At 10:55 AM 11/29/01 -0500, Wesley Webb wrote:

>Can anyone tell me if it is possible make a via or pad that is connected 
>to the Vcc net on the top and bottom layers only. TI do not want to 
>connect it to the inner Vcc plane as the decoupling caps are on the 
>secondary side of the pcb and I want the voltage to see them before the 
>leg of the IC.

First of all, normally the best strategy for improving high-speed 
performance is to minimize the inductance of the connections. The strategy 
described has been discredited. For best results, I recommend allowing the 
vias to connect, and they should be direct connect. It is often, also, not 
worthwhile to run stringers to the bypass cap, as I have often seen, indeed 
I used to do myself. I had the same idea as Mr. Webb. It was a bad idea.

With a multilayer board, the best source of clean bypassed power is the 
plane structure, particularly if the stackup has been designed to minimize 
the distance between the plane, 5 mils of prepreg is a common figure, so 
that the distributed capacitance is large. But even with more widely 
separated planes, it would be better to connect the vias to the plane.

So I now place vias as close as possible to the cap pads; I'd put them in 
the pads if it did not cause manufacturing problems. I leave just a very 
short distance of space between the cap pad and the via to create a thermal 
relief. I do the same with the component pads.

However, if the engineer still wants to disconnect the vias from the plane 
for some reason -- after all, that's what engineers are for -- use a Design 
Rule for plane connection.

Use a pad instead of a via (you can use vias but the specification of the 
via is less flexible. Place one of the pads and give it some distinctive 
and descriptive name like "NO-PLANE". Then open up the design rule dialog 

Filter Kind should be Component-Pad. The pad will be, in the list, 
"-NO-PLANE", i.e., the component refdes will be a null string. Select it. 
Set connection style for No-Connect.

If you already have placed vias, select them all by any of the various 
means. Then use Tools/Convert/Vias to Free Pads, then globally edit on 
selected pads to give them the distinctive name.

You can set many different kinds of custom handling of "vias" using this 
technique. You can make some vias direct-connect and some no-connect, and I 
think you can control this by layer....

Abdulrahman Lomax
Easthampton, Massachusetts USA

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