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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]
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