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Forum: Reliability & Maintainability Questions and Answers

Topic: Reliability & Maintainability Questions and Answers

Topic Posted by: Reliability & Maintainability Forum ( )
Organization: System Reliability Center
Date Posted: Mon Aug 31 12:47:36 US/Eastern 1998

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Original Message:

Posted by: Shi Ke ( )
Date posted: Mon Sep 10 11:45:46 US/Eastern 2001
Subject: temperature for Arrenius law
Hi: In Bellcore TR-332, we consider the temperature factor. We use the Arrenius law to relate 2 temperatures. For the temperature, it refers to the operating ambient temperature. If the temperature around the device has a very big difference. What temperature should we use. By the way, is it more reasonable to use the junction temperature of transistor or chip? Thanks


Subject: Ambient Temperature Calculations
Reply Posted by: B.W. Dudley ( )
Organization: Reliability Analysis Center
Date Posted: Tue Sep 11 14:19:08 US/Eastern 2001
In “Bellcore TR332", I found the paragraph that addresses the problem that you have postulated. The Bellcore, now Telcordia, model uses the “ambient” temperature as the accelerating factor. Ambient temperatures can and do vary depending on the location of the heat producing source and the ultimate heat sink. The Bellcore procedure states that one should measure the temperature around the device. As you have stated, if there is a big difference what should one do? My suggestions are two-fold; one can use the highest measured temperature so that a conservative failure rate value will be calculated or one can perform a detailed thermal analysis based on known factors. This calculation requires the determination of the junction temperatures, cooling mechanisms, material properties and heat resistance factors. Standard thermal formulas are available from engineering references. One source we recommend is; the “RAC Thermal Management Guidebook”. This handbook, available from our product catalog, has design information along with thermal equations and is handy reference source. Your last question regarding the use of junction temperatures in the model is appropriate. Analyses using these values should be more accurate with regard to overstress conditions. The Bellcore model does not use this value, so one is stuck with the ambient temperature condition.

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