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

[Stuart Brorson <sdb@xxxxxxxxxx>]

Forgetting about any minor SI advantages/disadvantages, stackup 1 has
the major disadvantage that the plane layers are unbalanced, which can
lead to board warpage.  That is, the planes will strain the top
layers of stackup 1 more than the non-plane bottom layers, which can
lead to bent, warped boards.  The problem would become acute over wide
temperature ranges.  I'd stick with stackup 2.

Stuart



> I need to specify a 8layer board. Fastest edges on the board are on the
> 600Mbit LVDS channels : about 500ps. I have two stackups in mind :
>
>                  stackup 1              stackup 2
> ---------------------------------------------------------
> L1                routing                routing
> Substrate
> L2                power plane            GND
> Substrate
> L3                power plane            routing
> Substrate
> L4                GND                    power plane
> Substrate
> L5                routing                power plane
> Substrate
> L6                routing                routing
> Substrate
> L7                GND                    GND
> Substrate
> L8                routing                routing
>
> Stackup 1 has the advantage that all routing on L5, L6 and L8 is
> referenced to GND. We need to push the LVDS channels over a board to
> board connector. On the board to board connector, there are GND's
> between separate pairs. So we keep the same reference to GND over the
> connector.
>
> Stackup 2 is the stackup I would normally use due to it's symmetry.
> Signals on L3 and L6 are referenced to both GND and a power plane. But I
> don't know why I would need to favour stackup 2 over stackup 1.
>
> Can you comment om which stackup is most preferred ? Or what are the
> drawbacks/advantages of one stackup to another.
>
> best regards,
> Jan
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