Electromigration-safe sizing

Sign the power grid off for electromigration (EM): over its lifetime, metal carrying too much current per unit width erodes and fails. The grid is a hierarchy - a high-current trunk feeds medium branches, which feed low-current leaves - and the current density in each is its current divided by its width, which must stay under the EM limit at every level.

Every level starts too narrow, so all three are over the limit (red).

• Current density under the EM limit at each level - density is current / width, and the trunk carries four times a leaf's current, so it needs a much wider stripe. Size each level to its own current.
• Routing closes - every stripe blocks routing tracks. One uniform width cannot win: wide enough for the trunk wastes so much metal on the many leaves that routing fails, thin enough to route leaves the trunk over the EM limit.

Match each level's width to its current with the least metal, then Submit design.