[SI-LIST] Re: Copper Pours

It is always troubling to see someone doing something- "just because". 
Hardly a good engineering practice.  Takes time if nothing else.


> [Original Message]
> From: Andrew W. Riley III <drew3rdof3@xxxxxxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Date: 4/24/2006 10:08:21 PM
> Subject: [SI-LIST] Re: Copper Pours
>
> Howdy,
> I do not even pretend to be an SI expert.  <=That can also translate to I
know 
> just enough to be dangerous.  However, I (unfortunately?) like the look
of the 
> ground fill most of the time.
> I stitch the fills on a 1mil grid, to avoid copying a pattern and pour
them at 
> no less than having a clearance of 25mils from all other copper or five
times 
> the distance to the reference plane, whichever is greater.
>
> Anyone have the time to poke holes?
> I know, another possibly dangerous rule of thumb.  But that practice has
been 
> done on designs that have data or clock speeds well under the 500MHz
range and 
> only when requested.  I have never received any negative feedback, yet to
see a 
> poor quality signal, or caused the loss a customer.
> Maybe I should try to account for impedance controlled traces using the 
> asymmetric stripline trace portion vs. the embedded microstrip portion of
a 2D 
> field solver when those traces are rarely on the next inner layer.
> It's all that I have available.  I know that I am just asking for a
lashing...
>
> For designs into the GHz range and above, I have successfully referred
the 
> advise of and worked with a signal integrity specialist.
>
> In any case, I thank all of you for sharing your previous and future
posts as 
> well as letting me be a part of this forum for so many years even though
I have 
> not contributed to its database.
>
> Cheers!
> Drew
>
> ----- Original Message ----- 
> From: Dennis Han
> Cc: si-list@xxxxxxxxxxxxx
> Sent: Monday, April 24, 2006 6:43 PM
> Subject: [SI-LIST] Re: Copper Pours
>
>
>
> I think this is one of those 2-layer board rules that people applied to
> multilayer boards without thinking about it.  The reason some people think
> copper pours are beneficial is there are still many boards designed with
no
> transmission line rules whatsoever.  The pours when done incorrectly, and
> most are, lower the impedance of the transmission lines to the point the
> lines are now matched to drivers that are in the 20-40 ohm range.  This
> helpsthe overall design, but there are still many impedance
discontinuities.
> Also, the copper pours have reduced EMI most of the time just because the
> transmission line impedances are reduced enough that the discontinuities
are
> less destructive and lower impedances radiate less.  The pours also tend
to
> cover up poorly designed reference planes, especially power planes.  The
> pours are unnecessary when a board is designed properly.
>
> Dennis
>
>
> Lee Ritchey wrote: Robert, Using copper pours in signal planes to reduce
EMI
> is one of those may urban legends that are not true. You are correct about
> their potential negative effective on signals. Who starts these strange
> rules, anyway? [Original Message] From: <Robert.Havlik@xxxxxxxxxxxx>[1]
To:
> <si-list@xxxxxxxxxxxxx>[2] Date: 4/22/2006 1:26:08 PM Subject: [SI-LIST]
> Copper Pours I understand copper pours are commonly used on PCB&#8217;s
for
> EMC shielding, power supplies, and in analog designs, but I have not found
> any resources with good advice on when a copper pour should or should not
be
> used. I can see that placing a copper pour connected to a ground plane
> aroundhigh speed signal traces would reduce EMI and also possibly signal
> coupling, but it also seems that having a ground in close proximity to a
> signal trace it could also change the characteristic impedance of the
> transmission line. If the copper pour is not a uniform distance around the
> trace, it seems it could potentially cause impedance discontinuities.
Also,
> Ihave seen both hatched and solid pours, and I have not seen anything on
the
> advantages and disadvantages of either approach or in what circumstances
one
> should be used over the other. Any advice on copper pour usage or good
rules
> of thumb would be greatly appreciated. Thanks. -Robert Havlik University
of
> Colorado, Boulder
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