The Influence of RF Substrate Materials on Passive Intermodulation (PIM)

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

© Taconic 2012 www.taconic-add.com

What is Passive IM ?

• Passive Intermodulation Distortion
 Created when two or more signals combine
to generate new frequency products

2

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 !
3

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
4

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 !

5

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

6

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
7

Typical PIM Causes

• Printed Circuits Boards
 Processing issues
 Laminate issues

8

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

9

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
10
© Taconic 2012
www.taconic-add.com

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


Copper Foil
Copper foil is of huge influence on PIM

Typical Laminate Construction：
Copper foil
PTFE Film
Prepreg

Prepreg

Prepreg
PTFE Film
Copper Foil
© Taconic 2012

www.taconic-add.com

Very Low Profile ED Copper Foil

“Drum” or “shiny” side

Standard ED Foil “Matte” or “treatment” side

Additional plating process applied to “grow”
microcrystalline copper structure to
increase effective surface area.


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)



Incomplete
etching

 VLP copper increases opportunity for
complete copper dendrite removal
 Etching process is isotropic
 “Cleaner/straighter” conductor side-walls


SEM Photo of Microstrip (C1 copper)

copper debris left in at the
bottom of the etched track

Reverse Treatment Copper Foil


Reverse Treatment Copper Foil

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

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Surface Protection

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


Surface Protection

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


Influence of protective coating
PIM [dBm]
Surface Max Min Max Min
Protection (fwd) (fwd) (bwd) (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 0.2 - 0.8 -81.5 -92.5 -94.5 -108.6
Silver micron
ENIG 0.05 - 0.1 -50.5 -53.5 -58.5 -61.5
micron Au/
3-5 micron
Ni

HASL 3-25 micron -103.9 -123 -112.3 -123.8

Frequency for measurements
f1 f2 IM frequency
960 MHz 927.5 MHz 895 MHz
960 MHz 932.5 MHz 905 MHz Source: Powerwave
960 MHz 937.5 MHz 915 MHz


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 is here to support your needs!!


The Influence of RF Substrate Materials on Passive Intermodulation (PIM)

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