Hey Brian - sounds like you're having a fun day. In general, the ability of a multilayer board to warp (or not warp) is governed by distance that the copper planes are separated from a theoretical center line in the laminate structure, and the mass of the copper. The two principle factors in this situation are: 1] the distribution of metal resulting from the design itself; and 2] how well the board fabricator is able to balance or equalize spacings of the metal in the laminate structure. Copper has a TCE of 17.6 PPM/degC. Glass in the fabric bundles has a TCE of about 7-8 PPM/degC. The resin in the laminate is the wild card. Finished, unclad laminates will have X-Y TCE's ranging from 6 PPM to 18 PPM below their Tg. Interestingly their X-Y TCE dimishes slightly above Tg. At any rate, these expansion differentials produce the force necessary to generate warp and twist. There are a number of techniques to minimize warp and twist during design: 1] keep the copper as thin as practical; 2] try to keep equal metal distribution on layers which will be equidistant from that theoretical centerline; 3] break up large plane areas by cross hatching, or with thin separation lines on the artwork, etc. Most board fabricators are well aware of the need to maintain flatness in their products, but it has to be supported by a design which will allow the fabricators work to succeed. In many cases, if you can pattern the physical warp and twist, the board designer and board fabricator can help eliminate the problem for the next build. I would also add, it is possible to cool a board in a laminating press, yielding an acceptably flat product, only to have it curl up like a potato chip when it goes thru reflow or wave solder. Good luck - Kelly ################################################################ TechNet E-Mail Forum 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's web site (http://www.ipc.org) "On-Line Services" section for additional information. For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312 ################################################################