FPGA Layout question

Hi Jason, Have a look at this thread.

Ten layers, three ground planes, route the power, microvias between layers 1 and 2. I've long since given up on power planes across the whole board. Think of all the different powers you need. 1.2, 1.5, 1.8, 2.5, 3.3 blah blah blah. Little local power copper floods are the answer. And yes, you should connect the three ground planes together with all the vias you can, especially in the FPGA area. This provides paths for the return currents of signals and power floods on layers adjacent to the ground planes. If you use microvias on the top layers, there's loads of space underneath for decoupling caps. It really works and it works really well. Really! Cheers, Syms.

Symon,

Thanks for the response, I read the thread, very interesting, and I'll have to speak with my fab house about microvias. You mention lots of room underneath the BGA for the decoupling caps, but if they are connected (similarly to the BGA) with thru-hole vias for ground then that would impact on the space as well, unless you are using blind vias for the decoupling caps on the bottom of the PCB? There would be lots of vias to stuff in there with the BGA grounds (thruhole vias) and the capacitor grounds and powers (thruhole vias).

I have thought that I might be able to go to an eight layer PCB using blind vias with the following stack up

1 - signal/component 2 - mixed pwr plane 3 - ground plane 4 - signal 5 - signal 6 - 3.3V power plane 7 - ground 8 - signal/component

If I use blind via (layer 1 -3) for power and ground for the BGA then I have lots of room to route signals on layers 4,5, and 8 provided I place decoupling caps on layer 1 around the package (FG256). However this then goes against the connecting ground planes theory, unless I add lots of ground vias to join the planes after I route the signals. I do have quite a few components for the 3.3V so I don't know if I want to give up that plane, however my layer 2 is a mixed plane layer (1.8 and 2.5 volts) with more than enough room to accommodate signals or more 3.3V plane. I'm not sure on which is better since I have not ventured into this many layers (having multiple ground planes) before.

thanks,

Jason

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Consider sticking with normal vias in your BGA. If you have a full matrix of balls for the part, you can route the pin escapes away from the center providing a crosshair of board are where decoupling can be added. Microvias seem a bit extreme for your first journey into multiple ground planes. If you use tiny 0402 caps with pick & place machines, you can stuff a bunch of caps underneath. It may seem like 0402s are a mess to get involved with but I would think this is a better ($$) route than microvias. You can have connections to both ground planes with all ground vias. For EMI and signal integrity, the multiple connections are necessary to keep the return current paths on one plane from having to find a return path to the other plane.

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Correct, you need to use through vias for the ground and power. But you need those through vias to connect the BGA balls up anyway, so you're getting double the use for each one.

Hmmm, layer 1 to 3 blind vias = expensive. More than one drilling process or more expensive double layer microvias. And bad SI because the return currents can't get from one ground plane to the other near the FPGA.

Give it up! You'll never look back.

If I was doing 8 layers, I'd do

1 - signal/component 2 - signal 3 - ground plane 4 - mixed powers 5 - signal 6 - ground plane 7 - signal 8 - signal/component

I'd have microvias between layers 1 and 2. You'll be able to route out every signal ball of a BGA256 on layers 1 and 2 without a single through via. Through via every power and ground ball, connect the bypass caps on layer 8. Of course connect ground vias to all ground planes.

Anyway, that's what I'd probably do. I hope it gives you some ideas.

Cheers, Syms.

Extreme? Naahh. In at the deep end! ;-) You'll never go back!

No, the through vias for all the signals have used up all the space.

Of course, 0402s are a given! 1uF ones. I mix them with 0805 22uF. Don't fall for all that different value bypass caps in the same size package crap. Also, the microvias save $$, they're cheaper than you think, and you need fewer layers remember. Talk to your pcb fab house.

Yep, spot on.

Then am I to assume that the capacitors on the underside of the PCB share the power and ground vias with the FPGA power and ground vias, or do the capacitors have their own power and ground vias. I always assumed that it was a "no no" to use the vias from the BGA power ground for the capacitors?

I realize that my design is not really pushing the envelope, but I want good design practice from the start.

Thanks once again for the tips,

Jason

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2. > Of course connect ground vias to all ground planes.

Hi Jason, Absolutely, it's a "yes yes" to share the vias for the BGA grounds/power with the bypass caps. It works great. Another site you might be interested in is

Click on 'bypass capacitors' for some interesting stuff. This article shows where to put your vias relative to the capacitors. Best, Syms.

Thanks for the posts, tips , and help. Great articles and web site links.

Very much appreciated.

Jason

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