I'm a design engineer, not involved the PCB industry other than as a consumer. I want to understand the mechanisms of bow and twist in multi-layer PCBs so that I can specify stackups that will stay flat during fabrication and assembly. I want is to understand the mechanisms well enough to be able to get away from the constraint of always building perfectly balanced PCBs with symmetric stackups. There are times when unbalanced stackups have advantages, and I'd like to have the flexibility to use them with confidence. I've heard a lot of different opinions on how to prevent warping. Copper balance, balance of laminate and prepreg thicknesses and types, copper thieving, solid copper ring around the perimeter of each layer, etc. Others say there is simply no way to predict whether a board will warp or not. There has to be some science to this. Have there been any detailed studies of the causes and prevention of PCB warping? Are the stresses that lead to warp always created during board fabrication, or can stresses be introduced later on during assembly or rework? Lots of questions. Any info or pointers to more info would be much appreciated. Thanks, Robert Sefton --------------------------------------------------- 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 847-509-9700 ext.5315 -----------------------------------------------------