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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.


Reply:

Subject: TO-247 Mounting
Reply Posted by: B.W.Dudley (bdudley@alionscience.com )
Organization: RAC
Date Posted: Wed May 1 9:57:28 US/Eastern 2002
Message:
After searching the web for a description of the mounting and heat sinking procedures for power FETS, I found a reference that has mounting guidelines for the Super 247. The address is; http://www.irf.com/technical-info/appnotes/an-997.pdf . This site indicates that the procedure that you are using is one that they recommend. The need for strain relief is not indicated in the guidelines. Consulting with design engineers, I found that strain relief, that is a half loop bend in the leads, is only used if extreme stress conditions are expected. These conditions would include use in a airborne rotary wing, airborne uninhabited or missile launch environment. Ground controlled conditions should not be a significant stress factor as the temperature changes should be insufficient to cause a solder fatigue failure which is the mode one should be concerned with.


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