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Using Vias in Multilayer HDI Boards

kazemiy974 , 01-25-2021, 04:17 AM
Hello friends,

I'm designing a stackup for my new board and some questions arose for me, and they are:
1.When are we allowed to use via on multiplayer boards and when should we not put via between layers?
2.As shown in section A in the image below, we can use via from layer 3 to layer 5, is that true??
3.According to section B in the figure below, are we not allowed to use the via from layer 3 to layer 6? Why?
4.What are the general conditions that we have to pay attention to in order to use via? For example, is it important for the underlying or superficial layer to be Core or Preperg, etc, or other reasons?


goncaloc , 01-25-2021, 07:00 AM
From the bottom

4 - Board thickness, via size, manufacturer capabilities, like via aspect ratio, minimum annular ring, and on witch class ( manufacturer class ) you want your board. Most of the times, manufacturer will look at your stack up and arrange it the best/cheap possible way, you define the dielectric thickness and they match it to the most fitting core or prepreg.

3 - Not that you are not allowed, ideally you want your board to be symmetrical from top to bottom. This will keep your board cost low, manufacturers can do vias like section B but be sure they will charge you for it.

2 - You can.

1 - Preferably you want to use "normal" vias every time, on a HDI board you will find your self some times in need to use uVias and burried vias for lack of space to use "normal" vias.
robertferanec , 01-25-2021, 09:25 AM
@kazemiy974 I would recommend to have a look how PCB is manufactured, that will help you to answer your questions. Have a look for example at video: https://youtu.be/67WhV0EDqCA

Understanding PCB manufacturing process will answer some of your questions. For example drilling. To build the stackup on your picture ... that may actually not be possible.

For example, Core is a 2 layer PCB: copper + izolation + copper = core. I guess it could be possible to have 1 layer core (I have not heard about it, but maybe it exists) and then you could do: Copper+izolation+prepreg+copper ... but ... I am not really sure.

The same for drilling. How would you drill 3 to 5? Standard drilling is always through hole, means, they drill through the whole PCB. If you would want to drill from 3 to 5, theoretically you could do it, but you would have to build this section first, drill it and then glue it with the other parts of your PCB. However in this case you could not have any buried vias ending or starting on layer 4, because how this would be manufactured?

I recommend to watch the video.
kazemiy974 , 01-27-2021, 11:07 AM
Thank you i understand all of things my need. @robertferanec @goncaloc
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