I’m running out of routing space on my PCBs and I’m considering switching to 4 layers.
From what I’ve read, in a 4 layer board generally the inner layers are reserved for power and ground planes…
If this is the case, I don’t really see how this would help with my routing issue, as power and ground traces represent a minority in my designs (well I guess once the ground plane is poured, it covers a large area…).
Are 4 layer boards mainly intended to provide noise immunity, by burying the power planes, or is it truly a way to jam more traces onto a PCB?
In general, what are your recommendations to get started with 4 layers, and to switch a design from 2 to 4 layers.
On a 4 layer board yes you will normally have the 2 inner layers as ground and power with no ground plane on the back.
This frees up the top and bottom layers as you no longer need to make sure you aren’t breaking up the ground plane.
When you get into a really tight spot you can use the inner layers which really does help out on a high density board.
So to summarise, you should have the inner layers for ground and power, connect everything to them using vias, then use the top and bottom layers as your first priority for routing but when you get stuck you can use the inner layers.
Also, try and stick with the same directions on each layer, especially the inner ones, this will make routing a little easier. For example one inner layer might predominantly go from top-left the bottom-right and the other might go from bottom-left to top-right. This sounds weird but in practice it will make routing easier.
It will be probably be cheaper if you use smaller design rules so that you can route your board with two layers. PCB Pool can manage 0.15 mm tracks with .15 mm spacing on double-sided boards.
leon_heller:
It will be probably be cheaper if you use smaller design rules so that you can route your board with two layers.
Leon
Thanks guys for the comments.
Leon, yes I hear you. With BatchPCB, it’s also cheaper to use 2 PCBs stacked together than using 4 layers.
As for smaller design rules, here’s the dilemma: I assemble my boards manually, and if I use narrower traces, I tend to delaminate them with the soldering iron.
However I understand that smaller design rules also includes smaller spaces between (wider) traces. My PCBs interface with higher voltages (around 300V) and it is not always easy to reduce the clearance between traces.
It is typical that the inner two layers of a four layer board are used from ground and Vcc but there isn’t a rule saying this must be.
You can make the inner two layer a signal layer if you wish.
One approach I’ve used successfully is have one of the inner layer the ground plane and the other a signal layer. This gives one more layer to route traces which can really help.
The down side of four layers is the huge increase in the cost.