Steve/Alcon:
- 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.
That is in the form of tests of PTH integrity that were conducted by Roger
Wilde 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 MIB Z thickness expansion toward both surfaces of the MIB.  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%.  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)."
- 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.
- 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.
- 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 Tce (or
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.
Regards, Jim Moffitt

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