Usually, 0.4mm pitch BGA circuit board is a bit difficult to do ,but we can make 0.4mm CSP, even,0.38mm CSP PCB ,we can make now.
0.4mm pitch BGA, means the space from BGA ball to another BGA ball is 0.4mm. This PCB is very small,10*10mm, and blind vias from L1-L2,it is 4 layer PCB ,0.6mm, half-hole plating.min hole 0.15mm,via in pad, 0.38mm pitch BGA. All the above features, we can see from below image.
Actually,we tried to made with board with standard blind PCB technology,but it is too difficult due to all difficult features put together.At last,we have to make this PCB with HDI technology and drill the blind vias with laser.that is very perfect.
We can make such HDI boards in 8-10 days.