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veresk313 , 07-20-2019, 03:25 AM

Hi everyone.

I'm currently working on an FPGA based development board. It's 2-layers PCB and has an FPGA in EQFP144 package, SDRAM chip and some peripheral interfaces (USB 3.0, HDMI, Ethernet).

So, I have a few questions:
1. What is the best way to place bypass capacitors? Should I place them on the same side of the PCB as the FPGA. Or is it better to locate them on the opposite side? If so, can I place caps under the chip to save some space or chip will influence the power plane?
2. Can I route signals under the SDRAM chip or again the chip will influence the signals?

milos.stankovic , 07-20-2019, 03:42 AM

Hi Veresk313,


I would consider following:
1. Try to use at least 4L board
2. Yes, you may put bypass capacitors under the chip (opposite side of the board)

Regards,
Milos

veresk313 , 07-20-2019, 04:17 AM

Thanks for reply.

Unfortunately, I'm limited to 2L board.

robertferanec , 07-21-2019, 11:10 PM

I agree with @milos.stankovic I would not route high speed interfaces such hdmi, ethernet, usb3 on 2 layer PCB.

- About layout and placement, have a look how they placed decoupling capacitors on the reference board. Usually you may find them on the bottom layer under the pins (usually I place them under the chips).
- I do not know much about your board and the chips, but I often route under chips, There may be some exceptions (e.g. do not route directly under crystals or be careful about power supply area), but generally there should not be problem routing under chips.

veresk313 , 07-21-2019, 11:26 PM

I was trying to keep the price of the PCB as low as possible. But then I calculated the PCB height and track width and spacing needed to get the 100 Ohms differential impedance. The values are ridiculous (either 0,2 mm PCB height with normal tracks and spacing or normal height and wide tracks and tiny spacing). Are impedance issues the main reason why you wouldn't advise to route high-speed interfaces on 2L PCB? Or there are some other issues with 2L PCB?

robertferanec , 07-24-2019, 07:06 AM

In 2 layer PCB also good solid GND (reference) plane is missing. PS: There are many issues with 2 layer PCB, simply to say, many of the issues are connected with not having SOLID GND plane placed as close as possible under the tracks.