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PIM Testing
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PIM Testing at
Rogers Corporation
Advanced Circuit Materials Division
Patricia LaFrance, Chris Caisse, & Allen F. Horn, III
Lurie R&D Center, Rogers, CT
John Coonrod, Ling Smith, & Art Aguayo
Rogers Advanced Circuit Materials Division, Chandler, AZ
Evan Yuan & Sharon Young
Rogers Suzhou & Shanghai, China
1
Discussion Outline
What is PIM?
PIM standard at IEC
Reverse vs. Forward PIM
Description of Rogers laminate PIM tests
PIM Test Repeatability & Statistics
PIM Processing Observations and Experiment
Conclusions
2
Intermodulation (IM)
In a ideal linear system (where V = IR perfectly) no new frequencies can be generated. If f1 and f2 are input,
then only f1 and f2 will be output.
If a system is non-linear (like a power amplifier nearing its saturation point) then IM products can be generated
at frequencies such as (f1 - f2), (2f1 - f2), (3f1 - f2), (3f1 -
2f2), (4f1 - f2), (4f1 - 2f2), (4f1 - 3f2)..
Only some odd IM products will occur in-band:
IM3: (2f1 - f2)
IM5: (3f1 - 2f2)
IM7: (4f1 - 3f2)
3
Intermodulation (IM)
4
Passive Intermodulation (PIM)
PIM is IM generated by a passive system or device, such as a filter, power divider or antenna.
Power levels are extremely low. Even a very bad PIM level of -133 dBc (referenced to 20 watts) is 10-12 W.
Typical sources of non-linearity:
Nearby ferrous metals
Metal particles on a planar circuit
Metal particles in connectors
Corroded connections
Stressed connectors
5
PIM is not a basic material property
Like insertion loss, antenna efficiency, gain, return loss, and many other important electronic performance
measures, PIM is a property of a circuit or system, not of
a material. It also depends on input power and
frequency.
However, there are material properties such as copper profile that when properly controlled, can result in
consistently lower PIM.
We build a certain Rogers internal standard circuit and test it at standard conditions to assess the relative PIM
performance of different laminate materials.
6
PIM is not a basic material property
3rd order PIM is always highest in power, so if you measure a value for IM3, you can be certain that IM5
and IM7 will be lower power.
IM3 depends on 3rd power of the current density.
Different circuit designs will exhibit very different PIM values, even with the same material and connectors.
Example:
Rogers 50 ohm TL PIM circuit on 0.75 mm thick RO4730JXR laminate exhibits a PIM of about -153 dBc.
A customer builds a filter on this laminate that exhibits PIM of -168 dBc
7
PIM Testing at Rogers Standards Development
PIM Test Methods are addressed by the IEC TC46 WG6 Document IEC 62037.
IEC 62037 does not address testing of copper clad laminate materials
In general, IEC recommends testing Reverse PIM when possible.
Rogers presently tests for PIM using a method recommended by Summitek Instruments now Kaelus.
8
Reverse vs. Forward PIM Measurements
If PIM is relatively high (e.g., -130 to 140 dBc) and emanates from multiple point sources, the reverse PIM
measurement can result in cancellation, while the
forward measurement does not.
For low PIM and/or distributed sources, this is not an issue.
Both Kaelus and IEC recommend measuring reverse PIM when possible due to difficulties with the low power
PIM signal and the high power base signals incident on
the duplexer in the forward measurement.
9
PIM Testing at Rogers Method 2001 to present
Rogers PIM Test Method:
300 mm long 50 ohm microstrip transmission lines on 1.5 mm laminate, stiffened by lamination to 1.5 mm FR4.
DIN 7/16 coax to microstrip connectors soldered to each end
Port 2 of the test sample is connected to a low-PIM load.
43 dBm (20W),two-tone, swept, reflected measurements with Summitek SI-1900b instrument at
1900 MHz
10
PIM Testing at Rogers Method
Rogers PIM Test Method - continued
Manipulate sample and connectors to get best stable PIM value over the swept frequency
range (1930-1990 MHz)
Report average PIM value at 1870 MHz of the up and down sweeps in dBc.
dBm = dBc + 43
PIM tester performance is checked before and after each test session with a PIM source
and a low PIM load
11
Summitek 1900b PIM Tester
12
DIN 7/16 Connector
13
Next generation on PIM Testing at Rogers
DIN 7/16 currently used for many connections in a BTS system, but most coax to microstrip connections are now
made through low PIM solder-plated braided jacket cable
and simple coax to microstrip connector.
Due to the very large growth in telecom industry since 2001 PIM testing equipment is much more widely used,
more user-friendly and less expensive
Rogers has added PIM testing capability at our Suzhou and Chandler locations, with Kaelus iQA-1921c PIM
analyzers.
14
Next generation on PIM Testing at Rogers
1.5 mm thick 50 ohm TL circuits with 0.141 flexible solder plated low PIM cables and light weight Coax-to-
microstrip soldered connectors.
We measure PIM versus time at 1870 MHz generated by fixed 1990 MHz and 1930 MHz tones and manipulate
connections and stress to get best PIM levels.
We have shown we get same values on all three (two new iQA-1921cs and our Summitek 1900B)
15
Kaelus iQA-1921c PIM Tester
16
New PIM test set up
17
Soldered connection from 0.141 low PIM cable
18
Rogers Reporting of PIM Testing
Material Sample #
Best PIM
IM3
dow
n
Best PIM
IM3
up
Average
up&dow
n
Best PIM
IM3
dow
n
Best PIM
IM3
up
Average
up&down
Grand
AVG
DH2
Lot X08924
w/ 1 oz.
LoPro
1 -154.9 -154.6 -154.8 -146.2 -146.7 -158.2
2 -166.4 -166.6 -166.5 -154.9 -154.2 -154.6
3 -155.9 -155.9 -155.9 -160.3 -160.9 -160.6
4 -159.3 -159.6 -159.5 -155.6 -155.5 -155.6
DJ1
Lot X08922
w/ 1 oz.
LoPro
5 -156.4 -156.2 -156.3 -153.5 -153.6 -153.6 -158.4
6 -160.2 -160.4 -160.3 -163.1 -162.1 -162.6
7 -160.1 -160.3 -160.2 -155.5 -155.9 -155.7
8 -159.8 -159.4 -159.6 -159.3 -158.8 -159.1
19
PIM Testing at Rogers General results
Weve purposely obtained a wide range of data: (-105 dBc to 175 dBc), with high profile foils, added ferrous metals, and conductive particles
to help understand the test.
As long as the cladding is pure copper and nothing is added to increase PIM, the major
controlling variable related to a laminate is foil
roughness.
20
Laminate PIM versus Copper foil Profile
(Rq - RMS profile - microns)
-170
-160
-150
-140
-130
-120
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Rq - RMS Profile - (microns)
PIM
(d
Bc
)
21
Repeatability of PIM Test
Major concern is test repeatability
Repeatability of a PIM test is about 3 dBc, without disconnecting sample.
This is consistent with what Kaelus specifies for repeatability of their PIM sources.
22
Repeatability of PIM Test
Based on four sample measurements, the repeatability of a laminate PIM test is about 6 dBc, including dis- and re-" connection when measuring in the -155 to -170 dBc range.
Repeated samples/measurements are necessary to understand material performance.
23
24
PIM Statistics
The repeatability of measurements affected by normal random variation improves by a factor of
1/n where n is the number of measurements.
We have shown that the 95% confidence limit of a single PIM measurement is +/- 12 dBc.
By measuring groups of 4 samples we improve the 95% confidence limit to +/- 6 dBc.
More samples would reduce confidence limit e.g., sets of 8 samples would be +/- 3 dBc
25
PIM Statistics
With a 95% confidence limit of +/6 dBc for the groups of four samples, significant
measurement-induced variation is more common than you might think.
As the following Monte Carlo simulation demonstrates, if a group of four circuits is
measured on consecutive days and the results
compared, the most likely outcome is that one of
the days measurements has higher PIM by 3 dBc or more
26
PIM Statistics
The Monte Carlo method is so-named since it is simulates a real gambling session.
Randomly select a group of 1000 values that fit distribution of mean of -157 and sigma of 3 (this is the
first measurement).
Repeat random number generation for second experiment.
If you average the differences between measurement 1 and measurement 2, it will be close to 0.
However, if you look at the absolute value of the differences, result is quite different.
27
PIM Statistics
Trial Number "Run 1" "Run 2" Difference
Absolute
Difference
990 -159.2 -154.3 4.92 4.9
991 -154.9 -149.0 5.86 5.9
992 -159.4 -156.3 3.08 3.1
993 -151.8 -156.1 -4.26 4.3
994 -157.5 -156.6 0.90 0.9
995 -157.7 -154.6 3.05 3.1
996 -155.3 -157.6 -2.33 2.3
997 -155.5 -151.4 4.12 4.1
998 -156.4 -152.6 3.80 3.8
999 -159.1 -153.9 5.21 5.2
1000 -160.8 -153.4 7.43 7.4
AVG -156.97 -157.05 -0.08 3.35
Std. D. 2.91 3.05
Last 10 Lines & Averages for PIM Statistic Simulation
28
PIM Statistics
29
PIM Statistics Conclusion:
If you really want to understand moderate PIM differences, many measurements
must be made.
30
PIM Testing for Production Control
With purchase of new PIM testing equipment, we are starting to regularly measure PIM as a
production reference test.
31
PIM & PROCESSING
OBSERVATIONS
32
PIM & Processing Observations
Due to the large measurement 95% confidence limit of +/- 6 dBc, it is very time consuming to
collect significantly significant data comparing
effect of various factors on PIM.
Nonetheless, over the years, some things seem to be significant.
33
Material & Connector Stress
Stress in the material, connector, and solder joint can have a large effect on PIM.
We manipulate the sample and joints during testing to obtain the lowest PIM values
34
PIM, current density, power level and thickness
The PIM values depend highly on the current density on the conductors, with higher density
leading to higher PIM.
Standard input power level for many test protocols (including Rogers) is 20 W (43 dBm)
on a 50 ohm transmission line on 1.5 mm
laminates.
Circuits that have a lower current density will exhibit lower PIM than the value on this test
circuit.
35
PIM, current density, power level and thickness
Example:
A 50 ohm transmission line on 0.75mm RO4730JXR laminate exhibits PIM of about -150 to -155 dBc with two 20 watt tones at 1870
MHz.
A customer routinely builds filters on 0.75 mm RO4730JXR laminate that exhibit < -168 dBc with
two 20 watt tones at 1870 MHz.
36
PIM, current density, power level and thickness
Example:
A 300mm long 50 ohm transmission line on 1.5 mm RO4730JXR laminate can typically exhibit a PIM value of -163 dBc.
A 300mm long 50 ohm transmission line on 0.75 mm RO4730JXR laminate can typically exhibit a
PIM value of -150 to -155 dBc.
37
PIM, current density, power level and thickness
RO4350B laminate with standard foil is a non-antenna grade that uses a high profile copper foil
and exhibits high PIM in our standard test.
However, RO4350B laminates can exhibit low PIM at lower power levels.
38
Material 20 W 10W 5W 2W 1W
0.060" RO4350B Std Foil -145/-102 - - - -
0.030" RO4350B Std Foil -133/-90 -137/-107 -144/-107 -147/-114 -151/-121
0.020" RO4350B Std Foil -135/-92 -138/-108 -137/-100 -145/-112 -152/-122
PIM Levels with two tone inputs at power level listed below (dBc/dBm)
PIM & Processing observations
As an experiment we also added small amounts of Fe powder (~1%) to the 0.08 mm thick dielectric layer in the
center of a 1.5 mm laminate.
Dk & Df were not affected.
However, PIM was -120 dBc
Without added Fe, PIM would have been
PIM & Processing observations
Metallic powder on dielectric surface causes very bad PIM.
Years ago, a colleague was going to show me how removing Cu oxide would improve PIM.
As soon as he touched the surface with sandpaper and generated Cu particles, there
was a large increase in PIM.
40
PIM & Processing observations
We routinely use sanding to generate a range of bad PIM materials for our gauge analyses.
Start with a better than -160 dBc material.
Sand it a little to increase PIM to -143 dBc
Sand it more to increase PIM to -125 dBc.
Wipe the surface with a solvent-soaked rag to remove the copper particles and one can see
the PIM improve significantly
41
PIM & Processing observations
Similarly if a sample is under-etched and nodules of copper are left in the dielectric, you
would expect to see bad PIM.
Customers have told us that this happens.
However, Rogers has not been able to recreate the effect in a controlled manner.
42
PIM & Etching Residue
Etching Residue on Surface causes poor PIM We had installed a new etcher at R&D. Rinse section was not
functioning properly. Coupons exhibited poor PIM.
After the initial testing, the PIM coupons were cleaned by wiping with 10% sulfuric acid and rinsed with DI water.
Significant improvement in PIM was seen.
43
PIM improvement with solder mask and immersion Sn
Samples are 300 mm 50 ohm TLs on 0.75 mm laminates:
RO4534 Lot A bare Cu PIM = -144 dBc
RO4534 Lot A SM & immersion Sn PIM = -158 dBc
RO4534 Lot B bare Cu PIM = -150 dBc
RO4534 Lot B SM & immersion Sn PIM = -156 dBc
RO4730JXR bare Cu PIM = -154 dBc
RO4730JXR SM & immersion Sn PIM = -159 dBc
44
PIM & Processing observations
Customers report that ENIG results in poor PIM.
45
PIM Processing Experiment
We set up an systematic experiment to determine the effects of various processing changes on PIM.
These included:
Slight under etch
Cupric chloride & ammoniacal etching chemistry
Combinations of solder mask, bare Cu, and tin plating
Plating finish
HASL, immersion silver and tin
Surface cleaning
Microetch vs. pumice scrub
46
PIM Processing Experiment
We were hoping that many of the factors that had lead to improvements in PIM in the past
would be consistently demonstrated.
Unfortunately, the control circuits of 50 ohm TLs on 1.5 mm RO4730JXR laminate exhibit PIM of better than -160 dBc, so nothing could be
consistently better.
None of the factors we investigated showed a significant difference.
47
Conclusions
PIM is not a material property. It is a circuit and system property that depends on many variables.
Due to the high variability of PIM when testing near the noise floor, multiple samples are required to differentiate
between relatively low PIM materials.
Laminate copper profile is a major variable affecting the PIM of microstrip transmission lines.
Rogers is committed to monitoring PIM performance of our antenna grade materials.
48
Rogers Low PIM PTFE Materials
Since copper roughness is normally the major variable, Rogers PTFE-based materials with low
profile rolled foil have always exhibited low PIM
values.
RO3003, RO3203 and RO3035 laminates are silica filled for low z-axis CTE, very low
temperature coefficient of dielectric constant,
and relatively high thermal conductivity.
49
Antenna Materials: PTFE based
Dk
10 GHz
Df
10 GHz
Thermal Conductivity
(W/m/K)
PIM
(dBc)
RT/duroid5880
Laminates
2.2 .04 0.0009 0.26
Rogers Low PIM RO4000 Thermoset Materials
The use of LoPro reverse-treated foil has allowed the development of a family of low PIM
RO4000 laminates.
RO4500 and RO4700 antenna grade laminates exhibit the rigidity, low CTE, low
TCDK, and low cost circuit processing that
typifies all RO4000 laminates.
51
RO4000 Antenna Materials
Dk
10 GHz
Df
(2.5/10)
GHz
Thermal Conductivity
(W/m/K)
PIM
(dBc)
typical
RO4533
Laminates
3.3
0.0020
0.0025
> 0.60 0.60 0.60
UL 94V0
0.40 0.40
Arlon low PIM AD WG-PTFE-ceramic laminates
In January, 2015, Rogers finalized the acquisition of Arlon, LLC.
Arlons woven glass-PTFE-ceramic filled laminates are widely used in telecom antenna
applications requiring low PIM.
The AD-C series is priced for high volume applications
53
Arlon low PIM AD WG-PTFE-ceramic laminates
Dk
10 GHz
Df
10 GHz
Thermal Conductivity
(W/m/K)
PIM
(dBc)
AD250C
Laminates
2.50 .05 0.0014 0.30
The information contained in this presentation is intended to assist you in designing with Rogers products. It is not intended to and does not create any warranties, express or implied, including any warranty of merchantability or fitness for a particular purpose or that the results shown on the design guide will be achieved by a user for a particular
purpose. The user should determine the suitability of Rogers products for each application.
RO4500, LoPro, RO3003, RO3203, RO3035, RO4534, RO4533, RO4535, RO4350B, RO4730JXR, RO4725JXR, RO4000, RT/duroid, The world runs better with Rogers.,
and the Rogers logo are licensed trademarks of Rogers Corporation.
Copyright 2015 Rogers Corporation. All Rights Reserved.
55
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