Platform forum

8 or 10 layers

mulfycrowh , 05-20-2022, 09:44 AM
Hi everyone,

I am working on a motherboard design.
I set it up with 8 layers (Signal 1, GND, Signal 2, Power 1, Power 2, Signal 3, GND, Signal 4).
I have about 150 differential pairs.
Among these 150 differential pairs, at present time, about 90 pairs are routed.
Still remaining are pairs between CPU and GPU, all pairs involving SSDs, Wi-Fi, LAN ...

I am facing trouble to route the remaining pairs.
I use Signal 2 and Signal 3.

The true question now is should I use 10 layers ?
As an example, I requested an instant quote by PCBWay.
For a PCB - 200mm X 300mm, I got the price, including shipping, of $500 for 5 samples with 8 layers and $826 for samples with 10 layers.

So we have to add 65% for 10 layers ?
Does it seem coherent ?

Did you already manufacture 10 layers PCBs ? What was the budget ?

If you had to add 2 layers, where would you insert them ? Between GND and Signal 2 and Signal 3 and GND ?
Would you insert GND planes ? In that case we would have 12 layers...

Thanks a lot.
mulfycrowh , 05-23-2022, 01:45 AM
I went to 12 layers.
It is tricky but doable ...
robertferanec , 05-24-2022, 12:14 AM
That is an interesting difference, but yes, it looks like there is not much to do - I had a look at PCBWay calculator and I am getting the same numbers. For mass production it may be cheaper, but for prototypes the price between 500 - 1000USD for this kind of PCBs is what we normally pay.

I would add extra GND layers between signal and power, so it would be like: gnd-signal-GND-power. I know that may not help much with routing, but you may get smaller diff pairs if you gave GND from both sides. Also that would be good for EMC and SI.

12 layers is even better - if you need two extra signal layers.
mulfycrowh , 05-24-2022, 03:08 PM
Here attached is the stack-up.
What is important to mention is I selected a thickness for the board of 2 mm instead 1.6 mm.
Why ?
Because the thicker is your board the wider are your tracks.
With 1.6mm I got a minimum track width of 0.06 mm and it is not in the specs of the PCB manufacturers.
The minimum then can do is generally 3 mil = 0.0762 mm.
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