Hi Werner:
  	 	      I was referring to THT holes not SMT holes. This
is what some PCB vendors go by to get the final hole size with
tolerances after plating it. Their first choice would be to add 6 mils
and the last choice would be to add 5 mils to the nominal hole size to
find a standard drill # to perforate the board. 
	Best Regards,
	Ramon



-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier
Sent: Friday, April 27, 2007 2:50 PM
To: [log in to unmask]
Subject: Re: [TN] "plated shut vias"

Ramon,
What you suggest, "we added...6, 7, 8 or 5 mils...[to the]
specified...final hole size...to find the drill size", is very bad
practice. How do you know how much inner land ring width is left or
whether you have break-out? It is the drilled hole diameter that needs
to be used to determine via reliability [just look at all the data and
documents, like IPC-TR-578, IPC-TR-579, IPC-TR-486, IPC-D-279, they are
all in drilled diameters]; it is the drilled hole diameter that
determines the aspect ratio, it is the drilled hole diameter that gives
you an idea as to how difficult it is to plate good copper into the
hole.
Going with the finished hole diameter was necessary when we used
through-hole technology, but it is totally outmoded nowadays.

Werner



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