Layer Stackup - Page 1
Forum section: electronics / general

author: nsh1233
author_rank: Contributor
timestamp: July 17, 2014, 12:58:03 am
content: So these are the schematics of my project and i have not idea how to assign the layer stack up. I heard about the high speed layout and i am not sure if mine is needed? How do i tell if i need the high speed layout? Are there any thing that i shoulg take note of? I will appreciate it if you could guide me on how to assign my layer stackup.Thank you.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 17, 2014, 02:05:25 am
content: Those look more like OrCAD to me...

author: nsh1233
author_rank: Contributor
timestamp: July 17, 2014, 02:15:14 am
content: These are not drawn by me. Just to show the schematics

author: Laertes
author_rank: Regular Contributor
timestamp: July 19, 2014, 07:26:15 pm
content: Quote from: nsh1233 on July 17, 2014, 02:15:14 amOkay. Still, your question isn't really related to Altium Designer as a tool(unless I misunderstood), it's a question about PCB design guidelines. So maybe post something like this in the projects forum in the future.Though, I admit, I don't quite understand the actual question. Could you clarify what you mean by 'assign the layer stack up'?I think that you need to consider very little about the high-speed aspects of the design, more about the analog-digital aspects. You seem to have a few rather precision analog lines going to the connector but also have I2C and digital connections. You would probably want to split your ground between analog and digital and make sure to not route any digital traces too close to the analog ones(and put ground pours inbetween them, as well).

author: nsh1233
author_rank: Contributor
timestamp: July 21, 2014, 01:01:45 am
content: Quote from: Laertes on July 19, 2014, 07:26:15 pmPreviously i am not sure what signal means as they always say(for example) Top Layer, GND, Signal 1, Signal 2, Vcc, Bottom Layer. So I thought that certain routing must be on Signal 1 and 2.So now i have started routing, i noticed that the digital and analog ground is connected. So how do i separated them?

author: lgbeno
author_rank: Frequent Contributor
timestamp: July 21, 2014, 03:34:51 am
content: From the looks of your schematic, I don't think that you have any high speed nets, these are just signals with high frequency content like in the 100's of MHz or GHz range. As far as stack up, it's best to route on a layer adjacent to a ground plane and also have the power plane adjacent to a ground plane as well. It all depends on your number of layers that you use too.If you are doing a first time layout. I would not recommend doing a gnd split, if you don't know how, you could really mess it up and get worse results.

author: nsh1233
author_rank: Contributor
timestamp: July 21, 2014, 05:03:22 am
content: How will it affect my pcb if i'm not doing the gnd split?And can i also use the polygon pour for ground instead of assigning a layer for it? and what do i do to the empty layer for ground? (My stackup: Top, gnd, signal 1, signal 2, vcc and bottom)

author: free_electron
author_rank: Super Contributor
timestamp: July 21, 2014, 06:02:36 am
content: you are missing basic knowledge.first of all : how many layers will you use ?board thickness ?prepreg thicknesses and copper thickness ?this can easily be done on a 2 layer board. , maybe a 4 layer would make it easier.as for splitting ground. this is a very difficult topic and cannot be explained in half a page. the goal is to keep return currents constrained to their supplies.you need to analyse current flow in this system.

author: DerekG
author_rank: Frequent Contributor
timestamp: July 21, 2014, 06:33:27 am
content: Quote from: nsh1233 on July 21, 2014, 05:03:22 amRemember that Mr Google is your friend. Spend a half day to a day reading everything you can about analogue/digital/power ground planes & ground splitting. These are the things you should have learnt about when you studied at Uni.Look for professional & not amateaur sites so that you get the good oil & not the snake oil.

author: nsh1233
author_rank: Contributor
timestamp: July 21, 2014, 07:26:03 am
content: Quote from: free_electron on July 21, 2014, 06:02:36 amBut i have a board size that i have to follow which is 3.1 by 5.7 cm so this cannot be done in a 2 layer board so im doing a 6 layer board.And yes i don't understand many of these things because i just started using this software and this is a school project which is not really related to my course. And i'm still studying in Polytechnic. So i'm asking for help here as this needs to be completed in 3 months and i have other boards as well. (+ soldering, testing, report, presentations)Sorry if i posted on the wrong category

author: Laertes
author_rank: Regular Contributor
timestamp: July 21, 2014, 02:17:55 pm
content: Quote from: nsh1233 on July 21, 2014, 01:01:45 amThe layer name assignments are completely arbitrary, you may chose them to your liking in the Layer Stackup Manager. Altium by default calls inner layers 'Signal 1', 'Signal 2' and so on, but you don't have to use any layer for signal routing. You normally designate two layers to power and ground in such a design and use the other four layers for routing in any way that fits, so whether you use top, signal 1, signal 2 or bottom to route that one signal is mostly irrelevant. Just try to avoid using too many vias on one signal and give the highest speed nets preferece while routing, so that they may be routed with as few vias as possible(ideally none). Vias have an inductance and that can cause problems on high-speed lines.For power layers, Altium offers you a feature called a plane layer that is automatically filled with a pour of one specific net(typically either power or ground). Personally, I tend not to use that but rather put individual 'Signal' layers in the stackup and put polygons for the power nets onto them, as that approach is much more flexible.Quote from: nsh1233 on July 21, 2014, 05:03:22 amYou can use a polygon pour for any net you want. You could put a split ground plane onto the ground layer if you didn't assign it as a plane layer(as I stated above). However free_electron and Igbeno are probably right, split grounds are a complicated topic, it might be better to ignore that for now. Maybe you could get one of your tutors to go through the design with you and explain to you where and why and how split grounds can be applied...

author: DerekG
author_rank: Frequent Contributor
timestamp: July 21, 2014, 11:09:47 pm
content: Quote from: Laertes on July 21, 2014, 02:17:55 pmThis is an excellent idea. If you lay out the planes in the way that your lecturer prefers, you are more likely to maximise your marks.

author: free_electron
author_rank: Super Contributor
timestamp: July 22, 2014, 05:15:27 am
content: Quote from: nsh1233 on July 21, 2014, 07:26:03 amThen you are basically screwed.Begin with drawing a clean schematic and creating all footprints.This board is packed with stuff. It can be done on 4 layers. Top layer, ground plane, power/ signal grid and bottom routing.Once you have all footprints : do a placement excercise.

author: lgbeno
author_rank: Frequent Contributor
timestamp: July 22, 2014, 02:37:37 pm
content: Quote from: free_electron on July 22, 2014, 05:15:27 amI would not say screwed...If you have already made the jump to 6 layers and are ok with the cost of doing so, thats your choice. Can this be done with 4, most likely... Will it be easier to route 6 layers, of course. Will the results be better with 6 layers, probably not, but maybe in some instances. You can always start with a 4 layer stack and add 2 layers if needed or start with a 6 layer stack and remove 2 layers if you don't end up using them. All depends on what motivates you.For a 4 layer board, you can't go wrong with a top/gnd/pwr/bottom stack where you try to do the majority of placement and signal routing on the top, decoupling, a minimal number of jumpers other stuff on the bottom. Analog circuits always have a nice signal flow so this should be fine.6 layer you could do top/gnd/sig/sig/pwr/bottom or top/sig1/gnd/pwr/sig2/bottom like you mentioned. Each have pros and cons. I would probably choose the latter if it were me, the con is that you can have signal coupling between top and sig1 you you are not careful.In general, @free_electron is right and saying that floor planning and component placement needs to be done first, place components how the signal flows throughthe schematic and try to reduce the snarl of rats nest as much as possible to minimize intersecting rats.When you do that, post a screen shot and then talk more about stackup.

author: free_electron
author_rank: Super Contributor
timestamp: July 22, 2014, 04:58:47 pm
content: i still say he is screwed. he's never done a board in his life, has never used the software and will do a 6 layer, tightly packed board...raw -> cooked. he's dead.

author: lgbeno
author_rank: Frequent Contributor
timestamp: July 22, 2014, 05:24:17 pm
content: We all start somewhere, it's ambitious. We'll see where/if he can take it. If anything he'll get calibrated to what it takes. I like ambitious.