Also, I have used 'Ny-lock' or nylon patch treated machine screws in place of any type of 'biting' or surface deforming device like a star washer or split-lock washer under pan head style machine screws. These pre treated fasteners can prevent loosening of the fastener under vibration or even from thermal excursions. The nice thing about them is they can be loaded into a screw feeder machine and installed automatically because the thread locker is cured and dry and don't fall off the screw thread during handling. This type of treated fastener will limit the torque required to retain the screw in the treads ultimately retaining the board to its hardware mounting points...without galling the top surface of the board all up. http://www.nylok.com/products_trueblue.htm Depending on the application, 'trilobular' style screws are a potential option too. They grip the mating threads because they are not perfectly round and bite into the threads as the screw fastener is engaged. Or there are also lock-tite liquids or 'sticks' that can be very effective. But they are hand applied and cannot be machine inserted for high volume applications to the best of my knowledge. http://www.taptite.net/powerlok.asp It would be extremely advisable to avoid breaking the fibers in the board by over applying torque to the fasteners. That sort of injury to the sealed integrity of the board could potentially allow moisture incursion into the interior of the epoxy laminate and eventually causing adhesion or delamination failures. Under vibration the failure of the mechanical fastening feature would be likely be next to follow. http://www.boltdepot.com/fastener-information/US-Recommended-Torque.aspx Torque values for fasteners used with printed circuit materials would be a really great research project and white paper... anyone want to tackle it? :) Best regards, Bill Brooks PCB Design Engineer, C.I.D.+ Tel: (760)597-1500 Fax: (760)597-1510 Datron World Communications, Inc. Vista, California -----Original Message----- From: Stadem, Richard D. [mailto:[log in to unmask]] Sent: Thursday, October 12, 2006 12:02 PM To: [log in to unmask] Subject: Re: [TN] Compression Limit on FR4? Hi, Jack I can tell you that typically the torque setting used for hardware on pwb assemblies is between 1 to 6 in/lbs. More than that tends to deform the fiberglass over time. Of course, it all depends on the type of board material, layer count, etc. It also depends on the application. Realize that a lot of relaxation occurs during any type of thermal excursion, and the hardware will loosen up. It is always a good idea to use a Bellville washer in conjunction with a serrated or star washer to continue to hold the torque. It is also a good idea to use Glyptol or some other threadlocker. -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Jack Olson Sent: Thursday, October 12, 2006 1:54 PM To: [log in to unmask] Subject: [TN] Compression Limit on FR4? Does anyone know of any guidelines or recommendations that suggest a maximum limit on compressing board material? Can any damage occur by over-torque'n screws, for example? I've never heard anyone discuss that... Jack --------------------------------------------------- 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 -----------------------------------------------------