL 217FN2 for PCBs.

Now the Questions we have are

Q1. How do I compute the Failure Rate for the PCBs using PRISM.
Q2. In MIL 217FN2 we have Model for PTH and SMT Version of the PCB. My Query is that since we take the solder joint factor, Cycling Rate Factor for most of the components we are using in PRISM, do we need to separately calculate the FR of the PCB (SMT Technology).


Reply:

Subject: PRISM Model for PCBs
Reply Posted by: David Dylis (ddylis@alionscience.com )
Organization: Reliability Analysis Center (RAC)
Date Posted: Wed Feb 9 12:56:27 US/Eastern 2005
Message:
Care must be taken when combining failure rates from MIL-HDBK-217 and PRISM since the MIL-HDBK-217 provides failure rates in operational hours and PRISM provides failure rates in calendar hours. Traditionally, reliability prediction failure rate models have been based on the operating time of the part, and the values were typically stated in failures per million (or billion) operating hours. The PRISM RACRates models (and the empirical data contained in the databases used in the PRISM tool) predict the failure rate in units of failures per million calendar hours. This is necessary because it represents the common basis for all the failure rate contribution terms used in the PRISM methodology (e.g., operating, nonoperating, cycling, and induced). If an equivalent operating failure rate is desired (in units of failures per million operating hours) steps need to be taken to make this conversion.

To convert a PRISM analysis from Calendar to Operating hours the resulting PRISM predicted failure rate (based on the expected duty cycle) should be divided by the duty cycle in its decimal form (e.g., A 50% duty cycle = 0.5). It would be incorrect to assume a 100% duty cycle for the system (unless the system actually operates at 100% duty cycle) as factors that influence reliability, such as cycling and non-operation, would be not be properly addressed.

Q1. How do I compute the Failure Rate for the PCBs using PRISM.
The PRISM tool incorporates extensive field reliability databases that can be used to assess the failure rates of components where PRISM predictive models do not exist. PRISM also allows user input of data. If a user has field experience or test data, this data can be factored into an analysis. The name for a PCB in PRISM is “Circuit Card Assembly, Unpopulated”.

Q2. In MIL 217FN2 we have Model for PTH and SMT Version of the PCB. My Query is that since we take the solder joint factor, Cycling Rate Factor for most of the components we are using in PRISM, do we need to separately calculate the FR of the PCB (SMT Technology).
Solder joint contributions will be included for any components where a PRISM RACRates model was used. Solder joint failure rates will need to be calculated for any components predicted by MIL-HDBK-217 or other sources.


Reply:

Subject: PRISM Model for PCBs
Reply Posted by: Rajeshkumar
Date Posted: Wed Nov 23 6:05:26 US/Eastern 2005
Message:
While dividing the PRISM failure rate by duty cycle, for small duty cycles ie less than 10%, failure rate is increased by 10 times,which is very high for certain applications. How can we consider this dutycycle factor in converting the calender hours to operational hours?


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