Typically BGAs have a central square of balls which are all connected to ground. What is the best way to hookup the PCB tracking for the central ground matrix.
The simplest, and lowest inductance, seems to be to put down a solid square of copper covering the central ground ball pads, and pepper the square with ground vias. But will this cause a soldering problem with the manufacturer
Some of the Xilinx app. notes discuss board layout: the techniques should be adaptable to other devices. (Sorry, I don't have the exact link to hand). Generally, don't expect to solder direct to a big square of copper: you'll never get it hot enough. The preferred way is to use a proper-size pad for each ball, with vias in the diagonal spaces between those pads (there is room: the package designers made sure of that:) You can use X-shaped runners between the pads & vias. Your solid square of copper lives on a lower plane, & extends at least to the outside of the BGA. Decoupling caps. (plenty of them) are commonly put on the backside of the PCB, as close as possible to the power pins. Of course, a multi-layer PCB is essential: you won't get the signal lines out on a 2-layer, even if you could make the grounding work.
The manufacturer of the device you are using probably has a document that describes what you need to do. But generally, you have a pad that the ball connects too, and then there is a short track to a via to an internal plane. Between the pad and via you need solder mask, otherwise during soldering the solder ball can melt and disappear down the via !! and you end up with an open circuit. Browse the applications section of the manufacturer's web site of your part - a detailed document is probably there somewhere.
Just use the standard dog-bone shaped pad, ball on one end and groundplane via on the other, like any other bga connection. Works fine.
John
Have something to add? Share your thoughts — no account required.
Ask the community — no account required