Rated Power and VSWR Improvement of Termination Resistor with ...

March 28-31, 2011 CARTS USA 2011 Jacksonville, FL
Rated Power and VSWR Improvement of Termination
Resistor with Innovative Design and Integrated Matching
Network
Abstract
A. Rahman, F. Olinger, and M. Howieson
Thin Film Technology Corporation
1980 Commerce Drive,
North Mankato, MN, 56003, USA
Phone: 507-625-8445, Fax: 507-625-9175, email: arahman@thin-film.com
Matching RF and Microwave signals to components within a circuit has become more
challenging for circuit designer as operating frequency and power handling requirement
increase. Termination resistors with excellent power handling capability with superior
voltage standing wave ratio (VSWR) characteristics are needed for microwave circuit
components in various wireless and telecommunication application. In this work, we
extensively studied to improve the power handling capability and VSWR of termination
resistor while maintaining the smaller size. We optimized the energy transmission within the
component and significantly improved the efficiency of energy transfer between the
transmission lines and the terminations. We discussed techniques of improving power
handling capability and reduction of thermal fatigue to the component’s structure. By using
innovative techniques of heat transfer we appreciably increased power handling capability
while keeping the product size as minimum. We improved the package technology and
reduced the chance of solder cracking due to thermal fatigue. We constructed matching
network using integrated microstrip line at the beginning of the resistor material. We
optimized impedance matching by varying length, width and pattern of transmission line as
well as using defective ground structure (DGS) technique. We minimized the detrimental
influence of component footprint and resistor material characteristics effect on signal
reflection.
Considering the necessary microwave parameters and required thermal performance, we
designed and manufactured Pb free high power termination resistor. Several sizes of resistor
are manufactured with rated power of 20 Watt, and 100 Watt which provides excellent
VSWR characteristics for frequency up to 4 GHz. Resistors are manufactured for 50 ohm
impedance; however, methods mentioned in the study are applicable to any impedance.
Introduction:

The wireless telecommunication, broadcast and radar industries rely on high power
transmission of the radio waves to reach subscribers or measure the environment. Ever
growing multimedia data communication constantly increases data traffic to the systems. As
the wireless revolution extends, component requirement in higher frequency, elevated
operating power, smaller in size, and improved performance demands on resistive devices
grow even more stringent. Several research works have been done on increasing power
handling capability in smaller size resistors. Other work was done on 50 Ω microwave
termination resistors for up to certain frequency. Simulant high power resistor development
technique was discussed in [1]. Based on proven ceramic resistor technology, a new type of
power resistor has been developed to offer significant advantages over conventional wire
wound and thick/thin film devices [2]. Power-dividing-based microwave power thin film
resistor (MPTFR) that exhibits high operating frequency and high power load was discussed
in [3]. Thick film and wirewound resistors were studied for high temperature storage and
thermal cycle [4]. Ceramic component of high energy resistors ware studied in [5].
Microwave transmission line with dielectric and ground plane and its importance was argued
in [6].
In this work, we studied how to improve the microwave termination resistor’s power
handling capability while keeping the size smaller. One of the eminent reasons of power
resistors failure is not being able to keep surface temperature below the level of component
failure mode. We analyzed how to remove the heat as the component surface temperature
increases. We explored techniques on efficiently heat dissipation through the resistors. Then
we applied this heat removal technique for microwave termination resistors. For higher
frequency compatibility, we used integrated impedance matching network on the resistor
surface to achieve excellent return loss characteristics. We used High Z Low Z transmission
line method to realize the impedance matching network. Based on the simulation results we
fabricated the chip power resistors. We provided return loss characteristics frequency of
response results and thermal profile of the fabricated termination power resistor. Thermal
profile demonstrates the true power handling capability of the resistor.
Improving rated power efficiency in chip resistor:
The biggest challenge to improve power efficiency in chip resistor is heat removal. As the
power gets increased, the product gets extremely hot and starts to degrade its performance.
Excessive generated heat can change the resistor value during hot temperature. If the power
resistor is extremely hot for a longer period of time, the heat can cause irreversible change in
the resistance. Resistor eventually starts entering in the failure mode and finally quits
performing. We worked on several different ways to efficiently reduce the heat. Since the
product size is exceptionally small to remove excessive generated heat, we improved several
different parameters of the product to remove heat. We worked on mainly three key areas to
reduce heat from the product surface. We improved the product to distribute the uniformly to
entire resistor and improve current density. Then we lowered the thermal resistive interface
between the chip and printed circuit board. Finally we improved the rate of heat transfer from
the resistor to the circuit board.

Improve heat dissipation:
The dominant form of heat removal from a chip power resistor is by conduction through the
circuit board from which heat is removed from the larger area of the board, ultimately by
convection. We significantly enlarged the resistive material area in the chip to uniformly
distribute the heat in all part of the resistor. As a result, we eliminated one single point of
heat transfer. Using distinctive thin film sputtering process, we maximized the resistive
portion of the chip, therefore resistive material is distributed uniformly throughout the
product. Consequently, heat is removed throughout the product instead on one single point.
Accordingly, no single point gets too hot to make the part damaged. Hence excessive power
is dissipated in a smaller size chip without increasing the peak surface temperature
Bigger resistive area of the product not only helps with uniformly distribute the heat; it also
improves the current density. Just as overheating a resistor can cause irreversible changes in
resistance, exceeding the resistive elements maximum current density can as well – even if
the heat is sufficiently conducted away. The maximum current density refers to the
maximum tolerated current per unit of cross-sectional area perpendicular to the current of the
conducting resistive element before subjected to mass transport effects (i.e. electromigration).
Electro-migration results in the drift of atoms in the conducting material. The
drift of atoms is due to the continuous momentum transfer from conducting electrons.
Electrons have very little mass, so it takes a lot of them at once to carry enough energy to
make an atom from the structure of the resistive element drift. This effectively reduces at
some point the cross-sectional area of the conducting resistive element. As this process of
electro-migration continues, the probability of additional electro-migration becomes greater
and greater. For instance, applying a current source in excess of the maximum current
density of a resistor would lead to eventual resistive runaway.
The material properties of the resistive element determine the current density limit and play a
major role in the resulting absolute maximum power and pulse power ratings for a given
design and level of reliability. We used Nickel Chromium as resistive material. Nickel
Chromium has very high current density limits, which make them very suitable for surface
mount power resistor applications. In addition, since Nickel Chromium is deposited directly
onto the thermally conductive chip carrier with which they have an intimate interface, there
is virtually zero thermal resistance between them.
Improve thermal resistivity:
After convection process, the most significant path to remove heat is the component
footprint. Heat is generated in the resistor surface area then it travels through the footprint to
the circuit board. By optimizing the heat transfer through the footprint can improve the
power dissipation process. A lower thermal resistance interface will enable more efficient
heat transfer from the component, enabling lower surface temperatures at greater power

levels by making good use of the relatively larger area of the circuit board to dissipate the
heat.
We explored the ways to improve heat transfer through the footprint. We established a low
thermal resistance interface from the resistive element to the circuit board. We maximized
the termination electrode’s size. Therefore heat has more ways to transfer from the surface to
the circuit board. We also used high purity, high thermal conductivity chip carrier to further
lower the thermal resistivity. We also investigated special design consideration, to ensure a
balanced distribution of the generated heat.
Improve heat transfer rate:
We studied to improve the heat transfer rate through the component footprint. We realized
that the faster the heat can transfer from the electrodes, the less time the heat has to keep the
electrodes hot. If the heat cannot move faster through the electrodes and more heat is
generating in the chip resistor, then the electrodes keep getting hotter. As the temperature
rise, at some point the surface temperature could exceed the melting point of the mounting
solder and potentially makes a crack in the solder joint. This phenomenon can potentially
cause irreversible changes in resistance – as well as potential damage to the printed circuit
board. We significantly improved the rate of heat transfer through the termination electrodes.
We showed the result of the heat transfer improvement in Figure 1.
Fig 1 a. Conventional power resistor after 1000 cycle
Fig 1 b. TFT power chip resistor after 1000 cycle

Fig 1 c. Conventional power resistor after 2000 cycle
Fig 1 d. TFT power chip resistor after 2000 cycle
Figure 1. Power chip resistors solder joint picture after applying high temperature expected
to generate from dissipated power.
Figure 1a and Figure1c represent conventional chip resistor where heat transfer rate is
ordinary. It illustrates that excessive heat made the temperature rise more than the melting
point of mounting solder and made a crack in the resistor joint. As a result the product went
into failure more. Figure 1b and Figure 1d represents our new improved chip resistor, where
heat transfer rate is optimized. This figure demonstrates that for exact same heat, generated
from applied power, there is no change in the solder joint. As a no result, no damage is done
in the chip resistor. For this test, we used thermal shock cycles. Test condition was -55˚
temperature applied for 30 min , followed by room temperature applied for 3min followed by
+155°C applied for 30min, followed by room temperature applied for 3min, repeating to
1000 cycles and 2000 cycles on FR4 0.6mm thickness PC board.
Thermal profile comparison:
After making the several changes in heat removal process we fabricated the chip power
resistor. Then we compared the temperature differences between conventional chip resistor
and our new chip power resistor. Figure 2 illustrates the thermal profile of the comparison
between two chip power resistors. In the figure, the thermal profiles shows the surface
temperature differences between a conventional chip power resistor and our improved chip
power resistor, at thermal equilibrium. The horizontal line across the thermal images titled
REGION (extending across electrodes) is the position for the profiles shown in Figure 2.
The chip power resistors compared are identical sizes (2525, 6.3 x 6.3 mm) and mounted to

identical PCB’s (1 x 3 inch FR-4) with the same amount of power applied in a static
environment (no air flow). Each was coated with a flat black paint to eliminate any
emissivity differences that might have biased the infrared images.
Figure 2a. Thermal profile for conventional
type chip power resistor
Figure 2b. Thermal profile for improved
chip power resistor
Fig 2a illustrates the presence of the single point in the chip surface. For the given power,
temperature rises from all sides of the part to the middle of the part. In the middle, peak
surface temperature rose as much as 180˚ C. Fig 2b illustrates the surface temperature for the
same given power. Here temperature is distributed uniformly to the whole chip. Surface
temperature never rises more than 137˚ C. In the figures, green color represents the printed
circuit board temperature. In Figure 2a, PCB temperature is around 80˚ C; while in figure 2b,
PCB temperature is around 100˚ C. The ratio of the differences between electrode and PCB
temperatures (from the thermal profiles shown in Figure 2 demonstrates how much more
efficiently the new power resistor removes heat from the resistive element to the PCB. The
far more uniform surface temperature of the new power resistor is a further testament of this.

The sample PCB used for this test are far from thermally optimized, as can be seen from how
cool the board temperature is around the parts compared to the surface temperature of the
electrodes, for the purpose of more clearly illustrating the differences. For thermally
optimized PCB, the improved power efficient chip resistor are designed to tolerate current
densities that equate to as much as 70W – that is if the PCB can sufficiently dissipate the heat
with a reasonable surface temperature.
Improving Voltage Standing Wave Ratio (VSWR) in chip resistor:
One of the most important parameter of the 50 Ω termination resistor is impedance match
with the RF/Mirowave signal. If the termination resistor posses an RF impedance, which is
not 50 Ω, significant energy reflects back into circuit by the unmatched termination. This
signal reflection elevates the voltage standing wave ratio (VSWR) in the circuit. This
phenomenon degrades the circuit efficiency. RF engineers require that the VSWR be as close
to unity as possible. VSWR is an indicator of how well the parts absorb energy and not
reflect it in the opposite direction. Unity VSWR indicates complete impedance match
between transmitted and reflected signal. As the operating frequency gets increased
termination resistor must have good VSWR for ever higher frequency. Current system design
environment requires termination resistor to have excellent VSWR specification up to 3.0
GHz.
We have used several innovative techniques to improve the impedance match for the
termination resistor. First we developed the power resistor for the termination resistor. We
used above discussed technique to make high rated 50 Ω power resistor. Then we designed
integrated matching network. We used High Z and Low Z transmission line method for
impedance match. Even though serpentine and L-shaped matching network can provide good
matching result, however, it takes more space in the resistor surface. By using High Z Low Z
transmission line method, we kept the chip size significantly smaller, which lets maximize
the resistor surface area. Higher resistor surface area helped us to uniformly distribute the
heat so the product can handle higher power dissipation.

Figure 3a. Design Layout of the termination power
Figure 3b. Return loss simulation result of the
resistor designed termination resistor
Since we extended the resistor surface area, it adds up significant capacitor value, which
degrades the VSWR performance. Due to the additional capacitance we changed the
integrated matching network. We utilize the influence of excessive capacitance to improve
the VSWR. We also use Degenerative Ground Structure (DGS) to further improve the
VSWR characteristics. In DGS, we intentionally change the ground path to improve the
VSWR characteristics.
Figure 3a and 3b shows the design and simulated result of 20 W RF power chip resistor
respectively. Figure 3a demonstrates that the resistor is maximally extended for optimized
power dissipation and usage of High Z low Z transmission line method for impedance
matching network. Figure 3b shows the return loss plot of the designed termination resistor.

Simulation plot illustrates that return loss value is -16.5 dB at 4.0 GHz. From VSWR and
Return Loss calculation, one can find that VSWR is 1.35:1 at 4.0 GHz frequency.
Component fabrication and measured results:
Based on the excellent theoretical and simulation work to design 50 Ω RF termination
resistor, we manufactured termination resistor with 20W and 100W power handling
capability. We used high purity Aluminum Nitrate (ALN) as substrate due to its higher
thermal conductivity. Its thermal conductivity is 170 – 180 W/meter ˚C. For resistive element
we used Nickel Chromium as thin film material. Epoxy-resin was used as overcoat. For chip
termination, we used pre-tinned (Sn100, Matte) material over Nickel (Ni) barrier.
Fig 5a. 20 Watt power resistor in RFevaluation board
Fig 5b. 100 Watt power resistor in RF evaluation board
For RF measurement purpose, the manufactured resistor was mounted in Rogers 4350B 10
mil thick printed circuit board. PCB along with the component was then placed in TFT’s
evaluation board. Figure 5a and 5b illustrate 20 Watt resistor and 100 Watt resistor mounted
in measurement module. As the resistor performance we achieved tolerance of less +/- 1%.
We realized TCR level of +/- 25 parts per million (PPM) for these chip power resistors.

Figure 6a. Return loss characteristics of 20 Watt power
resistor in RF evaluation board
Figure 6b. Return loss characteristics of 100 Watt power
resistor in RF evaluation board
Figure 6a and figure 6b illustrate Return Loss characteristics of frequency response of 20
Watt and 100 Watt power resistor respectively. Return loss plot of frequency response
illustrate that return loss value is around -20 dB for both the parts at 3.0 GHz. From VSWR
and Return Loss calculation, one can find that VSWR is 1.16:1 at 3.0 GHz frequency. At 4.0
GHz frequency, VSWR is 1.43:1 for 20 Watt power termination resistor and 1.95:1 for 100
Watt power termination resistor respectively.
Then we measured the true power handling capability of both resistors. We applied power
and observed the surface temperature of the resistors. First we applied the rated power for

oth resistors. Then we increased the power to measure how much power is needed for the
peak surface temperature to reach 200˚C.
Figure 7. . Thermal profile of the 20 Watt and 100Watt chip power resistor.
Figure 7 illustrate the thermal profile of the peak surface temperature of 20 Watt and 100
Watt new chip power resistor. From the figure, we can observe that for 20 Watt power
resistor, at 20 Watt applied power, maximum surface temperature is 101˚C. For 100 Watt
power resistor, at 100 Watt applied power, maximum surface temperature is 124˚C. This is
much lower temperature for the resistor to enter in failure mode. One can also observe that
for 20W resistor, it requires 37 Watt power for the peak surface temperature to be 200˚C.
Similarly for 100W resistor, it requires 150 Watt power for the peak surface temperature to
be 200˚C. This figure confirms that these chip power resistor can handle more than one times
and half of the rated power.
Conclusion:
In this work, we studied termination resistors and importance of impedance matching
network and heat removal process of termination power resistor design. We investigated
several innovative ways to efficiently remove heat from the resistor surface, resulting
minimizing peak surface temperature to a level that will not cause resistor to enter in failure
mode. We removed the heat by uniformly distributed the heat around the chip surface area
and improved the heat transfer rate through the footprint. We showed the enhancement of
heat removal technique with surface temperature thermal profile. Then we investigated
impedance matching network technique and implemented High Z Low Z transmission line
technique to successfully match impedance up to 4.0 GHz operating frequency. Simulation
result shows excellent return loss characteristics of the power resistors. Then we
manufactured 20 Watt and 100 Watt RF termination power resistor. We measured the

frequency response of the fabricated component. Return loss plots of the measured results
proved exceptional VSWR characteristics of both power resistors. Then we tested the parts
for rated power. We applied various power and tested for highest peak surface temperature
rise. We provided thermal profile plots of both 20 Watt and 100 Watt resistors. Thermal
profile demonstrates the outstanding power handling capability of both RF termination
power resistors.
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