Smart devices are smaller and smaller ,but smarter. This has promoted the continuous progress of PCB technology，any layer HDI,laser drilling,back drilling ,etc. We must embrace new technologies and growing with it.
Below is an example what we did.it is a 4 layer board with 1.0mm,but the blind via is from L1-L2.
The blind via is only 0.075mm in the BGA pad. via in pad is must to do at the BGA area.
min hole 0.075mm, laser driling
blind via ,via in pad technology.
gap between hole edge to trace is only 0.128mm.
You may say,oh ,it is not a very difficult printed circuit. But,actually,the key technology is the gap between the hole edge to the trace is only 0.13mm (5mil).Usually,the gap should be more than 0.2mm,but there is no space and the trace is only 0.076mm (3mil). hole is 0.075mm, laser drilling