Hi Jim,
>- The only objective evidence that I have ever found is that which we
>considered when developing the criteria for Rev. C of J-STD-001 and A-610.
Item 1: >That is in the form of tests of PTH integrity that were conducted by
Roger
>Wild in the early & mid 70's.  In one area he makes the statement "Another
>PTH stress concentration that can reduce PTH reliability is partial solder
>filling of the PTH's.  The PTH condition may concentrate normal PTH
>expansional stresses into a specific area of the PTH's, with a result of more
>rapid 360 degree type PTH barrel crack propagation in severe thermal cycling.
>Plated-through-hole barrel plating stresses are also increased when the MLB
>is adhesively bonded to a stiffener or heat sink.  This bonding prevents
>normal MLB z-thickness expansion toward both surfaces of the MLB.  In this
>bonded configuration, the PTH's furthest away from the bonded surface will
>experience higher thermal deflections and stresses."
>His tests included thermal cycling of PWB that had a controlled amount of
>solder in each PTH.  Solder fill was controlled to 0%, 25%, 50%, 75%, and
100%.
Comment: Roger's observations are as true today as then. I have consulted on
a number of such failures. They do occur in the more severe field
environments, such as industrial, military, and automotive. I have not seen
it in telecommunications or computers.
Item 2: >In one section he notes that: "multiple joint failures were found
>between 201 and 260 thermal cycles, all failures (fractured solder joints)
>were only on connectors with 0% PTH solder penetration (capped solder
>joints).  Although cracking was significant, there were no failures with
>at least 25% solder penetration (0.035" PTH solder plug)."
Comment: Roger's observations pertain to the solder joints involved, not the
PTVs.
Item3: >- Reading his reports one is compelled to conclude that solder fill
of 25% or
>more will be acceptable for PWA's in all but the most thermally challenging
>environments.
Comment: This conclusion is NOT warranted in this form. The solder joints
will be acceptable--this means that single-sided boards with unsupported
holes should essentially be called Class 0 [zero]--but he PTH is not for the
more severe, not just "the most thermally challenging" environments
Item 4: >- I think that this series of reports (including the Colin Lea
report that
>approves of blowholes and voids) can be downloaded from the IPC FTP site.
Comment:  I am in Hong Kong right now, so I can not look up Colin's report to
see the context.
Item 5: >- I suspect (fervently hope) that PWB fabrication technology has
improved
>since the mid 70's and that PWB's fabricated today have less z-axis CTE
>and better control over the elongation and elasticity of copper deposition.
>Is it time for someone to run a series of tests with newer board
>material and determine that we have been over-specifying PTH solder
>connections for years.
Comment: While Cu plating technology has improved it was in response to
smaller via diameters and thicker PWBs. Thus, as far as Cu strength and
ductility in the holes nothing  much has changed. Higher Tg's have helped,
however.

Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL  32174  USA
Phone: 386-437-8747, Fax: 386-437-8737
E-mail: [log in to unmask], Website: www.engelmaier.com

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