[SI-LIST] Re: Cross-hatched reference planes
- From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
- To: "Ed Linke" <ed@xxxxxxx>, "John Stephens" <john.stephens@xxxxxxxxx>
- Date: Fri, 6 Apr 2007 10:18:53 -0700
Ed,
I agree with reason 1.
However, reasons 2 & 3 are not necessary. I've copied John Stephens of
Merix on this who is among the best fab engineers on the planet and he can
attest to this from the fab side.
Lee
> [Original Message]
> From: Ed Linke <ed@xxxxxxx>
> To: <leeritchey@xxxxxxxxxxxxx>
> Cc: Jeff Seeger <jseeger@xxxxxxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
> Date: 4/6/2007 10:03:23 AM
> Subject: Re: [SI-LIST] Re: Cross-hatched reference planes
>
> Hi Lee and All,
>
> The cross hatching or degassing ( venting) was done for a number of
> reasons in the past.
> 1- This was to allow the bonding of the resin from one layer to
> better bond with the resin of the adjancient layer. Resin will bond to
> itself easier than to other material.
> 2- In addition the cross-hatching also help to stress releive the
> copper plane to help to eliminate warping of the assembly.
> 3- It also allowed for gases to escape during the laminating process
> which is done under high temperature and pressure. If the gases are not
> allowed to escape you will get local air pockets that promote
> delimination and have reliability issues especially if vias are in this
> region.
>
> As for the Southeast Asia practice, it is probably due to shops that
> are not well controlled due to the humidity. If the raw material is not
> well controlled and absorbs mositure from humidity you will get the
> gases forming as the moisture from the humidity boils and turns into a
> gas during the lamination. This will cause the delamination talked about
> above if the product is not cross-hatched.
>
> Now, since this is an SI-list discussion, the cross-hatching
> practice is not desirable from a SI standpoint with high speed signals.
> Therefore most new high speed designs have eliminated the cross-hatching
> wherever possible. I still feel it is a good practice to place
> cross-hatching where possible. We normally do our layouts with the
> cross-hatching and then remove the cross hatching where there are
> adjacient signals on neighboring layers. So the plane contains some
> cross-hatching but not 100%. I would estimate that it is in the 40 - 50%
> range.
>
> I hope this helps.
>
> Ed. L.
>
> Lee Ritchey wrote:
>
> >I think the venting that was mentioned in an earlier reply was from
someone
> >who thought my responses were venting frustration with a bad practice in
> >PCBs. yes, it could be called venting, except for teh CAD departments who
> >have to try to make Gerber files that include this type of plane
structure.
> >
> >I don't know what the venting being discussed in this latest string of
mail
> >referes to.
> >
> >For those who saw my response as simple venting, I apologize. For all
> >those engineers and designers who have been asked to create cross hatched
> >planes, maybe this helped.
> >
> >For the record, the only fab shops I have seen ask for this in the last
> >five years are located in Southeast Asia. Wonder why they do that?
> >
> >Lee Ritchey
> >
> >
> >
> >
> >>[Original Message]
> >>From: Jeff Seeger <jseeger@xxxxxxxxxxxxxx>
> >>To: <si-list@xxxxxxxxxxxxx>
> >>Date: 4/5/2007 12:35:30 PM
> >>Subject: [SI-LIST] Re: Cross-hatched reference planes
> >>
> >>Andy wrote:
> >>
> >> > what is "venting"?
> >>
> >>In built-up multilayers, which start with a traditional core
> >>laminate, subsequent dielectrics are applied in liquid or semi-
> >>liquid state. Unbroken copper areas inhibit cure. Relief is
> >>in the form of "vent" pads appearing as stray anti-pads, or as
> >>a cross-hatch shape.
> >>
> >>Sorry that carpal tunnel limits my replies. Or maybe that's a
> >>good thing.
> >>
> >>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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