After finished the whole circuit design，before deliver to layout department , the circuit designer should make layout plan. The important thing among the layout plan is power plan. How to evaluate the power plan quality? Now
two rules should be followed:Power metal Width:
(1)after long wire route, Power metal's IR drop should less than 20 percentage of supply voltage.
(So you should calculate how much current will flow through the Power Metal, it is often the sum of banch current.)
(2) The average current of per um Power metal should less than 1 mA.
The other rules is also need to notice: (1) How much current can flow through one via ?
Please check the TLR file of given process, it will explain at the Electomigration Rules column. Please calculate the number of via is sufficient or not.
Why need to concern the number of via?
For example, If The Main Power supply metal is metal three at the vertical direction, and Metal two is supply power to every cell at the horizontal direction. If Metal two need to input 20mA current, but you only have one via2 which connect Metal3 and Metal2. If one via2 can only flow through 10mA current, Now how to ensure the Metal2 power supply source. The principle is similar as water pipe network. The Main pipe and banch pipe both are big enough to flow sufficient water , but if water faucet is very small. Now the water faucet will grow to be a bottleneck.
The another factor should be considered.You should have strongly understand layout. you can evaluate the width of circuit layout and overall arrangement. Only this, you can confirm whether the Power wire have adequate place to layout.