Welcome to the CADSTARguys Blog - Information, hints, tips and my waffle on the CADSTAR Printed Circuit Board design suite.

Please note that all names used are completely fictitious and any thing written is my own personal opinion or knowledge and not related in any way to either my employers or their customers (or Zuken).
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Monday, 15 July 2013

Should my Inner layers be Powerplane or Electrical?

How the inner layers are defined is often a cause for confusion amongst new users of CADSTAR when it comes to power planes.

Why is this so?


The inner power layers are often referred to as power planes, a GND plane and a VCC plane etc.

I have only been "doing PCB's" since about 1992 so am too young for some of this but as I understand it historically the inner plane layers would be defined as a "powerplane", all is OK so far however a powerplane in CADSTAR (and most other CAD packages at the time) was displayed in the negative - that is the copper is not shown and the gaps in the copper (around the pad) are shown instead.

This stems back to ye olde days of 8086, 256k RAM and other such cobweb covered stuff that has less power than my calculator.

A positive plane was too much-too slow-too big for technology then so they were negative and the PCB manufacturers inverted them when plotting or some other unseen wizardry.

So we set the inner layer to be a powerplane in CADSTAR - whats wrong with that?
Well, if you have a single net solid plane on an inner layer you do not see where the copper is, you also do not easily see where the copper is not. Gaps in the plane are harder to visualize and pins that are not connected also harder to see.

By having the layer as a positive plane we can see exactly where the copper goes or it doesn't, we can also see the pads that do not get connected to it.

OK - so positive copper is better in the power planes - then why do we still set those layers to be negative planes you ask? This is what is often so confusing.

The reason for this is that when routing a connection can be terminated on a plane when it passes through it. That is to say that if you route a track and place a via that goes through the plane then the track will be terminated and not continue as a track on that layer. If the layer was "Electrical" then it would continue to be routed as a track.

So in order to make the task of routing the board quicker and easier we set the inner power layers to be powerplanes while we are routing, then once routing is finished we change them to be electrical and pour positive copper on them within copper pour templates.

It's simple really and now that you know this, it becomes second nature to start with powerplanes and then change them to electrical and pour copper when routing has finished.


4 comments:

Hobbs said...

But is it possible to choose whether to view the copper in positive or negative and if not can we have this feature? Routing problems aside, simply viewing the design at a later date is a lot more difficult if the copper is in negative.

CadstarGuy said...

Yes it is, if you read the last 2 paragraphs....

A negative plane is used purely for routing, then you use copper pour templates as shapes to define the area and flood fill it with positive copper.

Hobbs said...

Sorry, i don't think i explained myself very well.
Can you change the copper from positive to negative or vice versa on a power plane after the design has already been laid out?

CadstarGuy said...

I'm pretty sure that if the layer is a "Powerplane" then when you post process you can choose either positive or negative output - not that I have ever done this in 25+ years of using CADSTAR- that's ancient technology but like most things we still call it a "powerplane" even though its just the same as any other signal layer.

If you have routed as a powerplane for ease, changed the layer to "Electrical" and poured positive copper then if you then want to change it back to a "Powerplane" to do a lot of rework then the best thing to do is unpour the copper to clear the design, change to powerplane, reroute and so on.

Changing from powerplane back to electrical will clear any previously poured templates on that layer. So yes you can change the layer type during the design - although its not something you would really want to do that often.

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