miner_tom , 12-14-2018, 08:12 PM
Hi Robert, I am going though your Advanced Layout Course now and I really love it! Although at Altium I am a novice I have spent many years supervising layouts and have spent more time than I care to remember in EMI labs finding the source of radiation. Please comment on the CPU board that was included in the course. I'm not saying that I can do any better but i would love to hear your commentary.
I have the example CPU board and in the picture below there is a small section of the area under the BGA. I have some issues with the length of the total path that the charge will need to travel. If you look at the picture attached, and please forgive my attempts to draw in "Paint", observe the green line. It starts at the H14 Pad and simulates where charge would be "likely" to go in a path from the voltage pad to the ground pad of the BGA. Starting at H14, charge (lets not quibble about positive charge vs negative charge) flows, at best, to the via then to the cap positive pad, then through the cap (not really, but lets go with it for the purposes of the argument) then to the GND pad of the cap, up through the GND via and finally ends at the GND pad of the BGA. That seems like a lot of path to travel. I have seen boards radiate at say 333 MHz where the radiation was stopped by tacking a 201 Cap in the right place.
Second, I am assuming that either layer L2 or L3 is GND (near as I can make out).Look at the yellow line starting from H14 again. That is in reality, an alternative path for charge. Ok, that might not be so bad because there is another cap, which shares the via. But, I question the connection of the Poly line that connects the vias. If there is a GND plane then this Poly line connection can be removed and a potentially large return path for charge can be avoided. I have blocked out this section of the Poly with green lines. Again, if there is a GND plane, I think that this connection should be removed.
I really hope to learn from your comments. My question is strictly on the basis of learning and as i have mentioned, i have seen just about every radiation problem that there is to see and I hope to avoid them in the future.
Please seen the attached picture.