Oops, sorry for the blank post.  My finger went too fast.

It takes more torque to loosen the screw because you have to overcome the clamping forces that are being applied to the parts in question.

For plastic bodied parts (TO-247, TO-220, etc.) with holes in the bodies you should be using either a Belleville washer and screw or a flat washer and a square-cone screw.  The washer is needed to distribute the load forces of the screw head.  You also need to determine just how much torque is necessary to obtain the desired clamping force.  Keep in mind that if there is a hole in the device for mounting, you can never fully clamp the device for maximum thermal transfer under the device die because the screw cannot go through the middle of the die.

We have found two solutions. Some manufacturers have come up with "hole-less" packages.  These have no mounting hole and require a spring type clip.  For these hole-less packages we use two parts from Aavid (this is not a marketing plug), one clip screws to the heat sink and one the clip snaps into the a special Aavid Max Clip® heatsink.  There is an alternative plastic screw down style clip, but we were not happy with the long term clamping forces after high temperature heat cycles.

What are you using for thermal transfer interface?  If you are using one of the phase change thixotropic materials like Isostrate® you should look into heating the device/heat sink assembly to 60°C before applying power to the device as you can only achieve maximum thermal transfer after the thixotropic material has gone through its pahse change temperature.  This can be accomplished by heating the assembly in an oven for 15-20 minutes or if it is attached to a large cold plate, run hot water through through the cooling tubes.

Hope this helps.

Phil

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Victor G. Hernandez
Sent: Thursday, May 05, 2005 5:25 PM
To: [log in to unmask]
Subject: [TN] Torque

Fellow TechNetters:

   Can someone share their experience with the topic of TORQUE.   How is
the torque valve of hardware differ from the
value of Un-torqueing that same hardware.   Is the torque valve higher
than the un torque value or vice versa.

   Power FETs can be damaged when the plastic package exhibits a hole
where the bolt is inserted.
Not the typical TO220 or TO66 plastic style.

Victor,

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