I know this topic has been discussed here before, but an article I just read
by Paul Reid put a different twist on it
http://www.pcb007.com/pages/zone.cgi?a=69569&_pf_=1

I wanted to ask about a paragraph where he says:

"We know from many years of reliability testing that a board with
non-functional pads removed tends to be more robust than the same board with
non-functions at every layer. Occasionally, designers will have
non-functional pads at every internal layer. In most applications this
produces a reduction in reliability with an increase in barrel cracks in the
central zone of the PTH. It appears that this produces a number of anchor
points along the PTH and failure occurs in the barrel. Customers who remove
non-functional pads for increased PTH reliability reduce the "anchor" point
and stress is transferred to the knee of the hole."

This is a very important point for me, because I have always heard it
explained a different way. My (unfounded unscientific) understanding was
that fabricators wanted to remove them to save drill bit wear (especially
for high-volume boards in benign environments), but designers often want to
keep them in because the extra ribs provide more support (especially for
harsh environments).
This article suggests that keeping inner layer pads is LESS reliable.
The reason it is important to me is that our boards are expected to survive
20 years in an automotive environment, we have been allowing unused pads to
be removed, but some have suggested we retain them for lead-free processing
temperatures.
We haven't cared about inner-layer pad removal until now, but soon we will
be required to design for RoHS compatibility, and we were about to start
specifying that they be retained. Am I misunderstanding these results?

Jack


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