Hi Richard,
You are talking about a problem previously termed 'post separation' and now
'innerlayer separation. It occurs mostly at layers 2 and N-1, sometimes also
at 3 and N-2, and has to do with large thermal excursions during soldering
processes which cause large thermal expansions in the epoxy of the FR-4
boards. The epoxy expands, causing large z-axis movement causing upwards
rotation of the innerlayer attachments to the PTH/PTV barrels as well large
radial compressive forces on the PTH/PTV barrels. This can cause separation
either at the plating interfaces or in the copper foil of the innerlayers.
Solution options are: reduction in the maximum temperature of the soldering
process, using high Tg epoxy, using grade of copper foil with higher
ductility.

Werner Engelmaier
Engelmaier Associates, Inc.
Electronic Packaging, Interconnection and Reliability Consulting
23 Gunther Street
Mendham, NJ  07945  USA
Phone & Fax: 973-543-2747
E-mail: [log in to unmask]

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