Layer stack copper balance for 4-layer PCB - Page 1
Forum section: electronics / general

author: v8pete
author_rank: Newbie
timestamp: September 18, 2015, 10:03:05 pm
content: Up to now I've only ever designed PCBs with a maximum of 2 layers, but my current design is a fairly dense SMT layout for which I'm planning to go with a 4-layer board. The usual layer stack that my PCB design package defaults to is top tracking layer/GND Plane/VCC Plane/ bottom tracking layer. The Ground Plane layer is very useful, but a VCC plane is not very useful to me! My design has a number of voltage rails, eg +5,+12,-12,+8,-8, and what I'd really like to do is use the "VCC Plane" as a tracking layer primarily for the power rails. I'm not sure if this is that simple though, as I remember in the past that you need to be careful when having an odd number of plane layers as this can unbalance the physical construction of the board, leading to warping of the PCB. Or would it be better to use the VCC Plane layer as just a general tracking layer, ensuring that it has a reasonable amount of general tracking on it - would that help keep the layers in better balance? Any guidance appreciated!

author: nctnico
author_rank: Super Contributor
timestamp: September 19, 2015, 12:41:37 am
content: I think board warping is something from the past and caused by uneven cooling after soldering (*). Then again there is probably nothing against putting a ground copper pour in unused areas. Just be carefull with matched impedance traces and high voltage clearances. In many of my multilayer designs I have an odd number of routing layers but no problems with warping.

author: v8pete
author_rank: Newbie
timestamp: September 19, 2015, 09:11:37 am
content: Cheers for the reply, very useful to know! I remember the PCB guys at work talking about it quite some years back, so as you say it's probably a manufacturing issue that isn't such a problem these days, and I guess I could as you say use some copper pour areas to balance things back up a bit.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 19, 2015, 09:45:40 am
content: You can still (and should) use very wide traces, or pours/polygons, for the various supplies. Count the number of branches and take priority, signal quality, etc. into consideration, and prioritize the most important ones. You may consider routing the lower priority ones on the top or bottom as well, to relieve congestion on the one layer. Though routed runs may be more susceptible (to noise and disturbances), which doesn't help if they are sensitive rails, but that can be addressed by adding more filtering (ferrite beads, more bypass caps). There's always a way to go about it.Tim

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 19, 2015, 01:47:33 pm
content: Quote from: v8pete on September 18, 2015, 10:03:05 pmI usually design 4 layer boards, I always designate one layer (usually the 2nd from top or 3rd from bottom) as a ground plane, but the only time I dedicate an entire plane to a supply rail is for RF or for very low noise designs (both of which I haven't done in years).So my layer stackup usually looks like Top - Signal; Top Inner - Ground; Bottom Inner - Signal; Bottom - Signal. I tend to route supply rails on either the top or bottom layers and high speed signals on either the top or bottom inner layers. Less critical signals or power traces go to the bottom layer.

author: v8pete
author_rank: Newbie
timestamp: September 19, 2015, 08:52:31 pm
content: Many thanks for the great replies - gives me confidence that it should all work out ok. Spent a few more hours on the placement today, so should be all ready to route the design later in the week. Pretty pleased with how it's going so far - the density isn't too bad at all, and the layout has a nice clarity to it that I was worried would be hard to achieve with SMT compared to conventional board designs.

author: nctnico
author_rank: Super Contributor
timestamp: September 20, 2015, 02:53:40 pm
content: Most of the work is getting the components placed right (with some pin swapping / connection optimisation). Once that it done routing usually is a piece of cake.