Using a flat washer can help immensely. The square cone screw is in the SEM screw family. It has a "conical" shaped permanently attached washer on the screw. It is designed for temperature cycled applications, like heat sinks, where repeated temperature cycles can cause normal hardware to loosen. It needs a metal surface to bind against, otherwise it will cut into the soft plastic case of the device. -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Bloomquist, Ken Sent: Friday, May 06, 2005 10:25 AM To: [log in to unmask] Subject: Re: [TN] Torque Hi Phil, I have to disagree with your "more torque to loosen" theory. When we do torque testing the torque meter will read something like 24 in/lbs to torque. Then when we loosen the screw to do another test it only takes about 8 to 10 in/lbs to break it loose. We've also tested this using a recording torque wrench and got the same results. Now tell me about this "Square Cone Screw"? I went on the internet and could only find "Square Cone Washer". The washer sounds really interesting. We have a problem on occasion of breaking the plastic shoulder washers we use to install TO-220's. I wonder if this washer would help. Have you used them? Does using just a flat washer help? Thanks, KennyB -----Original Message----- From: TechNet [mailto:[log in to unmask]]On Behalf Of Phil Nutting Sent: Friday, May 06, 2005 5:19 AM To: [log in to unmask] Subject: Re: [TN] Torque 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. Hope this helps. Phil --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 1.8e To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 ----------------------------------------------------- --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 1.8e To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 -----------------------------------------------------