[SI-LIST] Re: 6 Layer microvia stackup

Hello Andrew,


I really doudt if this stack-up is doable...

Maybe u can manage a 50um(2mils) prepeg, but I doubt if the PCB
manufacturers would support a core of thickness 50um (2mils).


Assuming that you take care of the thickness issue, I am not really sure
that you would be able to achieve good=20

impedance control for your traces in SIG1 & SIG3 layers. The reason for
this being that your GND would be more or less

continuous in Layer-4, but I doubt if you will be able to do the same on
the top layer. If you still want to continue with=20

this stackup, you can ensure impedance control by routing signals on
SIG1 & SIG2 such that the GND on the TOP layer=20

encapsulates these traces without any break in between.


It is highly necessary for you to maintain orthogonality on your signal
layers as there is a very nice capacitive coupling

between them.


Best Regards,
Dhiraj=20

=20

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Andrew Seddon
Sent: Wednesday, January 11, 2006 5:54 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] 6 Layer microvia stackup

Hello,

I'm currently designing a really dense PCB and was wondering if anybody
has comments with regards to the signal integrity of the following
stack;=20


1 GND   =09
                50um    ER1 3.6
2 SIG 1                                         75um trace for 50 ohm
                50um    ER1
3 SIG 2                                         100um trace for 50 ohm
                100um   ER2 4.2
4 GND
                50um    ER1 3.6
5 PWR
                50um    ER1 3.6
6 MISC


Basically the plan is to do all routing on layers 2 and 3. The top layer
is mostly BGA's and floods to a pretty solid GND plane. All lands on
layers 1 and 6 are micro via in pad. So microvias from layers; 1-2, 2-3
. These can be stacked to create a 1-3 via.

So I have a few worries about this stack;

1. Can't route SIG1+SIG2 orthogonally. The impedance calculations are
based on each layer being calculated as an independent offset stripline.
Is this valid and will there be a lot of cross talk on traces running
underneath each other.

2. Getting controlled impedance on tracks that switch between 2 and 3
could be tricky.


Lastly I was wondering if anybody knows a good reference for micro via
stack's. It seems that you basically have to throw away the stacks used
for conventional via's. For example if you have 3 micro via layers on
each side
(3+n+3) where do your planes go?

Best regards,

Andrew








------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List FAQ wiki page is located at:
                http://si-list.org/wiki/wiki.pl?Si-List_FAQ

List technical documents are available at:
                http://www.si-list.org

List archives are viewable at:    =20
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu
 =20

------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List FAQ wiki page is located at:
                http://si-list.org/wiki/wiki.pl?Si-List_FAQ

List technical documents are available at:
                http://www.si-list.org

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu

Other related posts: