Not to take us too far off topic but my opinion is give me information on
what you want the end product to be. let me and my experts develop the plan
to product that for you.
With ever tighter boards these days, we need every advantage to keep yields
up. We have seen a few drawings come across denoting drilled hole size, and
in most the designer did not think to factor in accumulative tolerance. let
me say that again, accumulative tolerance.
Design 101 - For those that may not understand let me describe it this way,
lets say my process is capable of maintaining a +/- 0.001" tolerance
(laughing at the idea right now, but lets proceed). This does not mean that
you should make your drawings with an overall tolerance of +/- 0.001".
For example; you design a pretty tight 18 layer board, lines at 4/4 , 0.062
overall thickness. And you add the words "layer to layer registration shall
be 0.001" or less".
Beginning with layer image, on the 18 layers, I am allowed 0.001" shift,
beginning with layer 1, and ending at 18, lets say they all shifted 0.0005"
in the Y axis, that would be 0.0085" variance from layer 1 to 18. Would
0.0085" present a problem??? Well of course. accumulative tolerance.
Lets say then you had layers at 0.001" off in one direction, and the drill
off 0.001" in the opposite, would this present a problem? Well it might not
be an issue internally but one you applied solder mask, and then for grins
say solder mask is off 0.001" from the drill in the opposite direction as
well, well if you've designed copper defined BGA's you've just put up to 2
mils of solder mask on the edge. But this is within the 0.001" (accumulative
Now here is where the heartburn gets you.there are few shops that can
maintain 0.0025" tolerance let alone the 0.001", most shops will accept
0.003" tolerance (and possibly suffer high yield loss).
So in closing (and because I am just tired of typing). Tell the fabricator
exactly what you want, including what you can tolerate, what you cannot
tolerate, be reasonable in your request, be receptive to their feedback, and
if you designed it lousy, do not take offense, learn from those who are
trying to make their living off your design as well.
From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Wednesday, July 09, 2008 3:47 PM
To: [log in to unmask]; [log in to unmask]
Subject: Re: [TN] PCB Inner layer shifting
That is the reason vias should be specified as to drill diameter, not
finished hole size.
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