David
The point at which one begins to consider the skin effect an important consideration is somewhat arbitrary of course dependent upon path length. Since the skin depth is inversely proportional to the square root of the frequency the behavior is sub linear which of course works to our advantage.
On the other hand, I consider an 8% error in the impedance calculation to be catastrophic since most OEMs are specifying a tolerance of no more than 10% and in many cases 8% or even 5%. If we are to build to the requirements of the customer our calculations must be very accurate, which is why I use a field solver, since process variations will consume most of the tolerance allowed by the customer.
Best regards
Lee
J. Lee Parker, Ph.D.
JLP Consultants LLC
804 779 3389
----- Original Message -----
From: David Greig<mailto:[log in to unmask]>
To: [log in to unmask]<mailto:[log in to unmask]>
Sent: Saturday, December 10, 2005 3:14 PM
Subject: Re: [TN] External Plating - Impedance Calculation
Lee
The frequency at which "skin depth" of Cu equals 0.5oz(17.5um) is only ~13MHz. For 1oz(35um) ~ 3.5MHz, so a 2oz track or foil in
free space will start to be less useful above 3.5MHz.
Becomes pretty significant for power circuits (remembering of course the 0.4% TCR of Cu!), and somewhat limiting if you need to
push GHz signals any distance(just as well dielectric losses other things become more of an issue in the GHz, or we would all be
hung up over "skin effect")
Getting back to the original Q, assuming a covered microstrip over return path plane then the difference in characteristic
impedance for 0.5oz and 2oz will be in the order of about 8% (rough guess). Most of the signal will be propagating in the space
between the trace and plane so the track thickness is not terribly important
Best Regards
David Greig
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Lee parker
Sent: 09 December 2005 22:09
To: [log in to unmask]<mailto:[log in to unmask]>
Subject: Re: [TN] External Plating - Impedance Calculation
John
I believe the Polar web site shows the typical geometry of the electro-magnetic flux lines extending from the signal trace to
the reference ground. They are distributed along the sides of the trace from the top of the copper to the dielectric. I always
use the entire copper thickness of the trace for the impedance calculation. You may have in mind the skin effect which is a
different issue altogether. If the frequency is in the range of gigahertz the skin effect becomes important when calculating the
ohmic contribution of the resistance ("R") since most of the current is transported within the outer portion of the trace; not
the core, the effective copper thickness is greatly reduced.
Best regards
Lee
J. Lee Parker, Ph.D.
JLP Consultants LLC
804 779 3389
----- Original Message -----
From: Roger Mouton<mailto:[log in to unmask]<mailto:[log in to unmask]>>
To: [log in to unmask]<mailto:[log in to unmask]<mailto:[log in to unmask]<mailto:[log in to unmask]>>
Sent: Friday, December 09, 2005 4:04 PM
Subject: Re: [TN] External Plating - Impedance Calculation
John,
Let me add to what Jeffrey has said by pointing out that the plated thickness uniformity must be reasonably close so that the
values you are assigning will have validity. I've seen plated thickness uniformity vary from 1.1 mils to over 3.0 mils on the
same board. I hope others will weigh in on this because I'm told plated thickness can significantly affect the impedance
values.
Roger Mouton
EIMC - Advanced Plating Technologies
949 481-5194
www.smartcatshield.com<http://www.smartcatshield.com/<http://www.smartcatshield.com<http://www.smartcatshield.com/>>
[log in to unmask]<mailto:[log in to unmask]<mailto:[log in to unmask]<mailto:[log in to unmask]>>
-------------- Original message from Babcock John <[log in to unmask]<mailto:[log in to unmask]<mailto:[log in to unmask]<mailto:[log in to unmask]>>>: --------------
> Hi All,
>
> When figuring impedances with the field solvers...for the external layer
> copper thickness does one enter the finished plated value or the
> unfinished?
>
> If you specify a top and bottom layer as 1.5 oz plated to 2 oz, which
> value would you put into the field solver for the impedance calc?
>
> I would figure you put in the finished plated value...but not positive.
>
> Thanks!
>
> John
> PCB Designer III
> Isothermal Systems Research
>
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