[SI-LIST] Re: Stripline more lossy than Microstrip?

  • From: "Eric Bogatin" <eric@xxxxxxxxxxxxxxx>
  • To: <jeff.loyer@xxxxxxxxx>, <olaney@xxxxxxxx>
  • Date: Tue, 28 Aug 2007 10:35:04 -0500

Jeff-

The article you mention is my Feb 2006 PCD&M column, "Microstrip or
Stripline". This is posted for free download on my web site,
www.BeTheSignal.com, under the Feature Articles and columns tab. Look on the
right side of the page for the columns.

Bottom line is microstrip, due to some of the field lines in air, does have
about 30% lower dielectric loss than stripline. However, if the lines are 5
mil wide, this 30% lower dielectric loss may contribute less than 10%
difference in total loss below 3 GHz. Add a little solder mask which can
have dissipation factors of 0.04 and the difference in attenuation is even
less.

This analysis was done with ADS, and is a very straightforward simulation. I
also have a section on this effect in my book in the chapter on lossy lines.

--eric

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Dr. Eric Bogatin, President
Bogatin Enterprises, LLC
Setting the Standard for Signal Integrity Training
26235 w 110th terr
Olathe, KS 66061
v: 913-393-1305
f: 913-393-0929
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www.BeTheSignal.com 
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-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Loyer, Jeff
Sent: Tuesday, August 28, 2007 10:10 AM
To: olaney@xxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Stripline more lossy than Microstrip?

Title changed and old thread deleted.
 

OOPS - you're right.  I had "microstrip" and "stripline" backwards in my
last e-mail.  I meant to question your assertion that stripline is more
lossy than microstrip.

 

Thanks for the links, Orin.  Looking these over and my old data
(including postings to this forum), here's what I think:

1)       The IEEE paper is for microstrip w/o soldermask.  My own data
agrees that microstrip w/o soldermask is slightly less lossy than
stripline.

2)       Regarding the 2nd link (Eric B.'s posting), I'll ask Eric to
share his source data for the assertion that microstrip provides less
attenuation than stripline (perhaps he's referring to the same paper).

3)       I'm including some verbiage from a previous posting of mine
below, including empirical data demonstrating that the difference in
loss was negligible.  In looking at the data again, there does seem to
be more loss on the stripline at 20 GHz, but not until then (the
difference between microstrip and stripline at frequencies less than
20GHz are in the noise range).   

 

Overall, I think the difference in loss between the 2 is not significant
enough to use as a criteria in layer selection.  Though I would stress
again that, in the practical real-world case, you might well be able to
use wider traces on microstrip and thus reduce your loss significantly.
That has to be balanced against more crosstalk and worse impedance
control, etc.

 

Enjoyed the chat - cheers!

 

NOTE: I apologize to those besides Orin, since I know this data below is
going to be converted into "plain text" and the table will look like
garbage.  With a little work, you can turn it back into reasonable data.

______________ Previous posting verbiage ___________________

 

Eric's comments got me looking at my data again (that lead me to believe
the losses of microstrip and stripline are equivalent).  That data was
based on simulations (XFX -> Hspice), which my past experience has shown

to correlate well to measurements.  Assuming the trace width and
thickness is kept constant (adjusting the dielectric thickness to
achieve the proper impedanct), it showed little difference between
microstrip and stripline for our current frequencies of interest (say,
up to ~5GHz).  I also validated this using ADS, but would love to see
anyone's conflicting data.  I should point out that this is with
soldermask, which I'm assuming most designs will be forced to use.
Similar experiments with pure microstrip (no soldermask) showed
microstrip to be slightly less lossy than stripline.

 

I also went searching for empirical data comparing microstrip vs.
stripline (with trace width/thickness kept constant), and could only
come up with differential insertion loss data (which my experience shows
to be about the same as single-ended) from some previous test boards.
Again, this data suggests there isn't a significant difference in
insertion loss between microstrip (ms) and stripline (sl).  

 

designator  ms/sl   length          SDD21 (dB/inch)     SDD21 (dB/inch)
SDD21 (dB/inch)    SDD21 (dB/inch)     SDD21 (dB/inch)

                                   (inches)        @ 2GHz
@ 5GHz                   @ 10GHz                     @ 15GHz
@ 20GHz

tx                     ms       3.70                -0.31
-0.64                          -1.09         

ty                     ms       6.72                -0.33
-0.68                          -0.93                           -1.41
-1.95

tz                     ms    14.73                -0.29
-0.65                          -1.24                          -1.78
-1.94

sx                    sl        4.70                 -0.32
-0.84                          -1.39                         -2.82
-2.89

sy                    sl      10.70                 -0.31
-0.70                         -1.39                          -2.40
-2.48

sz                    sl      20.00                 -0.32
-0.69                         -1.10                          -1.72
-2.18

 

When I look at this data, here's what I see:

(1) at 2GHz, there's no difference (as expected - conductor loss
dominates)

(2) at 5GHz, "sx" has significantly more loss than the rest - but hard
to believe it's solely because it's stripline.

(3) at 10GHz, 2 of the striplines exhibit more loss, but the longest
stripline ("sz", which is probably the most reliable, due to its long
length), shows the same as the microstrips (actually less than the
longest microstrip, "tz").

(4) roughly the same trend as (3) continues up to 20GHz (the data from
the shortest microstrip, "tx", was too noisy to make sense of).

 

The dimensions (in mils) were:

ms: h=3.5, w=5, s=5, t=1.4; microstrip had soldermask on it.

sl: h1=h2=6, w=5, s=5, t=1.4

Standard FR4 was used.

______________ End Previous posting verbiage ___________________

 

Disclaimer:

The content of this message is my personal opinion only and although I
am an employee of Intel, the statements I make here in no way represent
Intel's position on the issue, nor am I authorized to speak on behalf of
Intel on this matter.

Jeff Loyer 




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