Help with Drill files for board with blind vias (Solved)

[Psi]

Hi,

I'm getting a 6 layer board made which includes blind micro vias.

It all seems to be fine inside Altium however when i generate the drill files for each layer i notice that some holes are not shown on layers where i would expect them.

For example, i have a BGA on top layer using vias-in-pad (filled and plated).
- I have some vias from Top layer down1 to L1 (lets call those typeA)
- I also have some from Top layer down2 to L2 (lets call those typeB)

Altium generates TX1-4 files
TX1 is the first substrate to be drilled,  TX2 the second etc.

Currently TX1 is showing Type A drill holes and TX2 is showing type B drill holes.
This seems wrong, surely TX1 should show both A and B?

Any ideas?

The only thing i can think of is maybe this is normal and the two layers get glued together and then drilled together, hence TX2 would drill through TX1 at the same time. 
That seems quite prone to errors depending on the order the fab hours does the process.

[Psi]

Here's a pic showing the TOP drill file (TX1) and the next one down (TX2).

[julianhigginson]

What's the arrangement of core/prepreg layers in the stackup? Does adjusting those change the drill files?

[T3sl4co1l]

Well... have you asked a fab what they'll do with it?

Seems to me, you don't need to double-drill holes that go through both layers.  Consider the manufacturing process: they can't drill mid-layer holes, then laminate over them -- they'll get plugged with resin and have to get drilled out anyway.  They aren't gluing together layers of rigid material, the layers are the glue itself (resin-impregnated cloth, stacked to the desired height, smooshed and cooked until done).  So, it only works if the drilling is done after lamination.  Yea?

So, by that logic -- assuming it's right -- ask the fab to be sure! -- they would only need one list of drills to X depth, and another to Y depth.  This appears to be what Altium has generated.

Does that help?

But don't take my word for it, I'm not a PC fab house, do call.  Really, do it now.  Go!

Tim

[Psi]

Calling a board house isn't going to help. i think this is more an altium export question. Also trying to explain this concept to someone in china would be hard.

What's the arrangement of core/prepreg layers in the stackup? Does adjusting those change the drill files?

Not as far as i know

Seems to me, you don't need to double-drill holes that go through both layers.  Consider the manufacturing process: they can't drill mid-layer holes, then laminate over them -- they'll get plugged with resin and have to get drilled out anyway.  They aren't gluing together layers of rigid material, the layers are the glue itself (resin-impregnated cloth, stacked to the desired height, smooshed and cooked until done).  So, it only works if the drilling is done after lamination.  Yea?

hm.. if what you say is true, how will the board house know (from my files) that i have a via starting at layer 1 (top) and ending at layer 3.

There would be
L1
prepreg1
L2
prepreg2
L3

For a via between L1 and L3...
The gerbers would show the L1, L2 and L3 pads for it, and the NC drill TX_ files would show a hole in prepreg2 but nothing to say they should be joining L1 to L2 through hole as well.

EDIT

So, by that logic -- assuming it's right -- they would only need one list of drills to X depth, and another to Y depth.  This appears to be what Altium has generated.

Hm.. further reading your post. 
You are saying that TX1 -> TX4 are not actually 'drill map drawings' but rather 'drill depth ends here' drawings?  I guess that might make sense. Can anyone confirm this is the case?

If this is the case i'm not sure how they would handle the other direction, drills from L6 up to L5, without it also looking like a drill From L0 to L6

[Psi]

OK, i figured out the problem

Hopefully this will help someone else.

Problem is caused by trying to create a blind via that spans more than 2 layers, eg via from L1 to L3.

Altium lets you do this and will happily create you a drill pair from L1 to L3

The problem is that the blind via ".TX_" drill outputs files are matched up to drill pairs, not to actual prepreg/cores like you'd expect.

The result is, if you have a drill pair from L1 to L3 you will get a single drill file for this, it doesn't also include that on the L1 to L2 file.

The solution is somewhat obvious when you consider this is a stacked via.  You need to create two separate vias and stack them in the same location, one from L1 to L2 and one from L2 to L3.

Then the TX_ output files make sense.

Staked microvias need to be stacked... who knew 

[T3sl4co1l]

Ah yeah, buried rather than blind vias, that would matter

Tim