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Normally DRM (design Rule MAnual) for the techno that u use, provide the maximum allowed Idc, Irms and Ipeak for each of the metal wiring levelsat junction temperatures of X°C.
well you need to do Electromigration analysis on the metal line for
calculating the required metal width that can withstand the current to pass through it.
You can calculate Irms, Ipeak and Idc and put it in the formulas mentioned in
the DRM. you will exactly know how much metal width you need to maintain.
Added after 37 minutes:
Actually as you are only concerned about Power lines...VDD and VSS, you can calculate the Idc current . You can calculate the DC current by putting current sources for all the devices and find the current consumed by each circuit and then multiplied by voltage , you get the power consumption in your block. You can also calculate the average current by the CVF formula (capacitance*Voltage*frequency). Then use DRM to find out the metal width required..
Please do IR drop analysis too if your design block is power consuming. That will make sure whether your design block is getting good power supply all throughout..
Procedural flow for Electromigration analysis
(1) Do some analysis on your current consumption of the circuit, either by transient simulation or static IR drop analysis. however, be careful the IR drop is static meaning all devices are ON, so the analysis is over-exaggerated or run dynamic IR drop anaylsis
(2) Check with the PDK, most of the time, some figure of merit is around 0.7-0.8mA / um width of metal
(3) Do simple calculation for the required width
(4) Beware of peak current which can be a problem if you only do static calculation. Make sure your peak current won't happen so often and put de-couple cap enough on chip and off-chip to reduce the dynamic EM effect. Otherwise, your metal wire will still be melt out
(5) Finally, always a rule of thumb, everything over-design by 20-30%, by doubling the metal usage, put as many as vias as possible, no sharp 90 degree angles high current carrying metal, do metal slotting.
(6) Re-arrange the whole chip so that the thermal distribution is even
Hi,
As our friends said, first we have to check how much current (dc,rms,avg) will carry for 1um metal width. According to current requirement we have to calculate metal width.
Metal width=required current/current carrying for 1um metal width.
Apart from this metal width also calculated w.r.t voltage drop(resistance require).
metal width = required resistance/resistance per square (metal).
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