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electromigration + current

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Full Member level 5
Dec 24, 2008
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Hello all,

Can anybody suggesst the difference between uni and bi directional current..??
which rating is generally considered while considering electromigration...



This can be explained with an inverter.
If you consider PMOS, it is always sourcing the current and NMOS is always sinking the current. Thus, these are uni directional current.
If you look at the output terminal Y, current flows towards the load when PMOS is ON and out of the load when NMOS is ON. Hence, current is flowing in both the directions in the same line (Y) and thus it is bi-directional.
In this case, uni-directional current is higher than bi-directional current (Due to through current). So, use unidirectional current in this case.

Raviprasad K

Reversing currents also reverse (to some, less than total,
extent) the material drift caused by the electron "wind".

Unidirectional steady state is the worst case. If you can
take advantage of duty cycle factoring or an AC EM rule
then of course you would do so, provided that you have
the reliability data to support said rules.

Even bidirectional current may need finer treatment, when
you get to higher frequencies the time-signature of a
cycle is so small that material no longer moves at all
(or barely) but at low frequencies the material does move
during a cycle and any asymmetry of construction can
result in a "ratcheting" albeit at much lower than DC rate.

Hi Deepak,
Uni-Directional currents means only one direction example Vdd and Vss dc-power rails in which the current flows in only one direction . Thus constant flow of current in one direction forces the electrons (thus atom) to that direction . Thus Electrons migrate (move forward with the force of current) occurs in that rail . we will prevent this problem by calculating the width of rail corresponding to current flow properly.
For Bi-directional currents example "Signal rails" in which the current may be any direction either input or output . Here , the signal may be ac thus current flow will be similarly sine wave .Electromigration problem will also occurs to these rails due to Joule Effect (Self-heating) .
Electromigration problem is worse for Supply rails .


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