You have to look at cross-section-in-play at every step
of the way.
Fingering helps. So does pulling current up through
multiple vias onto traces routed perpendicular to width.
If you have terrible contact step coverage then this is
better than pulling current out the end because the last
contact on the way out, is a severe choke-point to all
the others further in. If your contact is allowed to be as
wide as min metal then you could see heinous cross
sectional area reduction at the contact sidewall, times
two, every one of 'em. That's a fail waiting to happen.
Visualize it as plumbing with a variety of pipe diameters
and this may lead you to "points to ponder". Anywhere
the metal width, metal layer or current changes, you
have to either disposition (e.g. any current below the
mA for min W for layer, you get to check off) or analyze
until you have collected all cases, to select the worst
(declaring success if passing Jmax) or fixing the layout
where not.
Long lines are not different than short as long as you're
above the Blech length (where mass migration does not
occur, below). These are what EM rules will be derived
from.
If you were conservative you might go as far as to study
the reliability reports(if accessible) to see how proper the
experimental conduct and analysis were. Seen some
on that count. Like running so much forcing current that
a conductor would be high above ambient, but using oven
faceplate temp for analysis. I had to follow ridiculous Met2
rules for years until I could convince somebody to read my
damn memo showing all that. Then they just let me use
industry (MIL) standard Jmax instead, never did anyone
go and repeat the reliability experiment. Lazy lying bastages.
Depending on customer / end-use you may receive more or
less attention to your methods and results. Heads up.