At 04:06 PM 9/12/2010, you wrote:
> Bigger??? Why do you need an annulus on an inner layer that doesn't > connect to a trace? Depending on the fab technique, you may need an annulus just to fill the gap between fr4 layers so that the electroplating is reliable. Either that, or you need to keep *other* copper far enough away that the prepreg can fill the gap.
I guess I'm not following. Prepreg is the insulator used between copper laminate layers. I know of no requirement to "keep copper away" to allow the prepreg to exist or do its job.
There are always some gaps or cracks in drilled hole walls even within insulation layers; the sides of the drill holes are never uniform in FR4 because the material is not uniform. These cracks fill up with electroplating quickly because they are very narrow and don't cause a problem. The only issue I am aware from these cracks being filled with plating is the need for keeping copper away from the holes to prevent shorts. That is normally dealt with by a hole to copper design rule.
I have never been told by a board fab house that vias need to have an annulus on every layer to help with any problems. To allow for maximum density of routing, separate design rules are needed for hole to trace from via pad to trace and also different between outside layers and inner layers. I don't see that as a reason to use an annulus on each layer, to make the design rules work.
Rick
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