If the boards were built flat in the past, something changed when manufacturing the board. There's a good chance the board was cross-plied (the grain in the cores is different from the grain in the prepreg). If you know the type of prepreg used (7628, 1080, etc.) you can burn the board using a torch to expose the weave. Once you've burned the resin, you peel layers off until you get to the core and prepreg. You then count the bundles in one inch. 7628 for example has 44 bundles in warp (grain) direction and 32 in the fill direction. If the core and prepreg contain 7628, the number of bundles should be the same in the one direction. There could also be problems in the process of manufacturing the board. If there are heaters out on one side of the press platens, the rate of rise is different from one side versus the other. This will cause different stresses in the material. There's also some theory to stress relieve the core material before lay up by baking at 250F for 30-60 minutes. Did your supplier change this? Also some laminate suppliers stress relieve their material before shipment. Sometimes they don't if they get busy. Rick Babyak Process Engineer Proto Circuit Inc. 7 Ascot Parkway Cuyahoga Falls, OH 44223 330-572-3401 Fax:330-572-3434 [log in to unmask] ############################################################## TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c ############################################################## To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TECHNET <your full name> To unsubscribe: SIGNOFF TECHNET ############################################################## Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional information. If you need assistance - contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 ##############################################################