I need to route the 2nd inner layer of 0.4mm pitch BGA package to bottom signal layer in a 4 layer PCB board. I have seen that people use microvias on the BGA pads to route the traces to lower layers. In addition, i have also read that the aspect ratio of microvias should be around 1/0.8 (diameter/depth) to meet manufacturer restrictions. This means that i cannot have microvia depth of more than 80um if my microvia depth is 100um. In a 4 layer PCB, a microvia from top to bottom layer will certainly have much deeper depth than 80um. How should i put a microvia in such situation then? Should i use a stacked microvia in this situation?. More importantly, if i have to use a stacked microvia then how can i define a stacked microvia in PADS layout?