If the inner layer copper thickness is >35 µm (and sometimes 35 µm), 
e.g., power planes, then you should keep the copper there for fear of 
resin starvation.

Brian

On 12/07/2010 18:52, Jack Olson wrote:
> 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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