Mr. Engelmaier,  (and TechNet Forum)

        Below, you touch briefly on the effect of non-functional pads (and
their absence) on plated through holes.
        Has any one documented the effect of non-functional pads, or their
absence, on plated through hole reliability?  (Sometimes our FAB vendors
have suggested eliminating non-functional pads to improve innerlayer
processing / yields)
        I would value your input.

        Best Regards,

        Ken Devitt
        ITT Avionics
        Clifton, New Jersey
        973-284-3880
        Fax 973-284-3736

>----------
>From:  [log in to unmask][SMTP:[log in to unmask]]
>Reply To:      TechNet Mail Forum.
>Sent:  Wednesday, August 27, 1997 8:30 PM
>To:    [log in to unmask]
>Subject:       Re: [TECHNET] [T] Annular Ring Limits
>
>Hi Les,
>I don't know about the 'Ring and Ball' experiment, but I do know what happens
>inside a PWB around the PTHs during heating to solder temperatures.
>(1) At those temperatures the epoxy is way above its Tg, is very soft, and
>should be considered a high-viscosity liquid as far as behavior is concerned.
>(2) For analysis as to what is going on around a PTH, the PWB can be
>considered an infinite plane in the x- and y-directions.
>(3) While the epoxy tries to expand in all directions, it is constrained in
>the x,y-directions by the glass reinforcements.
>(4) Thus, the primary expansion is in the z-direction at roughly 3 times the
>linear themal expansion, because all the volumetric expansion is funneled
>into the z-direction.
>(5) The PTH hole, however, allows the epoxy to also expand into the space
>occupied by the PTH if the PTH barrel wall is thin enough to by compressed.
>That is why in cross-sections of PTHs with 1 mil of copper plating and less
>one sees the PTH barrel bowed inward between innerlayer lands, particularly
>in prepreg layers (higher resin content, less cure) and when there are larger
>distances between lands (no non-funcional lands), and sometimes a space
>between the barrel wall and the reseded epoxy. If there were no copper
>plating, the hole diameter would decrease on heating.
>(6) Thus, the PTH barrel in in tension in the z-direction, with the largest
>stresses near the PWB center, the PTH barrel experiences compressive hoop
>stresses, innerlayer lands experience tensile stresses at their attachment to
>the PTH barrel, and innerlayer lands near the PWB surface experience bending
>moments due to land rotation resulting from the z-direction expansion of the
>epoxy surrounding the PTH barrel.
>
>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]
>