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The Influence of

RF Substrate Materials

on

Passive Intermodulation

(PIM)

Manfred Huschka, HKPCA 2012

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2

What is Passive IM ?

• Passive Intermodulation Distortion

Created when two or more signals combine

to generate new frequency products

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3

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 !

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4

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

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5

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 !

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6

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

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7

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

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8

Typical PIM Causes

• Printed Circuits Boards

Processing issues

Laminate issues

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9

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

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10

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

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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

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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

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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)

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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

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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)

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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

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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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