[SI-LIST] : more on pcb stack-ups

Peterson, James F (jfpeterson@space.honeywell.com)
Thu, 23 Oct 1997 12:52:57 -0400

all,

All of the feedback on my stackup has been a big help - thanks.
Hopefully the exchange helps others in their stack-up efforts. (The
stackup is detailed again at the bottom of this message.)

The thermal layer in the board really added fuel to the subject.....

Andy wrote (I responded):

> Layers Sig4 and Sig5 use the Thermal layer as partial reference.
(I hope it is a very small portion.)

> What's the AC noise between chassis and circuit ground?
(good question - but remember this plane doesn't directly connect to my
circuit ground, so I guess it would be one ground plane's noise
crosstalking into another. Which leads to the next statement.)

> Do you want to couple circuit ground to chassis, and vice-versa, by
way of this path?
(first question : don't care, second question : no, but I don't believe
these two grounds are very "far" from each other - I forgot to mention
that the backplane is connected to chassis ground. More SI feedback on
this one would be a good thing.)

> Will you add discrete capacitors from the Thermal layer to Gnd layers
> in areas of high via and pin density, to help the return currents pass
> between planes?
(yes, there will be areas of high via and pin density - but, no, I
wasn't planning on adding discrete capacitors between the circuit ground
plane and thermal ground plane. There will always be a low inductance
path, i.e. gnd plane, next to a signal layer. that should be enough,
right??)

> Do you really have four signal layers sandwiched between Gnd layers 11
> and 16 (assuming all 13 layers are mirrored as you indicated)?
(yes I do, but only 2 layers are trace layers and they are right next to
gnd planes. the other 2 layers are for pads that will be used for vias
traversing to the other 13 layers. I don't think this is significant
since all gnd planes will be "via'd" together all over the place.)

Below is the stack up for 1/2 of the board, the other half is a mirrored
image:
> 1 -- Pads/Breakouts only
> 2 -- Sig1
> 3 -- Gnd
> 4 -- Sig2
> 5 -- Sig3
> 6 -- Vcc
> 7 -- Gnd
> 8 -- Sig4
> 9 -- Thermal (a copper plane, but tied to chassis gnd, not circuit
gnd)
> 10 -- Sig5
> 11 -- Gnd
> 12 -- Sig6
> 13 -- Pads only

thanks again for your input,
Jim
Jim Peterson
jfpeterson@space.honeywell.com
Honeywell, Space Systems Division, M/S 934-5
13350 US 19 N., Clearwater, FL, 34624
813-539-2719