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PCB Design with FPGA (TQFP or BGA package)

NathS

hello all,

 it seems like at least 4 layers PCB design is being used for FPGA with TQFP or BGA package.

In case of TQFP, what would happen if I design a single-sided PCB with FPGA 144-pin TQFP? (Altera Cyclone, for example)

What would happen if I design a double-sided PCB with the same FPGA? making some signals going through the top layer and others through the bottom layer.

What are the differences such as good&bad of both 4-layer PCBs with the same FPGA using layer stack-order as follows:

signal1(top)
pwr
gnd
signal2(bottom)

or

pwr(top)
signal1
signal2
gnd(bottom)

My professor told me that on a single-sided design, we cannot use all the I/O pins because they are too close to each other.
I thought that as long as I follow the industry standard, even if it's single-sided, there would be no problem such as crosstalking.
So I was wondering why or why not the PCB with FPGA must be designed in multi-layers.

I appreciate your help,

Nath

  • Yes, Reason Ur professor had given is one of the reasons for multilayer for FPGA devices.In Single –side PCB (I assume single side PCB means one signal layer and one power plane), it will be difficult (PCB signal trace routing) to utilize all IOs to the maximum extend.As you know that shorten the rising edge, PCB traces will behave like transmission lines. Every signal requires proper return path to maintain good signal shape at receiver. So it is good to have continuous Power/Ground plane to achieve low impedance in PDN network.So that is why, we can go for multilayer (here 4 Layer will be good enough). 2 signal layers + 1 Power plane + 1 Ground Plane. About Stack up 1:       I recommend change in order of the layer assuming that major components are on top layer in the following manner.Instead of          S1-P-G-S2        to     S1-G-P-S2.It’s always good to have ground plane closer to the top layer or layer where major devices are present.- Critical signals can be routed without layer crossing (discontinuity so return path changes)- We can reduce significant amount of vias. (Compared to stack-up)Disadvantage of Stackup 1:Impedance tolerance will be more (It not possible to control PCB trace impedance exactly in fabrication. It will have larger tolerance compared with inner layers).So this point becomes advantage for the second stack-up.But I have not seen any design with this stack-up (because component placement in plane) till now.  For more details, I would recommend “Signal-integrity-issues-and-printed-circuit-board-design” by Douglas Brooks (Good beginner book) Signal integrity – simplidfied (by Eric) and High speed digital design by Howard JohnsonCheersR N

     

  • Added to above one, you can refer one forum's discussion with subject: 'ground planes at top / bottom layer ' (SI-List group in freelists.org) for more details.....

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