configure Micro vias

[prabhu]

Hello Everyone,

I am desinging my new flex/Rigid PCB inorder to fanout the 0.5mm pitch BGA i am planning to use the micro vias ,totally am using 8 layers pcb which consists of four signal layers and four Internal planes

my question

1) how to configure microvias properly and symmetrically to reduce the fabrications cost

first micro vias-> Top layer-> signal layer_1
second one-> signal layer_1->Bottom layer

or do i use this configuration for the drill pairs

first micro vias-> Top layer-> signal layer_1
second one-> signal layer_1->singal_layer_2
Third one-> signal layer_1->Bottom layer


please verify my configurations and also am bypassing the internal planes in the micro vias will it be an problem for the fabrications ?please suggest me your ideas

i am planning to use micro vias (replace all other through holes vias by micro vias) in order to reduce the cost of fabrications and feasibility for the fabrications whether it is advisable to do like this or not advisable?

Thank you
prabhu

[robertferanec]

How the stackup looks?

Keep in mind, that ideally you would like to use minimum hole in micro vias (because of very small pitch) - and do not forget, you can drill the micro via only as deep as is diameter of microvia (so if your hole in micro VIA is 0.1mm, you only can drill 0.1mm deep, if you would need to drill deeper, you would need to use bigger hole in microvia => you would need to use bigger micro vias).

[prabhu]

Hello Robert

Thank you so much for your kind reply i have attached the stackup configuration of my PCB i am not quite sure how to configure micro vias because in the internal plane we don't draw any tracks so how can i configure micro vias from the top layer to the bottom layer (L1-L8)

could you suggest me some possible ideas

[robertferanec]

Did you talk to your PCB manufacturer? They may not be able to manufacture this stackup.

Watch these videos. They may help you to understand how PCB is manufactured:
https://www.fedevel.com/welldoneblog...r-pcb-is-made/
https://www.fedevel.com/welldoneblog...u-should-know/

Drilling has some rules (or better to say - technological limits). Usually, you either drill through hole or you can drill up to certain depth (blind VIA), but then drill diameter is at least as big as the depth (because of plating process - the process when they put copper inside the VIA). If you are building buried VIAs (VIAs which are inside of PCB and you can not see them from outside), they first put together the middle of PCB, drill through it and then they "stick" other layers on top of it.

Also, when you are creating stackup, be aware how much each process cost. You may end up with a stackup which is possible to manufacture but may be much more expensive as if you correct it a little bit.

Example:
1) If you use Through hole PCB only, it can cost you 500 USD to manufacture
2) If you use Through hole + uVIAs + Buried VIAs, same size PCB and same number of layers as the Example 1 can cost you 3000 - 5000 USD