Forum: Reliability & Maintainability Questions and Answers

Topic: Reliability & Maintainability Questions and Answers

Topic Posted by: Reliability & Maintainability Forum (src_forum@alionscience.com )
Organization: System Reliability Center
Date Posted: Mon Aug 31 12:47:36 US/Eastern 1998
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Original Message:

Posted by: Glen Hanington
Date posted: Thu Apr 25 20:18:06 US/Eastern 2002
Subject: Heatsink mounted TO-247's, no strain relief.
Message:
My company is using a technique for mounting TO-247 power FET's which I believe to be unreliable. With the heatsink screwed to the top of the PCB the FET's are held to the aluminum heatsink with a metal clip that presses on the center of the device, providing good thermal conductivity to the heatsink through a Silpad. However, the leads from the FET's go straight down into the PCB and are soldered in place without any strain relief. Depending on the model, this distance can vary from 0.25" to 0.50". Do you know of any studies modeling this arrangement vs. others with strain relief, and whether it can promote internal fracturing of the die? Our units typically function in a laboratory or industrial type environment, but the heatsink temperature can get up to 85degC. I believe that the aluminum has a slightly higher coefficient of thermal expansion than the copper leads of the devices, but I don't know what forces would be generated and if it would be enough to crack anything. Since I just started working here I've not had the opportunity to review field failure data.

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