Power And Ground Line Widths

[Sparky5]

Hi

I am new to these forums, and I just have a questions on layout design.


When receiving a schematic from a design engineer with the currents required on the power and ground lines. (or any other line) How do you calculate the width of the line required using the design rule manual? What would I need to do?

If someone could give an answer and also an example calculation, it would be very much appreciated.

Sparky5

 

[erikl]

How do you calculate the width of the line required using the design rule manual?

See the metal sheet resistance values in your DRM. A good rule-of-dumb states 1mA/µm, see here or there.
Also this thread might be instructive.

 

[Sparky5]

See the metal sheet resistance values in your DRM. A good rule-of-dumb states 1mA/µm, see here or there.
Also this thread might be instructive.

Hi erikl

Thanks for your reply.

For example if I had received a schematic with various lines marked with their respective currents. Could you give me an example of how I would calculate the width using the sheet metal resistance please? Just a rough example to demonstrate the calculation.

Thanks

Sparky5

 

[Teddy]

if designer indicates what current flows through node correctly you should ask if it is DC or AC current.. But unless it is really high currents it's don't care.
To set wire width use just rule of thumb 1mA per 1um for all the metal except top metal. Top metal can be thick so can carry more. See the design rules for your process.
For current you don't have to consider sheet resistance.
In case designer is afraid about voltage drop across the wire then you have to use the sheet resistance calculation. To get reistance of wire : R = Rsheet(ohm/square) * Length / width
and from there you calculate the drop.
If you have to use vias in the wire path like switching from M2 to M3 etc check how much current can via or contact handle(again check the rules) Usualy per via it's about 0.4mA/via and for contact about 0.7mA/cont as a rule of thumb

 

[Sparky5]

if designer indicates what current flows through node correctly you should ask if it is DC or AC current.. But unless it is really high currents it's don't care.
To set wire width use just rule of thumb 1mA per 1um for all the metal except top metal. Top metal can be thick so can carry more. See the design rules for your process.
For current you don't have to consider sheet resistance.
In case designer is afraid about voltage drop across the wire then you have to use the sheet resistance calculation. To get reistance of wire : R = Rsheet(ohm/square) * Length / width
and from there you calculate the drop.
If you have to use vias in the wire path like switching from M2 to M3 etc check how much current can via or contact handle(again check the rules) Usualy per via it's about 0.4mA/via and for contact about 0.7mA/cont as a rule of thumb

Thanks for your reply Teddy

So the rule of thumb would be 1mA per 1um.

So for example, if I had a power line of 5 microns in width, that would be suitable for a current of up to 5mA

and for example if the line was 20um in length and I needed to calculate the total resistance of the line and the sheet resistance of the metal was 0.2Ohms per square. That would make the total resistance of the line, 0.8 ohms? 20/5 x 0.2 = 0.8 Ohms

Thanks

Sparky5

 

[Teddy]

Correct. - just ask what is the temperature spec for your part. 1mA/um is good for 110C but if you need to operate any higher it will go down by 40% at 120C and so on.

 

[Sparky5]

Correct. - just ask what is the temperature spec for your part. 1mA/um is good for 110C but if you need to operate any higher it will go down by 40% at 120C and so on.

Thankyou Teddy for your help, that has cleared things up.