About fan out of a 0.65 mm pitch BGA

[mulfycrowh]

Hi everyone,

I have to fan out a 0.65 mm pitch BGA.
The guides suggest using 90 to 100 um tracks.
My layer stack gives an impedance of 50 Ohm for 200 um tracks.
If I use 100 um tracks, I won't have 50 Ohm impedance.
What should I do ?
Thanks.

[WhoKnewKnows]

I recommend talking with your fab shop. 90-100 micron traces are going to be difficult and expensive. You'll likely only be able to escape the outer columns and rows on the same layer as the BGA. The interior pads will probably have to escape by microvia in pad to other layers. This would allow you to use wider traces.

[robertferanec]

WhoKnewKnows is right, 0.2mm for 50OHM is wide. It depends how cheap your PCB has to be, but you may want to bring the reference layer closer to your signal layer. It may be more expensive, but bringing GND closer to signal layer will give you also other benefits (e.g. less crosstalk, less emissions, etc)

PS: if you really have to use that stackup, you can use thinner traces under BGA, but you may want to make them wider once you are out. Not the best solution, but I have seen it in many designs - even many high speed boards use it.