[SI-LIST] Re: 0306 Capacitors
- From: steve weir <weirsp@xxxxxxxxxx>
- To: jseeger@xxxxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Mon, 15 Mar 2004 11:50:47 -0800
Jeff, you are interpreting my comments correctly. And yes, if we really
want to know what the inductance is across the process spread, we have to
evaluate the worst case. Now the interesting thing that comes out of all
of this is that it might be in some cases that because a sloppy
manufacturing process requires more capacitors and more vias to do the same
decoupling job, that a tighter manufacturing tolerance, like 0.003 run-out
yields a lower cost assembly. It depends on what that price premium is and
how much placed capacitors cost, real-estate included.
Similarly, it may be that it is more economical to plug vias than to add
space for SM dams. If this is true at all, it is more likely be the case
where the planes are close to the capacitor mounting surface, than on a 4/6
layer board.
Regards,
Steve.
At 02:32 PM 3/15/2004 -0500, Jeff Seeger wrote:
>,
> Let me take Scott's comment one step further and translate
> into "printed board guy" parlance:
>
> We have 5 factors:
>
> - Primary Drill (via outside diameter)
> - Finished Hole Size (via inside diameter)
> - Via pad size (allowable location for Primary Drill)
> - Annular Ring (min distance, Primary drill to Pad)
> - Fab Allowance (Pad - Annular Ring - Primary Drill)
>
> To translate Steve's clarification, I get that any increase
> in primary drill must by necessity cause in increase in the
> pad size for a given fab technology. Given properly spaced
> and consistent via to component pad, this moves the via
> further away from the sweet spot in 50% of instances, which
> will negate the benefit from the larger hole.
>
> To clarify my own point, an annular ring of 0 and a fab
> allowance of 5 is more limiting than it sounds. When the
> analysis sounds like it's sensitive to the mil wrt via size
> I suspect the potential location variability of 10 mils is
> the bigger concern.
>
> Best regards,
>--
>
> Jeff Seeger Applied CAD Knowledge Inc
> Chief Technical Officer Tyngsboro, MA 01879
> jseeger "at" appliedcad "dot" com 978 649 9800
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