www.isola-group.com/RF

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Selecting PCB
Materials for RF/MW
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assess the loss and power handling capabilities of families of laminates with variable copper weights.
Q: Automotive RADAR applications
FREQUENTLY ASKED QUESTIONS
Q: How do I know what type of copper I should specify on a laminate?
A: The choice of copper from an electrical performance standpoint largely
depends on the signal frequency and
data rate in digital applications. At high
frequencies the current is concentrated in a thin volume near the PCB copper surface and its flow must conform
to the rough contour, increasing resistance. This concentration of current to
the outer surface is frequency-dependent and known as “skin effect.” Copper foil can have peak-to-valley roughness on the order of 10 microns. At
1GHz the current is largely concentrated within a depth of 2 microns. For best
performance at high frequencies and
data rates, very low profile (VLP) copper
with peak to valley roughness of 2 micron is used. The effect of the different
copper profiles on the system or component performance can be modeled
using 3D electromagnetic field solvers
or through application of analytical expressions found in published literature.
Q: There are two major mechanisms
that contribute to signal loss in the
PCB. When are they important and
what dominates as frequency increases?
A: Dielectric loss and conduction loss
are the major contributors to signal loss.
In a PCB, dielectric loss depends on quality of resin, whereas conduction loss depends on the quality of the copper in
terms of conductivity and surface roughSponsored by
ness. Dielectric loss is directly proportional to signal frequency whereas conduction loss is proportional to the square
root of frequency. Over most frequency ranges in microwave applications,
both must be optimized for best performance. This is done by choosing the
copper profile and the lowest-loss resin.
As frequencies go higher, the dielectric
loss can in some cases dominate as it’s
directly proportional to frequency.
Q: When is it necessary to use a
high-performance laminate versus a
standard FR4 material?
A: This really depends on the specifics of the application including frequency of operation, length of transmission
lines, etc. Applications with low frequencies and data rates and relatively
short transmission lines may work fine
with standard FR4 type materials. Modern microwave and digital systems have
transmission loss requirements that cannot be met with standard FR4. Included
here are most RF/microwave antennas at
GHz frequency and digital applications
such as digital backplanes and highspeed digital switches operating at data
rates above 2Gbps.
Q: How do I determine if the lowest
loss material should be used versus
one of moderate loss?
A: This is determined through system
electrical analysis. Using the analysis results one can set loss budgets for each
of the components in the system and
choose the material that meets the requirement with some margin. This could
be the PCB product with moderate loss
versus the lowest loss, and generally
most expensive product.
Q: What RF/microwave laminates
are recommended for automotive
RADAR applications at 24GHz and
77GHz?
A: PCB materials for this application must
have low loss and stable performance
over the somewhat challenging automotive operating environment. For systems
operating at 24 GHz, it is desirable to use
thermoset PCBs having Df on the order
of 0.004. The 77GHz systems demand
greater performance, typically needing
materials with Df on the order of 0.002.
Materials with these characteristics,
which additionally offer process compatibility with lower-cost materials used in
power and ground layers, are available.
Q: Are there any tools available to
assess the electrical performance of
materials?
A: Tools used to assess performance
range from full 3D electromagnetic solvers to simple equations and rules of
thumb. The choice of tool really depends
on the fidelity of the answer needed. For
the purpose of sizing transmission line
traces, there are a number of tools available that will yield good approximations.
One example of an extremely capable
tool is the Impedance and Power Handling Calculator. This tool can be used to
are highly cost-constrained. What
can be done to balance cost and performance?
A: Cost and performance are balanced
in automotive RADAR PCBs through the
use of hybrid constructions where the
high-performance materials are used
where necessary and more traditional
laminates are used for the other layers.
To realize the maximum cost savings the
materials that comprise the PCB stackup must be highly compatible in terms
of processing, not requiring special process steps the other layers don’t require.
Applications that are becoming more
mainstream include blind-spot detection,
parking assist, and adaptive cruise control. As this occurs, the importance of
cost containment increases.
Low-loss Laminate Materials
for RF/Microwave/mm-wave
Designs
The newest members of Isola’s growing RF/Microwave laminate family now
includes TerraGreen® halogen-free, very low-loss, thermoset materials, which are
available in a variety of laminate and prepreg offerings. This material is inexpensive
to process – improving your company’s bottom line, as well as the environment.
The revolutionary Astra® MT ultra low-loss thermoset laminates are a replacement
for PTFE and are suitable for automotive radar, mm-wave and AESA applications.
These materials have been used for hybrid applications with Isola’s 185HR and
370HR products.
I-Tera® MT materials are suitable for both high-speed digital and RF/ Microwave
designs. A full compliment of cores and prepregs allowing flexibility in design is
available in core thicknesses from 0.002” to 0.018”.
I-Tera® MT RF is available in 0.010”, 0.020” and 0.030” in 3.38, 3.45 and 3.50 Dk.
for RF/microwave PCBs?
A. The demand for PCBs for LNBs, DAS,
base stations and general cellular infrastructure will remain solid. Overall, there
will be increasing demand for PCB materials with performance optimized for
higher frequencies through the mmwave band. For example, the “unlicensed” 57-64 GHz band will create new
opportunities and also PCB demands for
commercial wireless. Up until recently,
this band has been largely unused. However today, products are being developed
that enable two-way wireless communication at rates only achievable previously with fiber optic cable communications.
Additionally, the market for RADARbased Automotive Safety Systems is increasing and designers are moving towards operation in the 77-81 GHz band,
with ongoing development at higher frequencies. These and other areas will drive
the demand for RF/microwave PCB materials that offer high performance and
cost competitiveness. n
I-Tera® MT I-Tera®
RF
MT
200°C
200°C
200°C
360°C
360°C
360°C
2.80 - 3.45
3.45
3.45*
0.0028 0.0031
0.0031*
0.0036
IS680
Q. What market trends do you see
Tg
Td
Dk @ 10 GHz
Df @ 10 GHz
CTE Z-axis
2.90%
(50 to 260°C)
T-260 & T-288
>60
Halogen free
No
VLP-2
Available
(2 micron Rz copper)
Stable Dk and
-55°C to
Df over the
+125°C
temperature range
Optimized global
constructions for
Yes
Pb-free assembly
Compatible with
For use in
other Isola products double-sided
for hybrid designs
applications
Low PIM < -155
dBc
Yes
200°C
390°C
3.45*
Astra®
MT
200°C
360°C
3.00
0.0030*
0.0017
TerraGreen®
2.80%
2.80%
2.90%
2.90%
>60
No
>60
No
>60
Yes
>60
No
Available
Available
Standard
Standard
-55°C to
+125°C
-55°C to
+125°C
-55°C to
+125°C
-40°C to
+140°C
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
* Dk and Df are dependent in resin content NOTE: Dk Df is at one resin %. The data, while believed to be accurate and based on analytical
methods considered to be reliable, is for information purposes only. Any sales of these products will be governed by the terms and conditions of
the agreement under which they are sold.
www.isola-group.com/RF
Isola, I-Tera, Astra TerraGreen and the Isola logo are registered trademarks of ISOLA USA Corp. in the U.S.A. and other countries. All other trademarks mentioned herein
are property of their respective companies. Copyright © 2014 Isola Group. All rights reserved.