At 09:14 AM 12/15/98 -0600, Leonard Toohey wrote:
>Remove unused pads on vias.
>
>Leave unused pads on through holes that will receive a pin that
>gets soldered.

A single solder side pad can readily break free of the board, especially
under repair conditions but also with simple mechanical stress. Once there
is a pad on the other side and the hold is plugged with solder, it becomes
quite difficult to imagine such a mechanical failure; I don't think I have
ever seen one where there was a good solder joint; the pin/pad/solder
assembly acts like a rivet. It is difficult for me to imagine that two mils
or so of copper on an inner layer is going to significantly increase the
strength of the assembly. If the connection would fail without the inner
layer pads, it would probably fail with them. So I'd like to hear a
justification for what Mr. Toohey suggested.

By the way, on a telephone equipment (central office) manufacturer's
boards, we used to remove unused *top* side pads on double-sided boards,
particularly on ICs where we needed to run traces between the pins on the
component side. Presumably because the component still had double-sided
solder pads on some pins, and these parts were not subjected to mechanical
stress anyway, it was not considered dangerous to remove those pads.

As to vias, again, I do not understand how unused pads on an inner layer
are going to improve the integrity of the hole plating, so I would agree
that unused via pads can be removed.

The software I use, Protel 98, makes it easy to remove all unused inner
layer pads, and nearly every CAD system removes them on power and ground
planes anyway. (Have we ever heard of a complaint that a non-power via or
pad failed because it did not have a pad on the power or ground plane, or
that they *should* have such pads? I've done space-flight qualified boards
with software that did not make it easy to remove the inner layer pads, and
the fabricators, following NASA guidelines, removed them.)

To individually select the pads to remove, as was suggested by one writer,
seems unnecessary and tedious.

[log in to unmask]
Abdulrahman Lomax
P.O. Box 690
El Verano, CA 95433

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