One of the reliability concerns with multiple excursions of PWBs to soldering
temperatures is the structural reliability of the PTH/vias both in terms of
barrel cracking and innerlayer separation (post-hole separation). The more
excursions and the higher the temperatures the higher the stresses.  Smaller
hole diameters, thinner plating, thicker PWBs are more susceptible.

I teach a workshop "Design for Reliability and Quality Manufacturing of
Plated-Through-Holes and Vias", to be given next at NEPCON West'97 and IPC
Expo that deals with the subject you are asking about. Following is a short
course description: For high-density electronic assemblies the diameters of
plated-through-holes (PTH)/vias are becoming progressively smaller, while
board thicknesses might increase.  This has led to PTH/via failures after
assembly or in the field, because the standard manufacturing processes can be
inadequate to produce high quality and yields.  High yields and long-term
reliability can only be assured by a deliberate design effort that considers
the specific damage mechanisms and stresses, resulting from further
processing and operational use, threatening the integrity of the electronic
assembly.  Knowledge of the material properties and fatigue behavior of the
PTH/via copper barrels, and assembly processes and use environments are
necessary for the successful 'Design for Reliability' of high-performance,
high-density electronic assemblies.  
The design information presented has been included in the industry document
IPC-D-279, 'Design Guidelines for Reliable Surface Mount Technology Printed
Board Assemblies'. 

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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