[SI-LIST] Re: Advice needed for how to make my PCB thicker
- From: Istvan Novak <istvan.novak@xxxxxxx>
- To: Henrik Gildå <henrik@xxxxxxxxxxxxxxxxxxx>
- Date: Wed, 02 Mar 2011 18:23:17 -0500
Henrik,
If your board works well and the only reason to make it thicker is
mechanical stability, you could also consider adding a stiffener, a
frame or tray.
If it works, dont mess with it.:-)
Regards,
Istvan
On 3/2/2011 5:03 PM, Henrik Gildå wrote:
> Hello SI-gurus!
>
> As a hobby project I've designed my own graphics card because I wanted
> to. :) Among other things it features a six layer board and the
> protypes seem to work very well, but the PCB is only 1mm thick. I've
> realized that I need to make it thicker before I produce it to a
> selected few who have expressed interest in buying it. I've been
> thinking about two ways to make the board thicker and hope I can get
> some input from you about what I might have missed regarding the pros
> and cons for the two alternatives. :)
>
> This is what the stackup looks like today:
>
> --- TopSignal (Features Spartan3 in BGA, two DDR2 memories at
> 200MHz, DVI transmitter chip with impedance matched routings, DAC at 240MHz)
> --- 66um FR-4
> --- TopGND (several digital GND planes, DC/DC GND, analog GND)
> --- 250um FR-4
> --- MidPower (1.2V for FPGA, 3.3V, 2.5V, 1.8V, 5V)
> --- 230um FR-4
> --- MidSignal (some DDR address signals impedance matched with signals
> on top and bottom layer, as well as some extra power planes that
> wouldn't fit in MidPower)
> --- 250um FR-4
> --- BottomGND (digital GND, DC/DC GND, analog GND)
> --- 66um FR-4
> --- BottomSignal (Features Spartan3 core decoupling caps, DDR2 data
> lines with controlled impedance, etc)
>
> Now I have come up with two alternatives to make this stackup thicker:
> 1) Increase the 250um between the TopGND-MidPower and
> MidSignal-BottomGND layer pairs to 550um each. This way I keep the
> concept of tight layer pairs at the top, middle and bottom.
> 2) Instead increase the distance between the MidPower and MidSignal
> layers a lot and decrease the 250um parts to 66um. This way I get three
> layers packed tightly at the top (TopSignal, TopGND, MidPower) and three
> layers packed tightly at the bottom (MidSignal, BottomGND, BottomSignal).
>
> In the first alternative this would increase the impedance of the DDR
> address signals in the MidSignal layer by around 10% according to my
> calculations.
>
> In the second alternative I think I would get even better high-frequency
> performance out of the power supply planes (MidPower) because the ground
> plane (TopGND) is so close. But the components and caps on the bottom
> layer would have very long vias to reach the power plane.
>
> Any thoughts about this would be greatly appreciated. :)
>
> Regards
> Henrik
>
>
>
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