The stress of z-axis thermal expansion is greatest near the center of a PWB--that is why PTH barrels fracture there. The reason why inner layer separations occur predominantly on the outermost inner layers is the combination of the radial stresses from the expanding resin and the bending moments from the land rotations on the inner layers because the resin can more freely expand in the z-direction away from the PTH. 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. * ***************************************************************************