[SI-LIST] Re: Question on Rogers4350 loss
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: jose_moreira@xxxxxxxxxxx
- Date: Fri, 08 Apr 2005 20:48:07 -0400
Jose,
If I understand correctly, you are saying that you measure a lower loss
tangent than modeling and simulation predicts. There are several
possibilities:
1. The measurement is correct, but your simulations over predict the
specified loss of the material
2. The measurement is correct, and the material does not act as specified
3. The measurement is not correct.
Let's assume that the measurement is correct and the simulations are
correct, therefore the material is lower loss than the manufacturer
specifies.
Here's what I think is happening. RO4350B is a composite with glass
reinforced hydrocarbon/ceramic. It is composed of:
* An unnamed hydrocarbon (some sort of thermoset plastic)
* Amorphous Fused Silica (<75%)
* Continuous Filament Fiber Glass (<30%)
* Titanium Dioxide (0 to 7%)
The fiberglass used is standard 1080 E-glass. Here are the dissipation
factors for each component of the laminate:
* Amorphous Fused Silica tanD= .00033, Er=3.9
* 1080 E-glass = tanD= .0066, Er=6.6
* Titanium Dioxide tanD=.0017, Er=100
* unnamed hydrocarbon (who knows)
The specifications for RO4350B are:
* tanDelta = 0.0037
* Er = 3.48 +/- .05
In order to obtain a low Er and low tanDelta, the thermoset plastic must
have both a lower Er than all the other materials combined and a fairly
low tanDelta. My uneducated guess is that with different thicknesses of
material, multiply plys of 1080 glass are used along with different
percentages of Silica and the thermoset material. Then, to equalize the
macroscopic Er, a pinch (0 to 7%) of Titanium Dioxide is thrown in for
good measure. Each thickness of laminate has a different mixture of
materials. If the materials were totally uniform then the tanDelta and
Er would be uniform. However, because of the inclusion of 1080 E-glass,
there will be localized variations of these parameters along the X and Y
axis.
From the materials data we know that the average Er is 3.48 +/- 0.05
and the worst case specified tanDelta is 0.0037.
Now at 10 GHz, Rogers uses the IPC-TM-650-2.5.5.5 stripline resonator
method for measuring Er and tanDelta. In this method a sample of
material with an etched copper resonator is sandwiched between two
copper plates 1 inch wide. A trace is etched the appropriate resonant
length on a sample of material, and then it is sandwiched with 2 other
samples of dielectric between the plates. For this method to be
accurate and repeatable, a fairly wide trace is used, so the the
macroscopic behavior of the material is measured. (i.e. The trace is
really wide so that there is a good average of the composite material
properties.)
Once done, Er is specified, along with a worst case maximum tanDelta.
Now, this method is sensitive to the size of the sample being used. The
larger the sample, the better the average. Rolled vs electroplated
copper will also give different results due to surface roughness.
However, there are two clues in Rogers literature.
"It has been reported in some microstrip applications , a delta of
0.2 in dielectric constant has been observed for both RO4003 and
RO4350B based on actual circuit measurements and circuit modeling
comparisons."
"Dielectric constant typical value does not apply to 0.004
laminations. Dielectric constant specification value for 0.004
RO4350B material is 3.36 +/-.005.
Clearly, the fiberglass weave is having an effect on the Er of the
material. If this is the case, then it will also have an effect on the
dielectric loss of the material.
Interestingly enough, 1080 glass has a weave pattern of 60 x 47 threads
in the warp and weave directions. This corresponds to fiber pitch of
16.7 mils in one direction and 21.3 mils in the other. Unless you know
how your board was placed on the panel and how the panel was oriented
with respect to the original fiber weave roll direction, it's hard to
say which your trace will lay on if it the trace is oriented in either
the x or the y directions.
Jose, in the case of your trace width of 21 mils, it is well matched to
weave pitch of 21.3 mils. This means that if you happen to route in that
direction you will see a good average Er and loss tangent, similar to
the specification measurements made. If you route in the other
direction, there is a mismatch between your trace width and the 16.7 mil
fiberglass pitch. In this case, your Er will be dependent upon how your
trace lays w.r.t. the glass. You may see a higher percentage of fiber
or you may see a higher percentage of silica and plastic. A higher
percentage of fiber will raise the average Er and raise the average
tanDelta. And, if your routed your trace diagonally you will see yet a
different result.
Given the non-homogeneous nature of RO4350B, you cannot count on a fixed
Er or a fixed tanDelta. These will change with the size of the trace and
it's position on the laminate w.r.t. the underlying fiberglass. The
numbers that Rogers reports for their material are truly only valid for
their particular test method. We would need for them to report the
actual size of the sample that they used and it's orientation with
respect to the fiberglass reinforcement for a more full understanding of
it's true performance limits.
best regards,
Scott
Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax
http://www.teraspeed.com
Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC
jose_moreira@xxxxxxxxxxx wrote:
>Hi
>
>I would like to raise a question regarding to the dissipation factor =
>(tan d) of Rogers4350.
>
>My question is related to the problem posted by Ming Tsai some time ago =
>on the difference between measured and simulated loss of a microstrip in =
>Rogers.
>
>In my case I have microstrip with 21mil width and 10.2 inch length. When =
>I simulate the trace with the Rogers4350 specd value (~0.0037 at 10Ghz) =
>I get a insertion loss that is much higher that the measured values. On =
>my simulation I include a surface roughness factor of 1 micro-meter =
>which I would say is Ok. I also add the maximum guaranteed insertion =
>loss of the connectors. The connectors on my board are 2.4mm and I use =
>and optimized footprint obtained thorough 3D simulation by the connector =
>manufacturer.=20
>
>When looking for an answer to this problem I came across a presentation =
>from Chad Morgan from AMP where there is a slide with the measured =
>dissipation factor for several dielectrics and I see that for Rogers =
>they were measuring around 0.008!!.
>
>When I use this new value on my simulations I start to get some =
>reasonable correlation between simulation and reality.
>
>Has anyone looked into the real dissipation values for Rogers4350 and =
>other dielectrics (e.g. Rogers4003C) that you get on real boards??
>
>Best Regards
>Jose Moreira
>
>_____________________________________________________
>
>Jose Moreira
>Application Consultant
>STS Center of Expertise
>
>Agilent Technologies R&D and Marketing GmbH & Co.KG
>Herrenberger Strasse 130
>CCST-BUS
>D-71034 Boeblingen
>Germany
>
>Email: jose_moreira@xxxxxxxxxxx
>______________________________________________________
>=20
>=20
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