Answer to query by Andrew P Magee at Rogers-MCD. Your analysis assumes uniform strain and stress distribution, which is ok for first-order analysis and I use this technique myself for reliability estimates. However, this assumption is flawed and needs correction because it assumes that the layers remain parallel during thermal expansion-- they do not. The most obvious sign that they do not is land rotation. Remember the outer surfaces are free boundaries. The resin expands in 3-D; only the glass reinforcement prevents significant expansion in x and y. This creates large hydrostatic pressures around the PTH/via; this can be seen by the bulged in Cu barrel walls between inner layers. If one performs a 2-D FEA one finds that the stresses and strains are maximum in the center of the PWB. Work on this has been reported by Iannuzelli, DEC, and CALCE, U. of Maryland. A 3-D FEA shows that neighboring PTH/vias support each other (closer spaced PTH/vias experience less strains) and that the stress/strains in the 45 degree to the x,y-directions is largest in a square array. For more detailed information see IPC-D-279, Design Guidelines for Reliable Surface Mount Technology Printed Board Assemblies, Appendix B. I offer a one-day workshop "Design, Manufacturing and Reliability Issues of Plated-Through-Holes and Vias". Werner Engelmaier Engelmaier Associates, Inc. Electronic Packaging, Interconnection and Reliability Consulting 23 Gunther Street Mendham, NJ 07945 USA Phone & Fax: 201-543-2747 E-Mail: [log in to unmask] *************************************************************************** * TechNet mail list is provided as a service by IPC using SmartList v3.05 * *************************************************************************** * To unsubscribe from this list at any time, send a message to: * * [log in to unmask] with <subject: unsubscribe> and no text. * ***************************************************************************