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problems using long metal wire

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rameshiloveu

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Hi,

what problems we may face if we are using long metal wire in analog/digital layout?

some that are in my mind are :

there will be voltage drop ---> current drop ---> reliability breaks ---> antenna violation occurs---> more delay

Please correct if am wrong and also please add additional points if any ?

Thanks and Regards,
 

depends also on frequency -> substrate loss or coupling or changed impedance transformation if taken into account.
 
there is the possibility that your signal will not reach the receiving end, because of the large parasitic capacitance had not been fully charged or discharged between signal transition.
 
Other than those that are listed here, Noise coupling can be a big issue for an analog block driving the long line till the destination. If the long line is passing near any digital switching blocks, then it would affect the signal on the line. For this the driving block should have good drive strength i.e. low impedance.
 
cant stress enough the risk of antenna effect. plasma etch will charge up that line making metal deposition unreliable. The result will be high ohmic / open circuit vias.

An example where I have experienced this is a long metal line coming out of a bandgap to multiple PFET gates (to mirror the bandgap current). Antenna affect did
not manifest itself until part got into the field which needless to say cost $$$$$$ to replace!

To avoid this your layout tool should have specific DFM rules to deal with it. Check them out and if you are not comfortable with them, probe further.
 

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