How to set different isolation gaps between polygons and different thermal widths on the same polygon pour?

[paullevene]

Hi everyone,

I’m having trouble getting Fusion 360 Electronics to respect different isolation gaps between polygon pours.

 

I have a +24V polygon pour (pink) and a GND polygon pour (red).

• The +24V pour is set with an isolate of 1 mm.

• The GND pour has an isolate of 0.2 mm for everything else.

The issue is that the 1 mm isolation between the +24V and GND polygons isn’t being respected. The gap to pads, tracks, and vias follows the settings correctly, but the gap between the GND and +24V pours doesn’t change — they sit much closer together than 1 mm.

I’ve tried changing this through both the polygon properties and the design rules, but neither seem to affect how these two regions interact.

On top of that, I also need to use different thermal widths within the same GND pour:

• Thicker thermals for high-current pads,

• Standard thermals for the rest.

However, Eagle seems to only allow one thermal width per polygon, and I can’t find a way to vary it for specific pads or nets within the same plane.

So my questions are:

1. Is there a way to force a specific polygon-to-polygon isolation gap (e.g. 1 mm between +24V and GND, 0.2 mm elsewhere)?

2. Is it possible to have different thermal widths for certain pads within the same GND polygon?

3. Or do I need to split the GND plane into multiple polygons to achieve this?

Any advice or best practices would be greatly appreciated!

— Paul

[paullevene]

 I will just add that the red GND polygon is Rank 6 and covers the entire Layer.  The 24V Pink polygon is Rank 1 and is inside the GND polygon region.
— Paul

[jorge_garcia]

Hi @paullevene ,

I hope you're doing well. The isolate parameter only applies to objects within the boundary of the polygon so it won't control the interaction between polys. If you want to control the separation between polys the best way to do it is with a custom copper clearance rule. That will give you the control you want.

Now you've already discovered that polygons can only have one thermal width. If you need different thermal widths within the same polygon you can achieve this by drawing small localized polys within the larger poly and set the thermal width of those to be thicker. The general rule is that large poly uses the thinnest desirable thermal width. The localized polys should use a larger width.

Let me know if you have any questions or concerns.

Best Regards,

[chuck_toddN7PTC]

@jorge_garcia 

Instead of drawing separate polys to have different thermal thicknesses, please add a component scoped rule similar to the example below.

The component scope could also be used for necking down trace thicknesses on the fly when within some pin clearance.

i.e.

If component is U3

and Poly is GND

then Thermal = 0.2mm

BR -Chuck

[paullevene]

Hi Jorge,

Thank you for responding.  I have some questions:

"The isolate parameter only applies to objects within the boundary of the polygon": In the image provided you can see the objects outside of the polygon (tracks and vias) affecting the isolation inside the polygon based on the Pink polygon's isolation setting of 1mm.  So that statement is not valid.  

Why would you have the objects outside the polygon being respected for the isolation but not the other polygon's copper?

"If you want to control the separation between polys the best way to do it is with a custom copper clearance rule.": This I cannot get to work in anyway possible. Not with polygons. Could you please test this and report back on what would be the example for a working rule sufficient to allow for this? Perhaps I'm missing something obvious with designing the rule to be valid.

Another thought I had recently is that it would be good to have a "simulation test area" to run that shows the designed rules are working from the rule manager. You could insert two signal/object names and it would render the test view.  On the left side it should shows a window with a valid test (pass) and or the right it shows a window with a invalid test (exception). Another example, if its a clearance rule it could have a slider and it moves the two tracks closer. You would see a live point at where it detects so you know the rule works or is being over ridden by something else. Just a thought.

— Paul

[jorge_garcia]

Hi @chuck_toddN7PTC ,

There are plans to eventually be able to scope the thermals but this is still a ways out. Thanks for confirming that this is needed.

Best Regards,

[jorge_garcia · Accepted solution]

Hi @paullevene ,

I hope you're doing well. You are correct on Isolate, I apologize for my mistake. Here's what Isolate controls as defined in the the documentation. 

Isolate (https://help.autodesk.com/view/fusion360/ENU/?guid=ECD-CLI-P#polygon)

Distance between polygon areas and other signals or objects in the Dimension or according restrict layer (default: 0). If a particular polygon is given an Isolate value that exceeds that from the design rules and net classes, the larger value will be taken. See also Design Rules under Distance and Supply, respectively.

Note that if you give a polygon an Isolate value that exceeds that from the design rules and net classes, small gaps can result between the calculated polygon and objects belonging to the same signal as the polygon itself, which may lead to problems during manufacturing! It is therefore recommended to leave this parameter at 0 in most cases.

Setting up a rule is pretty straight forward, see the image below

Just tested it and it works as expected.

Let me know if there's anything else I can do for you.

Best Regards,

[paullevene]

Thank you @jorge_garcia.

I can now get this to work when the polygons have different ranks using the rules set as you have shown. With the same rank it just highlights it as an exception with the DRC. I do feel however the polygon isolation setting on the poly should be isolating as per the documentation to other polygons with the larger value, but it isn't.

[jorge_garcia]

Hi @paullevene ,

When polygons have the same rank the rules "misbehave" (it's a bug that over many years has become feature). Because they get the same priority isolate won't work as you expect. If they have different ranks then isolate will behave as expected. In general there is only one scenario where overlapping polygons having the same rank makes sense and that's when dealing with split power planes.

Let me know if there's anything else I can do for you.

Best Regards,

[eng47WXL3]

Hi Jorge

    Isolate (https://help.autodesk.com/view/fusion360/ENU/?guid=ECD-CLI-P#polygon)

    Distance between polygon areas and other signals or objects in the Dimension or according restrict layer (default: 0). If a particular polygon is given an Isolate value that exceeds that

    from the design rules and net classes, the larger value will be taken. See also Design Rules under Distance and Supply, respectively.

The interaction between the polygons doesn't seem to follow the rule.
In the attached example, the "1.2V" polygon (red) has rank = 1 and isolation = 20 mils, the "GND" polygon has rank = 2 and isolation = 8 mils.
The "1.2V" polygon provides the correct 20 mils of isolation from the traces and vias, but the isolation between the polygons is only 8 mils, not only the lowest one, but also that associated with the lowest-priority polygon (rank = 2).

Best Regards

Roberto

[jorge_garcia]

Hi @eng47WXL3 ,

I hope you're doing well. Instead of using the isolate value I would recommend using a the Rules as I show in the accepted solution further up the thread. So set isolate to 0 for both polygons and define the polygon clearance as a rule as shown above.

With that said as an interesting experiment swap the rank of the polygons. I get a feeling the rank causes the isolate parameter to behave differently within the polygon versus outside the polygon. If I'm right what you'll see is that outside the 1.2V polygon you'll get the 20mil separation but inside it will be 8mils. If you have a chance try it, but the recommended and final solution will be what I wrote above.

Let me know if there's anything else I can do for you.

Best Regards,