Your boards are fairly dimensionally stable to begin with because of the low layer counts. Beyond that, the following are basics: 1) Using balanced constructions with respect to reference plane locations (equally distributed in Z axis) and features (openings in metal equal on all planes, etc. to assure equal mass). If not, you have a bi-metal strip situation resulting in potato chips instead of PCB's. 2) Using low resin to glass ratio prepregs and core materials (7628,7629, 2116, as examples) - the more glass the more dimensional stability in all axes with tradeoff being reduced innerlaminar bond strength for the first two glass styles. 3) Using like, or homogeneous glass styles (60's yarn as an example using 106, 1080, 2113, and 2116 materials) so yarn bundles "nest" together and effect little relamination induced stress (later being relieved thermally during soldering operations causing excessive warpage). 4) Using unbalanced constructions with respect to higher layer count MLB's wherein a resin rich glass style (106 or 1080) is laid up with a less resin rich material (2113 or 2116) to provide the best of both worlds as constructions with higher relative dimensional stability and high bond strength. As an example for a 5 core construction, use one ply of 106 with one ply of 2113 instead of using two plies of 1080 and so on. Earl Moon (also at [log in to unmask]) ################################################################ 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 ################################################################