When do we use Irms value from process manual?

[ms_90]

Whatever scenarios i have faced, i always used Idc value to determine the width of metal.
So, why Fab has provided the Irms value in the process manual?
when do we need it?



thanks in advance..

 

[johnjoe]

Quite easy, if you want to consider the metal trace width with ac-signals.

 

[dick_freebird]

You should have a different electrmogration rule for AC (zero
average), which Irms would index rather than Idc. Irms of course
works fine for DC signals against a DC rule, too. You just have
to make sure your basis matches your application.

Irms may also pertain to resistor power-density rules, similarly.

 

[ms_90]

Quite easy, if you want to consider the metal trace width with ac-signals.

i will give you an example
say for particular technology node you will find two current values ie Idc and Irms
so now Idc=0.2mA/um and Irms=0.8mA/um (i am specifying these values as an example)
Now for same metal width Irms is more than Idc. so, if there will be a AC signal n you are using Idc values to calculate metal width then you will get more wider metal and it will be good to avoid EM.

So in my experience i always used Idc for calculating metal width even though AC current is flowing through the net.

Now can someone please tell me when that Irms current is needed?

 

[johnjoe]

Sometimes you're limited in space and don't have the opportunity to take the larger width. Or in RF-applications, where an increased metal width has worse properties compared to narrow trace width. E.g. consider an on-chip transformer, you don't want to use wider width than necessary because of trace parasitics. And it becomes even more important with increasing frequency.In the case of dc-current or low frequency application, I agree with you, let's say for supply routing and if you have the space!

 

[ms_90]

thanks a lot johnjoe..