I havent yet designed 4 layer, only did 2. Now, i want to ask some questions.
first of all, i will have only through hole vias, cuz blind/burried are expencive. And just wanted to make sure that the following configuration line in DRU is correct:
(12+1516)
i will have BGA256 device with 1mm pitch. And the question is: when i put it in schematic, and go to board view, i see for example that it is being placed on top, and with all its balls being RED (top layer default color in Eagle). Now, how do i route a signal from TOP to BOTTOM exactly under the CHIP?
One possibility is to route it, and somewhere have a via to go to bottom, but… is it OK to have through hole via RIGHT under the BALL under the chip?
i tried to find any example projects files for Eaglewith any BGA device and could not yet, anyone has link? I dont care it can be any BGA chip project, i just want to c how its done?
You can’t have a via under a ball (unless you are using microvias or filled vias), or it won’t solder correctly. Either try routing out of the ball field to vias (not likely unless the package only has signals two rows deep) or use dogbones (figure-8 shaped pads, one side for the BGA ball and the other for a via). The vias end up in the space in between 4 balls. For power or ground balls, it is usually OK to tie two or so adjacent balls to one via.
A common way to route from BGAs is to out a short trace at 45° to a via that is in between the grid. Then on the bottom or inner layer route the shortest direction to the outside of the BGA.