topic Re: Why is the layer stackup so baffling? in EAGLE Forum

Why is the layer stackup so baffling?

engineeringNCMXB — Thu, 08 Feb 2018 05:17:41 GMT

I am finally ready to do my first layout with blind/buried vias.

The layers setup strings are pretty ridiculous and I am hoping anyone can shed some much-needed light on this. In the end - I am hoping to learn the syntax/concept behind this. The best option is for the Eagle team to make it easier to use, but that will not happen before tomorrow.

Lets say a 4L PCB with blind and buried vias.

Through 1-4
Blind 1-2
Blind 1-3
Blind 4-2
Blind 4-3
Buried 2-3

Although I will not use all of those, I am not able to define this. The hack method is not going well and I am not getting the concept or understanding it's limitations.

Anyone able to decode the syntax or point me to a resource that can? Thank you in advance!

Re: Why is the layer stackup so baffling?

rachaelATWH4 — Thu, 08 Feb 2018 09:39:36 GMT

@engineeringNCMXB wrote:

Lets say a 4L PCB with blind and buried vias.

Through 1-4
Blind 1-2
Blind 1-3
Blind 4-2
Blind 4-3
Buried 2-3

Although I will not use all of those, I am not able to define this.

So I will start with saying you probably don't want to specify all those via combinations, it may make your board expensive to build. However, here is how you'd do it.

Note the layer numbering in EAGLE is such that the stack-up goes from 1 on the top side and 16 on the bottom side and is symmetrical so layers are added in from both sides. In this case your layer numbers will be 1, 2, 15, and 16. If you don't do this the EAGLE Standard licence limit won't allow it to work. Don't ask me why they changed it from earlier versions of EAGLE, they just did.

The syntax for this is really quite simple:

[15:[2:(1+(2*15)+16):15]:2]

See!

This will yield the following:

Ok, so I lied it's not actually very obvious unless you know what you are doing.

Note: I have not considered the thicknesses of core and prepreg material here. The core will likely be a lot thicker and the prepreg a lot thinner so layer 2 is nearer layer 1 and 15 nearer layer 16.

You need to think about how a PCB manufacturer is going to physically make the boards before you can specify an appropriate stack-up. You want a buried via from 2-15 so the manufacturer will start with a core material with copper on both sides, etch the two inner layers and then drill and plate the holes for the buried vias. Next they will add prepreg and a copper foil on each side and for the two outer layers and etch those. Then they will drill all the blind and through vias and plate those.

Now, think about your blind vias that go from 1-15 and from 2-16. The depth of those are such that it might actually be difficult for the manufacturer to adequately plate them so these are likely to drive the board cost up or make it impossible to build. I would consider just using the 1-2 and 15-16 blind vias which will result in the following configuration string:

[2:(1+(2*15)+16):15]

With the following result:

Now, because you want the buried via you have to have the core in the middle which pushes you towards a more difficult technology requiring drills to go only partially through the board. If you can live with just the blind vias and through vias then the process becomes much simpler and the drills are effectively all completely through drills. The reason being they can create the board using two copper clad cores sandwiched together on a centre prepreg as follows:

[2:(1*2+15*16):15]

With the resulting stack-up:

So from the above, a quick syntax summary:

+ is for a prepreg layer

* is for a core layer

( ) specifies a buried or through via

[t:...:b] specifies a blind/micro via

Best Regards,

Rachael

Re: Why is the layer stackup so baffling?

engineeringNCMXB — Thu, 08 Feb 2018 17:21:51 GMT

What a fantastic response! Thank you. Seeing the examples unfolded delivers some real clarity.

Just to be clear - I was not expecting to build a real PCB with the original stated stack, but rather have an example that would help me understand it well enough to make any stack I may want in the future. The second part of this is to make a sample layout and present it to my usual PCB fabricators to get DFM feedback and understand the practical restrictions.

This post was very helpful, now I am going to practice a few scenarios on my own to see if I actually get it.

Thanks again!

Re: Why is the layer stackup so baffling?

engineeringNCMXB — Thu, 08 Feb 2018 18:05:29 GMT

So let's see if I get this yet.....

In this example (may or may not be a good real-life stack, just an example)

[15:[2:(1+(2*15)+16):15]:2]

The highlighted vias correspond like this:

[15:[2:(1+(2*15)+16):15]:2]

Is that feature to string correlation correct?

Re: Why is the layer stackup so baffling?

jorge_garcia — Thu, 08 Feb 2018 18:10:57 GMT

Hi @engineeringNCMXB,

I also wanted to mention that if you go to the Control Panel > Documenation tree there is a PDF called layersetup_designrules that has some good examples as well.

You are understanding in the post above is correct.

Please let me know if there's anything else I can do for you.

Best Regards,

Re: Why is the layer stackup so baffling?

rachaelATWH4 — Thu, 08 Feb 2018 18:32:04 GMT

Yes that looks right

Re: Why is the layer stackup so baffling?

engineeringNCMXB — Thu, 08 Feb 2018 18:44:55 GMT

Thanks Jorge for chiming in.

One note on the concept - the programmatic syntax and command line interface in Eagle is powerful for those that dive deep and commit it to memory. For multi-role engineers like me (ME and EE and company owner) - I have to use a wide variety of complex software every day in addition to juggling multiple programming languages. The practical reality is that I really do not want to learn some obscure syntax for a task that I rarely do. On any given day, my capacity for 'learning curve' is maxed out with engineering challenges so I dream of easy software.

This is a good example in Eagle where it could easily be presented as a GUI defined task with the programmatic option for those that prefer that. The end user can choose easy OR fast.

As a general note - Eagle can do just about anything......eventually. But you have to slog through ULP's, and external calculator, command line syntax, etc. It is operationally very slow compared to any other software that I use in my professional endeavors.

To end on a high note.....the recent improvements are fantastic and VERY much appreciated. I have been using Eagle for about 3-ish years and really do not want to learn a new system. You really made a leap with the Fusion 360 integration. I finally decided to dump Solidworks and Mastercam after 20 years.

Please keep the improvements coming.