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The Influence of
RF Substrate Materials
on
Passive Intermodulation
(PIM)
Manfred Huschka, HKPCA 2012

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What is Passive IM ?
• Passive Intermodulation Distortion
 Created when two or more signals combine
to generate new frequency products
© Taconic 2012

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Why do we care ?
• Nonlinearity of passive components
causes power at transmit frequencies to
mix into the system's receive band
 Channels are blocked
 Phone calls can be dropped
Reduced Capacity = Reduced Revenue !
© Taconic 2012

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How is Passive IM Specified ?
• Absolute Power (dBm)
 The Absolute IM Power (in dBm) of the
Intermodulation Signal
• Relative Power (dBc)
 IM Signal Power relative to the Standard
Signal Carrier
 Example : -110dBm IM signal generated by
two +43dBm tones is a –153dBc IM level
© Taconic 2012

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How Is Passive IM Measured
• Multiple signals are injected to device
• High Power Amplifiers
• Highly Sensitive Receiver (LNA)
• Significant Filtering
 +43dBm Tx Power
 -140dBm Rx Noise Floor
 …………….means 183dB Isolation !
© Taconic 2012

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Putting it in Perspective
• Typically looking to measure
 -110dBm or better with two +43dBm
carriers (-153dBc)
• In Linear Terms…
 Distance to sun = 150,000,000 km
 -153dBc equates to approx 0.07mm
© Taconic 2012

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Typical PIM Causes
• Degradation Due to Environment
 Contaminated conductors & Interfaces
(Dirt, Dust, Moisture)
 Wind induced vibrations
 Temperature cycles
• Semiconductor Junctions (Assembly)
 Poor alignment of parts
 Inadequately torqued screws & fasteners
 Bad solder joints
© Taconic 2012

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Typical PIM Causes
• Printed Circuits Boards
 Processing issues
 Laminate issues
© Taconic 2012

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Typical PIM Causes
There are so many causes for PIM issues …
It is never related to one issue; it is usually a
stacking of smaller issues …
Let us look at the influence of LAMINATES
© Taconic 2012

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Laminates
• Not every laminate is suitable for
antenna pcbs!
 Tightly-controlled DK tolerance
 Dissipation factor < Df 0.0025 for most
grades @ 2GHz
 Mature manufacturing technology
 Lowest moisture absorption (<0.02%)
 Bromine-free & RoHS compliant
 Withstand >280°C assembly conditions;
lead-free compatibility
© Taconic 2012

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Low PIM Material Solutions
… for Base Station Antenna pcbs
(phase shifter, power splitter, power combiner, filter)
Grade DK Df @ 2GHz
TLC-32 3.20 ± 0.05 0.0018
RF-30 3.00 ± 0.10 0.0013
RF-301 2.97 ± 0.05 0.0012
TLA-6 2.62 ± 0.05 0.0012
TLX-8 2.55 ± 0.04 0.0009
TLP-5 2.20 ± 0.02 0.0006

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Copper Foil
© Taconic 2012
Copper foil is of huge influence on PIM
Copper foil
Copper Foil
PTFE Film
PTFE Film
Prepreg
Prepreg
Prepreg
Typical Laminate Construction：

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Standard ED Foil
Additional plating process applied to “grow”
microcrystalline copper structure to
increase effective surface area.
“Drum” or “shiny” side
“Matte” or “treatment” side
Very Low Profile ED Copper Foil
© Taconic 2012

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Copper treatment topography
(dendritic structure)
Rz DIN [micron]
C1 5 m-12 m
CL1 3-4 m
Rz is a measure of the roughness (or profile)
© Taconic 2012
Very Low Profile ED Copper Foil

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 VLP copper increases opportunity for
complete copper dendrite removal
 Etching process is isotropic
 “Cleaner/straighter” conductor side-walls
© Taconic 2012
Incomplete
etching
Very Low Profile ED Copper Foil

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copper debris left in at the
bottom of the etched track
SEM Photo of Microstrip (C1 copper)
© Taconic 2012

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Reverse Treatment Copper Foil

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Reverse Treatment Copper Foil

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CL1 copper shows
a lower surface
topography over the
plastic than C1
copper.
Note the “cleaner”
interface
Reverse Treatment Copper Foil

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Attack from air and chemicals leading to underetching
and discoloration
(85°C/85% rh/200 h)
Surface Protection

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Solder Mask leads to protection of side wall
from air and chemicals
Surface Protection

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Influence of protective coating
Source: Powerwave
f1 f2 IM frequency
960 MHz 927.5 MHz 895 MHz
960 MHz 932.5 MHz 905 MHz
960 MHz 937.5 MHz 915 MHz
Frequency for measurements
© Taconic 2012
PIM [dBm]
Surface
Protection
Max
(fwd)
Min
(fwd)
Max
(bwd)
Min
(bwd)
Immersion Tin 1.0 micron -114 -126.5 -113.5 -124.5
OSP 0.3 micron -92.2 -96.5 -99.5 -112.6
Immersion
Silver
0.2 - 0.8
micron
-81.5 -92.5 -94.5 -108.6
ENIG 0.05 - 0.1
micron Au/
3-5 micron
Ni
-50.5 -53.5 -58.5 -61.5
HASL 3-25 micron -103.9 -123 -112.3 -123.8

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Low PIM Laminate Solutions
Selecting the most suitable laminate
together with the most suitable copper foil
ensures that the most critical variables of
influence upon Passive Intermodulation are
met.
During pcb manufacturing process many more
critical variables have to get addressed …
© Taconic 2012
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