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]
  

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