Re: [SI-LIST] : Effect of low Zo for unterminated lines

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From: Raymond.Leung@qsa.idt.com
Date: Thu Oct 21 1999 - 16:36:56 PDT


Vinu,

Electromigration is sensitive to average output current. Therefore if you drive
a line of 60 ohm in 133MHz, say, the average stressing current would be
43/2=21.5mA on the pull-up device (base on your calculation, abeit the actual
output
resistance of the driver would be smaller); and of course the same
stress for the pull-down device. Some metal wires in the chip running with
alternate current is not an issue.

What I have suggested is to use a 50PF lumpped load as a reference to calculate
the average current limit, and that's the usual way a chip designer handle the
electromigration issue when the chip datasheet specifies a 50PF loading. (The
original posting is taking LVC245 as an example)

Raymond
---------------------- Forwarded by Raymond Leung/QSA/AU on 22/10/99 09:25
---------------------------

Vinu Arumugham <vinu@cisco.com> on 22/10/99 02:43:00

To: Raymond Leung/QSA/AU@QSA
cc:

Subject: Re: [SI-LIST] : Effect of low Zo for unterminated lines

Raymond,

I did a quick calculation and have a question. If we assume a driver is spec'd
at Iol=24mA at Vol=0.4V, the driver impedance is 0.4/24mA = ~17 ohm. The dynamic
current when driving a 33 ohm line is 3.3/(33+17) = 66mA. The dynamic
current for a 60 ohm line is 3.3/(60+17) = 43 mA. If the 33 ohm line is 9.2 inch
long, at a frequency of ~250MHz, the driver will have to constantly supply 66
mA. For the 16.7 in. 60 ohm line, running at ~133MHz, the driver will
have to constantly supply 43 mA. Which is the electromigration limit, 43 mA or
66 mA?

Thanks,
Vinu

Raymond.Leung@qsa.idt.com wrote:

> I think the main issue as far as the driver is concerned is its reliability at
> high current/temperature level. I would stick to the 50PF loading spec.
> then there should not be any problem of overheat at high speed or
> electromigration at high current level. Thus the estimated max. line length
> should be 9.2 inches for 33 ohm and 16.7 inches for 60 ohm case.
> Please be reminded the loading of the inputs of the other chips has not
> been considered.
>
> Raymond
>
> Chris Bobek <cbobek@cadence.com> on 20/10/99 03:08:52
>
> Please respond to si-list@silab.eng.sun.com
>
> To: si-list@silab.eng.sun.com
> cc: (bcc: Raymond Leung/QSA/AU)
>
> Subject: Re: [SI-LIST] : Effect of low Zo for unterminated lines
>
> This is a little off subject, but I made a slight mistake, the Zo is 33ohms.
> Here is a stripline portion of my stackup:
>
> 1 oz copper plane
> 4mil FR-4 (er = 4.5)
> 1 oz signal
> 4.4mil Prepreg
> 1 oz copper plane
>
> When I plug those values into Specctraquest, I get 33 ohms. When I use
Howard's
> equations from his book, I get roughly 30 ohms or so (I kinda forget). So, I
> believe this is correct.
>
> Chris
>
> Dave Hoover wrote:
>
> > All the PCB's I've built using 29 Ohms (GTL and Ram something)
> > was .018" lines .003" above a plane (microstrip).
> >
> > ??????? .006" was using a high Er' dielectric??????
> >
> > Dave
> > -----Original Message-----
> > From: Chris Cheng [mailto:hycheng@3pardata.com]
> > Sent: Tuesday, October 19, 1999 9:12 AM
> > To: si-list@silab.eng.sun.com
> > Subject: RE: [SI-LIST] : Effect of low Zo for unterminated lines
> >
> > are you sure you have a 29ohm line. its very hard to believe
> > you can achieve 29ohm with 6 mil traces on PCB.
> > chris
> >
> > > -----Original Message-----
> > > From: owner-si-list@silab.eng.sun.com
> > > [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Chris Bobek
> > > Sent: Monday, October 18, 1999 5:24 PM
> > > To: Si-list
> > > Subject: [SI-LIST] : Effect of low Zo for unterminated lines
> > >
> > >
> > > Hi,
> > >
> > > Because of several constraints on our stackup and line width, a 6mil
> > > stripline trace yields a Zo = 29ohms. When I simulate a 29ohm,
> > > unterminated line on our board (@ 10MHz), I see a pretty nice waveform,
> > > albeit with slower rise/fall times (which is ok for the nets in
> > > question). When I increase the Zo to 60 ohms, for example, I see a lot
> > > of ringing on the same net.
> > >
> > > >From a theoretical point of view, my guess is that this happens because
> > > as you widen the trace, there is more capacitance, which slows down the
> > > rise/fall times. Is this a correct assumption? Are there other factors
> > > as well that are going on?
> > >
> > > What I am most concerned about is the effect of a low Zo net on its
> > > driver. For example, if I have an LVCH245 driver (3.3V) driving a 29ohm
> > > line, will the low Zo "hurt" the driver? Or, is the only negative
> > > effect the slower rise time? This question is a general question for
> > > most ICs.
> > >
> > > Thanks for your help,
> > >
> > > Chris
> > >
> > >
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