[SI-LIST] : RE: Effects of thieving on SI and EMC.....

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From: Mike Saunders (mikesa@ia.nsc.com)
Date: Tue Aug 08 2000 - 12:34:49 PDT


Michael,

I don't have any quantitative numbers to give you or a tool that does this
automatically. However, here is my 2 cents based on my experience. I use
a diamond-shaped cross hatch shape (for continuity purposes) w/ ~200 MILs
gap to all other copper features, then tie it through vias to GND in
several locations. I do this on both microstrip layers, top & bottom.
This not only aids in the manufacturing process by balancing the copper and
helping to prevent warpage, but helps to electrically quiet down the board
and keep stray voltages, ie- noise, from propagating. This is great for
EMC. As for signal integrity, there should be only some minor (positive)
influences, assuming microstrip over solid GND, then surrounded by
theiving/balancing which is also tied to GND. The solid plane will carry
the returns, not the thieving. For pitch of the cross hatch, I usually
just use 0.125" - 0.250", unless I am trying to match to some seriously
dense routing. As far as technique, there's no need to try to cram it into
every small gap in the Cu; an overall balance is the desired result. This
process does take some time, since they are usually hand placed shapes that
are then voided, often resulting in isolated shapes which are no longer
connected to one another, so I can see why you would want to create an
auto-thiever. Another issue is to be careful that you work closely with
your board house. Often they will try to edit your thieving or, worse,
will add their own version if you left it off. Hope this helps, and if you
get something to work which is compatible with Allegro, let me know. ;)

--Mike

At 02:45 PM 8/8/2000 -0400, you wrote:
>A thieving pattern is a bunch of very small shapes
>added to a layer that helps equalize the plating
>across a given layer (increases manufacturability).
>An auto thieving utility or program uses a set of
>criteria to determine how and where to put these
>shapes. Unfortunately these programs are almost
>exclusively focussed on mfg issues and not SI or
>EMC issues.
>
>Usually if you see a bunch of 1/16" width diamonds
>on an artwork layer, this is thieving.
>
>FYI
>
>MG
>
>-----Original Message-----
>From: Ray Anderson [mailto:raymonda@radium.eng.sun.com]
>Sent: Monday, July 31, 2000 11:12 AM
>To: Greim, Michael
>Cc: SI LIST
>Subject: Re: [SI-LIST] : Effects of thieving on SI and EMC.....
>
>
>OK, I'll bite. Educate me, what is a thieving pattern,
>and an auto-thiever ????
>
>
>-Ray
>
>
>> Does anyone out there have a tool or reference
>> that would allow one to calculate the effects of
>> thieving patterns on signal integrity and EMC.
>>
>> I am trying to come up with appropriate rule sets
>> for driving an auto-thiever utility.
>>
>> Thanks for the help.
>>
>> MG
>>
>
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