FreeLists / si-list / [SI-LIST] Re: PCB layer stackup

["Zabinski, Patrick" <zabinski.patrick@xxxxxxxx>]

Jan,

From a power integrity perspective, you'll get the best performance if
the power planes are adjacent to ground planes.  In Stackup 1, there is
one
PWR-GND pair, but the other power plane is referenced to a different
power plane,
which makes decoupling less effective due to increased effective
inductance.
Stackup 2 has the power and ground planes separated even further, which
is
worse.

From a manufacturing perspective, I believe Symon is somewhat correct in
that
the layers do not necessarily need to be balanced/symmetric.  However,
balanced
boards are easier to fabricate and assemble to, which will save costs
in production quantities.  If you're dealing with low quantities, you
probably won't see much difference.  If you're in the 1000's to millions
of
boards, balancing/symmetry is important.

As for SI issues, true LVDS can reference ground or power planes without
much difficulty as long as the planes are properly decoupled.  That
said,
many parts stated to be "LVDS" do not fully comply with LVDS spec, so
you should look closely at the output buffer design to determine which
plane (power or ground) is most appopriate.

To get around these three issues, I'll offer an alternative stackup
that migth be worth considering.

 =20
>                   stackup 1              stackup 2     Stackup 3
> -----------------------------------------------------------------
> L1                routing                routing       Routing/Attach
> Substrate
> L2                power plane            GND           PWR
> Substrate
> L3                power plane            routing       GND
> Substrate
> L4                GND                    power plane   Routing
> Substrate
> L5                routing                power plane   Routing
> Substrate
> L6                routing                routing       GND
> Substrate
> L7                GND                    GND           PWR
> Substrate
> L8                routing                routing       Routing/Attach


Highlights from this third stackup:

        * Each power plane is directly adjacent to a ground plane, which
will
        improve/ease decoupling

        * the stackup is balanced for ease of manufacture

        * the routing layers are directly adjacent to ground planes

Good luck,
Pat
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