Via issue

[ican7]

Hi Robert , The reference design I am working on is 4 layers in total(Top signal layer, second layer with power and ground planes , third layer with ground plane and bottom signal layer). I noticed that some of the signal traces only go from top to the second layer but still a normal via connecting all layers is used for them. shouldn't those be the micro vias connecting the relevant layers? Also there is a BGA on top layer and it's ground pins are connected to ground with small vias but these vias again go all the way from top to the bottom as well. Shouldn't those be again microvias going from top to a certain dedicated or isolated copper pour or a plane ?

[robertferanec]

Hi ican7, for 4 layer PCB I do not normally use uVIAs.

I noticed that some of the signal traces only go from top to the second layer but still a normal via connecting all layers is used for them. shouldn't those be the micro vias connecting the relevant layers?

It really depends on what kind of signals. If they are very important or very high speed I would route them from the top directly to the bottom layer and draw most of the track on the bottom layer which has a solid GND plane as the neighbour layer.

Also there is a BGA on top layer and it's ground pins are connected to ground with small vias but these vias again go all the way from top to the bottom as well. Shouldn't those be again microvias going from top to a certain dedicated or isolated copper pour or a plane ?

That should not be a problem. I always use through hole VIAs for power and ground (e.g. many times you have to use decoupling capacitors close to them anyway).

I hope it helps.
- Robert

[ican7]

Thank you Robert but I would like to know why did they use vias connecting all layers for this particular signals traces because these signal traces only go down to the second layer and back on top layer and thats it.Would it be some incorrect translation during import?

[robertferanec]

I dont' know. In standard PCBs uVIAs are not normally used as they make the PCB more expensive.

[ican7]

So just to clarify,technically speaking ,if I have a signal trace routed only down to a certain layer , It doesn't matter if I use a blind via or a normal via unless I route a very high speed signal traces does it?

[robertferanec]

Yes, you are correct. For "standard signals" it is not required to use uVIAs and it doesn't matter. But in some cases it may be very useful, e.g. if your PCB is very small and your routing area is very limited, then you may want to use uVIAs because it will help you to save some space and route more tracks.

[ican7]

thanks Robert. Is micro via same as blind via?

[robertferanec]

Blind VIA - you do not drill through the whole PCB. This is definition from Wiki:

IPC standards ^[1][2] define microvias as blind or buried vias with a diameter equal to or less than 150 μm

For more detailed answer you can google some stuff, for example: https://en.wikipedia.org/wiki/Microvia

In reality, uVIA is usually Blind VIA, as normally to drill 150um hole you can not use material thicker than 150um (VIA ratio for Blind VIA is usually 1:1, it means, you can not drill deeper than is the hole diameter because you would not be able to manufacture it)