[SI-LIST] Re: Cross-Hatched Reference Planes
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: cadpro2k@xxxxxxxxxx
- Date: Wed, 04 Apr 2007 17:44:07 -0400
Mitch
Cross-hatched reference planes can be quite effective for signaling, if
engineered correctly. I've had need of them when designing high
performance flex circuits and found that with a little bit of
engineering intuition, followed up with some 3D fullwave simulations to
verify that intuition, it is possible to avoid problems. But, you have
to do the engineering, and the mesh holes (and or routing) must be done
deliberately. Unfortunately, CAD support for meshed planes is poor at
best, allowing little control of where the holes are placed with respect
to traces. This is what leads to significant impedance control and
crosstalk problem. (Pass two traces over the identical row of mesh
holes, across a significant length, and you'll see crosstalk skyrocket.
Or pass one side of a differential pair across mesh holes and the other
side across a solid plane, and watch the differential impedance and skew
get wacky.)
Except for flex circuits and packages, I've never had the need for
meshed planes. But in those instances, it is possible to engineer
quiet, impedance controlled systems with the proper attention to detail.
Scott
Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax
http://www.teraspeed.com
Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC
Mitch S. Morey wrote:
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> Date: Wed, 4 Apr 2007 10:43:01 -0700 (PDT)
> Subject: Re: [SI-LIST] Re: Cross-hatched reference planes
> From: "Mitch S. Morey" <cadpro2k@xxxxxxxxxx>
> To: leeritchey@xxxxxxxxxxxxx
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> And another response comes to mind when you get back to original question
> of "Cross-Hatched" and "Reference Planes"
>
> If you imply reference planes to be impedance associated reference planes,
> then: "Cross-Hatched Reference Planes equal disaster." If they are simply
> cross hatched power planes, that's a different matter. Still, like Lee
> mentions, with PCBs very select (few) cases are warranted.
>
> Good day.
> Mitch
>
>
>> Jeff,
>>
>> I agree that there are a few exotic places where laminate to laminate
>> bonding is needed and is accomplished using cross hatching as well as in
>> flex circuits to improve flexibility, which I think was the original
>> question.
>>
>> I responded the way I did because there are still those who want to see
>> this done to ordinary PCBs made from copper and FR4 class resins, where it
>> was needed long ago. I did several designs this way just to prevent
>> delaminataion. The industry fixed this problem long ago.
>>
>> I don't think there is much to debate here. My goal was to make sure that
>> this old sock did not creep back into standard multilayer board design.
>> And, yes, this is often necessary in multilayer ceramic IC packages.
>>
>> Lee Ritchey
>>
>>
>>
>>> [Original Message]
>>> From: Jeff Seeger <jseeger@xxxxxxxxxxxxxx>
>>> To: <leeritchey@xxxxxxxxxxxxx>
>>> Cc: <si-list@xxxxxxxxxxxxx>
>>> Date: 4/4/2007 8:27:09 AM
>>> Subject: Re: [SI-LIST] Re: Cross-hatched reference planes
>>>
>>> Lee Ritchey wrote:
>>>
>>>
>>>> All of those reasons, except the flex circuit one, went by the wayside
>>>>
>> many
>>
>>>> years ago. Wonder why they still linger around the industry like old
>>>>
>> socks!
>>
>>> Agreed venting and bonding concerns first appeared in 60s military
>>> specs but I'll wager that if you were to chat up a substrate foundry
>>> there is current call for venting.
>>>
>>> On the bonding front we were recently asked to provide some "hatch",
>>> understood to be for laminate to laminate, in an area of a board
>>> lacking vias. The board involved fussy materials and a challenging
>>> end-use environment. I don't believe it was an offhanded request.
>>>
>>> We'll simply have to agree to disagree on cross hatching as a way
>>> to work with supply- and signal- dense areas to minimize the layer
>>> count *and* bow and twist.
>>>
>>> I'd be happy to debate further but suggest IPC's Technet as a more
>>> appropriate forum for the bandwidth on DFM issues. Old socks can
>>> become new again, it can be useful to recall where we put them.
>>>
>>> Respectfully,
>>>
>>> --
>>>
>>> Jeff Seeger Applied CAD Knowledge Inc
>>> Chief Technical Officer Tyngsboro, MA 01879
>>> jseeger "at" appliedcad "dot" com 978 649 9800
>>>
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>
>
>
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