Stacking micro-vias and buried vias.

[palpurul]

Hello,

I'll start an HDI design which is going to be 10-layers with via in pad process (it's got a couple of fine pitch BGAs 0.4mm).

Since I am new to HDI tech. I am researching about it before I start. I learned about micro-vias (stacking them, staggering them etc), buried vias etc.

There are a few question that I'd like to ask. I understand that stacking micro vias requires extra steps in manufacturing, hence more expensive PCB. Is the same thing with stacking buried vias and micro vias? Does it also cost more than staggering them?

Here is a picture to demonstrate What I am trying to tell:



On the right a micro-via and a buried via are stacked. Does this require extra step or it's a standard process?

Thanks in advance!

[Pseudobyte]

I have never stacked a microvia on top of a drilled via.

You should know that in the photo they are showing technologies that are incompatible as drawn. Having mechanical blind vias terminate on the same layer as mechanical burried vias as drawn could never be manufactured. Each drill pair you add to a design will add an additional drilling and plating cycle. Depending on your stackup each additional pair may require an additional lamination cycle. 

Assuming you find someone that offers a stacked microvia on top of a blind via the process would be pretty much the same as stacking two micro vias. The drill would need to be plated shut (or maybe filled with epoxy then plated over) then they would laminate the next layers, laser the microvia and plate again.

In most cases it is preferable to stagger your micro vias.

[palpurul]

I have never stacked a microvia on top of a drilled via.

You should know that in the photo they are showing technologies that are incompatible as drawn. Having mechanical blind vias terminate on the same layer as mechanical burried vias as drawn could never be manufactured. Each drill pair you add to a design will add an additional drilling and plating cycle. Depending on your stackup each additional pair may require an additional lamination cycle. 

Assuming you find someone that offers a stacked microvia on top of a blind via the process would be pretty much the same as stacking two micro vias. The drill would need to be plated shut (or maybe filled with epoxy then plated over) then they would laminate the next layers, laser the microvia and plate again.

In most cases it is preferable to stagger your micro vias.

Yea, I've done some research many people told me not to do that, thanks for your answer.

Is there any reference design that uses micro vias? It's going to be my first time dealing with micro vias, I want learn more before I start.

[Pseudobyte]

Not sure of anything open source you can reference.

I would work following your chosen board house's requirements on µVias. If you need more specifics on µVias i would recommend picking up a copy of IPC-2226A.

[palpurul]

Not sure of anything open source you can reference.

I would work following your chosen board house's requirements on µVias. If you need more specifics on µVias i would recommend picking up a copy of IPC-2226A.

Thank you

As I said I am not going to stack micro vias. I decided to use via-in-pad for micro vias in the first layer, that's where I need them only really, and buried vias in between.

Something like this work for me:


I don't know if I need micro vias on the bottom layer too. I might use them if it doesn't add cost, but if it does add cost I would skip them.

I was thinking about using via-in-pad for capacitor pads to place decoupling capacitor under BGAs, but I've never seen an example of it. What do you think about it? Is it a stupid idea?

[Pseudobyte]

I don't know if I need micro vias on the bottom layer too. I might use them if it doesn't add cost, but if it does add cost I would skip them.

I was thinking about using via-in-pad for capacitor pads to place decoupling capacitor under BGAs, but I've never seen an example of it. What do you think about it? Is it a stupid idea?

I think there will be a marginal cost increase to run the laser twice (once for each side). But that should be minimal because the plating and pressing cycles are the same.,

As for via-pad should be fine as long as you are specifying that they are filled and plated over. (that will add cost)

[palpurul]

I think there will be a marginal cost increase to run the laser twice (once for each side). But that should be minimal because the plating and pressing cycles are the same.,

As for via-pad should be fine as long as you are specifying that they are filled and plated over. (that will add cost)

Thank you for your help.

I basically need to use via in pad process for the BGAs on the top, but I was also thinking about using them on the capacitor pads. Is this an acceptable application of via-in-pad?

[twospoons]

If you are already doing via-in-pad for the BGAs then there is no penalty for doing it to your caps as well.
Sidenote: I have done stacked micro-via/buried via, and it can be done but is much more expensive as the buried vias need filling, which is an extra process.  The PCB vendor even asked if I really wanted to do this, due to the extra cost.  And yes I did - dense 12 layer board, not a lot of options.

[palpurul]

If you are already doing via-in-pad for the BGAs then there is no penalty for doing it to your caps as well.
Sidenote: I have done stacked micro-via/buried via, and it can be done but is much more expensive as the buried vias need filling, which is an extra process.  The PCB vendor even asked if I really wanted to do this, due to the extra cost.  And yes I did - dense 12 layer board, not a lot of options.

Thanks for your feedback it really helps!