In net class constraint if track width is less than the default constraint track width then not able to set net class constraint to nets. It is taking the default constraint.
If net class track width is larger than the default constraints than it is working properly.
I suppose by “default constraint” you mean Board Setup -> Design Rules -> Constraints -> Minimum track width. In that case it could be said to be expected because it’s meant to be, as it says, the minimum width for the whole board, dictated by the minimum which the manufacturer can do.
If, on the other hand, you mean the Default netclass, it shouldn’t do so.
Can you describe the situation in more details?
First i have set Default constraint minimum track width of 0.25mm as shown in figure.
Then i have create the net class of 90_ohm impedance and set track width and track spacing of 0.15mm as shown in figure.
so now i go to route menu route differential pair and after routing i see the net track width is still 0.25mm instead of 0.15mm.
Similarly For differential pair gap if in default i have given minimum clearance of 0.2mm as shown in default constraint image and Net class constraint dp gap of 0.15mm. so when i route the differential pair gap remains 0.2mm instead of 0.15mm.
You have to take the design rules constraint into account. These are the minimal minimum values, they overrule all netclass values!
but what if my net class width is lower than the default constraint minimum width than why it is not taking net class value why it is taking default constraint.
Please give me clear answer.
They have different meanings.
You use net classes to set the width of tracks according to their function. Narrow tracks for signal lines, wide tracks for power and other area’s where there are high currents. Therefore you can set up different net classes for different groups of tracks.
The “Minimum track width” is intended to be used as an absolute minimum, for example for what your PCB manufacturer can make reliably, and this value is checked for all nets.
I think the clearest answer which can be given is that “minimum is minimum”.
If you need to override that value, say, only for a short distance because the design is already as tight as humanly possible and one track segment needs to fit through a small gap, and you believe the manufacture can do it despite what their capability table says, you can change the track width manually in track Properties and ignore the DRC error.
EDIT: the “default constraint” in the board setup constraints isn’t a “default”. You can give it a value which reflects the actual minimum track width of the board. It’s not a default value for the design, it’s the minimum value.
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