PCB Stack-up and Controlled Impedance in Altium Designer

[@enginstud8852]

I'm sure Phil like chatting with pcb manufacturers so much that he even asks them jow are the kids doing, and so on 😂

[@scottpelletier1370]

Helpful video, thank you. It is a great idea to ask fab house first. Otherwise, you'll end up with a question sheet about what they want to tweak on your layout to make it work... best to route your traces with as accurate info as possible to start with.

[@RixtronixLAB]

Nice info, thanks :) 👍

[@mate2logica]

Hello! In the case, for example, of working with a design where the manufacturer provides the specifications for an 8-layer board and gives the trace values for each impedance, is it unnecessary to use the impedance calculator? Instead, would you create a class for each differential pair and then set a rule per class specifying the trace values? For example, Rockchip, in its hardware design manual, specifies the trace values for each impedance considering an 8-layer PCB and indicates the layers where the differential signals should be routed. Another thing is my question: why should I use serpentine traces on differential signals?

[@lincolnpereiradovale3104]

And when the required impedance is unknown? What we do?

[@Dr.Bigglesworth]

Thank you for the video. When you said "import" the stackup information into Altium, I figured you really meant import. Not type it in manually. Does Altium support importing of any stackup data, like an IPC-2581?

[@Zachariah-Peterson]

I can answer this, right now Altium does not support IPC-2581 imports. The CAMtastic tool does support ODB++ imports, but I do not know if it does automatic layer assignments.

[@Dr.Bigglesworth]

@@Zachariah-Peterson Thank you for the reply. I'd strongly suggest this be a considered feature for the near future.

[@Dr.Bigglesworth]

@@Zachariah-Peterson Looks like the biggest issue with updating impedance profiles (say due to stackup changes) is the way Altium handles the length matching accordions. I can't find a way to change the trace width of accordions. Retrace doesn't work, even though the tracks and length match objects are selected, it only updates the tracks, not the accordions. There are claims you can do it with the PCB List panel, but there is no width parameter in that object there to edit. I tried adding it, but still didn't populate with a value, or allow an edit. It should just be in the parameter panel and it should allow updating/editing it, just like you do with tracks.

[@Zachariah-Peterson]

​@@Dr.Bigglesworth For the length tuning accordians, I have had to convert them to primitives in order for the query selector to work, then they can appear as tracks/arcs and their width can be modified just like with regular tracks.

[@Dr.Bigglesworth]

@@Zachariah-Peterson Yep, that's what I had to do. But, it would certainly be "useful" if the accordions could just be updated in and of themselves. That would make re-tuning easier.

[@Robert88KAL]

Do those rules still apply if you round up the corners? Also, if you change the dielectric, I see that the calculated width changes, but do the already placed traces change in width too? You raid that you round up to 2 digits but wont 3 digits perform a bit better overall in actual manufacturing? Since lithography is a optical process it is not constrained by a grid so I thought that a 3rd digit may help a bit (tho it may not be as exact . Also asking since if I remember well some PCB manufacturers gave 3 digit trace specifications and I am not sure if that is due to inch to mm conversion or just in order for us to know that the rounded up 2 digit width is ok.

[@Zachariah-Peterson]

If you change the dielectric thickness and then the calculated width changes, then you will need to change the width manually. This is because the impedance profile applies to anything in your PCB on that layer, you need to go into the PCB Rules and Constraints Editor to update the design rule for the nets/net class that uses that impedance profile. If you update the design rule, you will then see the nets that violate the updated rule as a DRC error. You can also select the traces you want to update with a query or select manually and re-type in the width. You could use the retrace tool to redraw the widths but be careful with this as it can mess up your via transitions and any length tuning. Usually it's fine on single-ended traces but I refuse to use it on differential pairs because of how it can mess up some length tuning and custom via entry.

[@alirezasalehi9149]

thanx