# On-Chip Bondwire Magnetics with Ferrite-Epoxy Glob Coating for Power Systems on Chip

Authors: Jian Lu; Hongwei Jia; Andres Arias; Xun Gong; Z. John Shen
Journal: International Journal of Power Management Electronics
 (2008)

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## Abstract

A novel concept of on-chip bondwire inductors and transformers with ferrite epoxy glob coating is proposed to offer a cost effective approach realizing power systems on chip (SOC). We have investigated the concept both experimentally and with finite element modeling. AQ factor of 30–40 is experimentally demonstrated for the bondwire inductors which represents an improvement by a factor of 3–30 over the state-of-the-art MEMS micromachined inductors. Transformer parameters including self- and mutual inductance and coupling factors are extracted from both modeled and measured S-parameters. More importantly, the bondwire magnetic components can be easily integrated into SOC manufacturing processes with minimal changes and open enormous possibilities for realizing cost-effective, high-current, high-efficiency power SOCs.


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## Body

## 1. Introduction

System on Chip (SOC) is an emerging trend of
integrating all components of an electronic system including digital, analog,
mixed-signal, communication, and sensor functions, into a single integrated
circuit. The SOC concept embodies what many believe to be the ultimate level of
integration: an entire system on one chip.
Since its emergence in the 1990s, the SOC concept has gained wide
acceptance in a broad range of applications from supercomputing to embedded
systems. The proliferation of the SOC concept into power management systems has
also generated a great deal of interest in the electronics industry. Power
management is a key enabling technology behind the digital revolution. Each
year hundreds of millions of power converters are manufactured and embedded
into computers, telecommunication equipment, consumer products, automobiles,
and industrial control systems worldwide. Unfortunately, the power management auxiliary
subsystem sometimes takes as much as 50% of the board space of the main electronic
system. For this reason, power
management SOCs that monolithically integrate all active and passive components
using low-cost semiconductor manufacturing processes will
provide an extremely attractive solution with significant improvement in performance
and unprecedented reduction in board space, parts count, and time-to-market.
The power SOC concept is particularly well received in several fast growing
power management markets such as point of load (POL) dc/dc converters, LED
drivers, and battery-powered mobile applications.Unfortunately, the development of
power management SOC’s is seriously hindered by a few major technical barriers including
integration of integrating magnetic passive components. The main challenge is
to find a cost effective means of integrating inductors and transformers onto
the silicon chip, while achieving adequate performance in terms of inductance,
dc series resistance, maximum saturation current, coupling coefficient, andQ
factor. Current research work on integrated magnetics for power SOC's has
predominantly focused on utilizing microelectromechanical system (MEMS) micromachining
technology as a postprocessing step after the completion of the CMOS chip
containing all power switching devices and control circuitry [1–8].
Sophisticated MEMS technology allows sequential deposition and patterning of
numerous layers of conductor, insulator, permalloy, or ferrite thin films to
form desirable inductor and transformer structures. However, the high dc resistance (typically
0.5 to 5 Ω) and poor Q factor (typically 3 to 8) of the MEMS
inductors/transformers severely limit the current handling capability and
efficiency of the power SOC. More critically, the large increase of fabrication
complexity and cost associated with the MEMS postprocessing approach raises
questions on its feasibility to facilitate large-scale commercialization of the
power SOC concept into the extremely cost-sensitive power supply market.In this paper, we propose and
investigate a new cost-effective approach to form on-chip magnetic components
utilizing existing bondwires with additional ferrite epoxy glob coating formed
during the packaging process.


## 2. Basic Concept

Wire bonding is commonly used in nearly all power
management IC packages today. As shown in Figure1, thin aluminum or gold
bondwires of 25 to 250 μm in diameter are used to provide necessary electrical
interconnection between the silicon chip and the package leads. It is well
known that the bondwires of IC packages typically exhibit a parasitic
inductance of a few nH and a resistance of several to several tens of mΩ. On-chip bondwire inductors were first
explored in RF integrated circuits by Craninckx and Steyaert in 1995 [9]. Although
bondwires act as natural inductors, the inductance and coupling effect are
typically insufficient for power converter applications. A higher inductance in
the range of several tens of nH is required for power SOC (in conjunction with
increased switching frequencies to a range of 10–30 MHz). Several US patents have addressed this
issue by suggesting the use of multiple bondwires connected in series [10–13] but suffer
from the drawback of increased chip area.Figure 1
A packaged IC chip interconnected to metal leads
through bondwires.We
proposed to add ferrite epoxy composite glob coating to the bondwires to
increase inductance and improve coupling effect as shown in Figure2. Bondwire
inductors and transformers can be made between the chip and package leads or
between pads on the chip [14–16]. Unlike
traditional ferrite ceramics, ferrite epoxy materials are essentially ceramic
magnetic powders mixed with a polymer binder and can be dried or cured at
temperatures less than 200C°.
These materials combine appropriate magnetic properties with a high
resistivity and high manufacturability [17, 18]. The ferrite epoxy glob core can be formed to
cover the bondwires during the SOC packaging process by brushing, squeegeeing,
dipping, dripping, inking, or other viable dispensing techniques using high
precision robotic tools similar to the commonly used electronic assembly
equipment such as solder paste dispensers.Figure 2
Concept of on-chip bondwire magnetic component with
ferrite epoxy glob core.Compared to the prior art MEMS
inductor technology, the proposed approach has the following advantages.(i)
All SOC components (i.e., control
circuitry, power switches, gate drivers, feedback compensation networks, etc.)
except for the magnetics are fabricated with standard silicon processing
technology, eliminating the need for costly post-CMOS MEMS processing steps.(ii)
The on-chip bondwire inductors and
transformers can be integrated into the power SOC packaging process with
minimal changes. This opens enormous possibilities for realizing
cost-effective, high current, high efficiency power SOC's.(iii)
Aluminum or gold bondwires, due to
their relatively large diameters, are much more conductive than the thin metal
films in MEMS inductors. Therefore, a much lower dc resistance and higherQ
factor can be expected for the bondwire inductors. Quality factor Q is the
ratio of reactive impedance to equivalent series resistance (or ESR), an
important parameter of inductor/transformer performance. High Q leads to low-power dissipation and
high efficiency of the power converter.(iv)
The electromagnetic field of a
bondwire inductor or transformer is mainly distributed outside the silicon
substrate. The Eddy current loss in the silicon substrate at high frequency, a
major concern in MEMS magnetics, can therefore be minimized.


## 3. Finite Element Modeling

The proposed bondwire
inductor and transformer structures were modeled and analyzed using the
electromagnetic simulation tool HFSS from Ansoft, Pa, USA [19]. HFSS is a high-performance full-wave electromagnetic (EM) finite element
simulator for arbitrary 3D passive component modeling. The objective here is to
investigate the effect of ferrite epoxy glob coating on the inductance value
and coupling effect of bondwires using EM simulation. Furthermore, EM modeling helps optimizing bondwire
inductor and transformer design in terms of physical dimensions of bondwire, ferrite
coating shape, coating position, coating thickness, and the ferrite material type
used.


### 3.1. Inductors

HFSS simulation generates a set ofS
parameters from the inductor structure which are then converted to a set of Y
parameters. The effective quality factor Q and inductance L can be then extracted
from the Y parameters by using the following equations: (1)Leff=−1ωIm(y11),Q=−Im(y11)Re(y11).Figure3 shows the modeled inductance value of bondwire inductors with a ferrite epoxy
coating thickness of 0 (bare wire), 10, 20, 30, and 40 mils over a frequency
range from 1 to 500 MHz. The bondwire is 17.5 mm long and 20 mil (0.51 mm) in
diameter. A permeability of 16 is assumed for the ferrite epoxy material in the
simulation work. The inductance value of
the bondwire increases from 14 nH without ferrite coating to over 70 nH with a
ferrite coating of 40 mil in thickness. A five times increase in inductance
value is therefore observed.Figure 3
Effect of ferrite epoxy coating thickness on the
inductance value of a bondwire based on HFSS simulation.
Measurement data is also included for comparison. The
bondwire is 17.5 mm long and 20 mil in diameter. Aμ value of
16 is assumed.A set
of measurement data is included in Figure3 for comparison with the modeling
result. A reasonably good agreement can be seen between the modeling and
measurement results. Key bondwire inductor design parameters include length and
diameter of the bondwire, and thickness and permeability of the ferrite epoxy
coating. It is observed in Figure 3 that the inductance of the bondwire
inductor increases with ferrite epoxy coating thickness since a larger ferrite
thickness leads to a smaller magnetic reluctance, greater magnetic flux, and
greater inductance. However, there are practical limitations on how much
ferrite epoxy coating can be applied into the IC package which has a limited
space.Figure4 shows the modeled inductance
value of bondwire inductors with a ferrite epoxy permeability of 1 (bare wire),
9, 16, 25, 36, and 49 over a frequency range from 1 to 500 MHz. Naturally, the
inductance value increases with increasing permeability of the ferrite epoxy
material. Iron oxide-based ferrite ceramic materials usually demonstrate a
relative permeability up to a few thousands. Ferrite epoxy materials with mixed
ferrite powders and organic binder tend to have a much lower effective
permeability in a range of 9 to 20 depending on the loading percentage of the
ferrite powders. For example, the permeability of the ferrite epoxy used in our
experimental study from Methode Development Company (Chicago, Ill, USA) is between 12 and 16. However, it is possible
that new high-μ ferrite epoxy materials
can be developed in the future. The simulation results indicate that the
inductance of a bare bondwire can be increased by a factor of 10 if a μ value of 50 can be used.Figure 4
Effect of ferrite epoxy coating permeability on the
inductance value of a bondwire based on HFSS simulation.
The bondwire is 17.5 mm long and 20 mil in diameter. A
thickness of 20 mils is assumed for the ferrite epoxy coating.The effect of bondwire diameter on its
inductance value is also studied. Figure5 shows the HFSS simulated trend of
inductance increase with decreasing wire diameter. However, it is observed that
the inductance increase is not significant. Furthermore, a small diameter of
the bondwire leads to a high dc series resistance and a small Q factor. Therefore,
it may not be an effective solution to increase the inductance by using very
thin bondwires in power SOC's.Figure 5
Inductance of bondwires with a diameter of 5, 10, and
20 mils based on HFSS simulation. The bondwire is 17.5 mm
long. There is no ferrite epoxy coating.In short, the EM simulation work indicates
that the bondwire inductance can be increased by increasing the thickness and
permeability of the ferrite epoxy coating, and to a less extent, by using
thinner bondwires.


### 3.2. Transformers

Self inductance, mutual coupling
coefficient, series dc resistance, andQ factor are among the most important
properties of a power transformer. In general, a high mutual coupling
coefficient and a high self inductance along with low dc series resistance are
desirable. We first study those parameters with a modeling and analysis approach.
Figure 6 shows a two-port transformer model that we use in our study for
analysis and parameter extraction. Lp,Ls, and M are the
primary self-inductance, secondary self-inductance, and mutual inductance,
respectively. The T-section model in Figure 6 uses three inductors to model the
mutual coupling between the primary and secondary windings. This model considerably
simplifies analysis of circuits containing transformers [20].Figure 6
A two-port transformerT-model.Figure 7
Simulated self inductance, mutual inductance, and quality
factor of a 15 mil spacing bondwire transformer.The turn ratioT, mutual inductance M,
primary inductance Lp, and secondary inductance Ls are
given by (2)T=LpLs,Lp(leakage)−=Lp−M=Lp⋅(1−k),Ls(leakage)=Ls−MT2=Ls⋅(1−k),M=kLpLs=kLp,ifLp=Ls, where k is the coupling coefficient (0≤k≤1) and can be extracted from modeling and measurement data.Transformer
parameters can be simply extracted from the two-port networkZ-parameters as the
following: (3)Lp=Im(Z11)ω,Ls=Im(Z22)ω,Qp=Im(Z11)Re(Z11),Qs=Im(Z22)Re(Z22),k=Im(Z12)⋅Im(Z21)Im(Z11)⋅Im(Z22).TheZ-parameters of the transformer
network can be converted from its S-parameters from electromagnetic field
simulation or network analyzer measurement. The S-parameters from HFSS
simulation of the two-port transformer can be described as the following.|S11|2 is the reflected
power from Port 1.|S12|2 is the transmitted
power from Port 1 to Port 2.|S21|2 is the transmitted power
from Port 2 to Port 1.|S22|2 is the reflected
power from Port 2.For two-port transformers, the
transmission coefficient is simplyS21 which indicates the capability of the
transformer to transfer power from the primary side to the secondary side.It is found from our modeling analysis
that the spacing between the bondwires has a great influence to the transformer
parameters. A small spacing improves the transformer's performance
significantly. Figure4 illustrates the simulated self-inductance, mutual
inductance, and quality factor of 1:1 bondwire transformer with spacing of 15 mil between
the two bondwires. A maximum Q factor of 138 is observed.Figures8 and 9 show the coupling coefficient k and power transmission parameter S21
extracted from HFSS simulation for a bondwire spacing ranging from 12 to 50 mils. It is observed that both parameters improve as the distance between the
bondwires decreases. This is indeed encouraging since the advanced wire bonding
machines used in today's microelectronics manufacturing can deliver a bondwire
spacing as small as 1 mil, which can guarantee excellent coupling between
bondwires.Figure 8
Coupling coefficientk for variable bonding spacing.Figure 9
Transmission coefficientS21 for various bondwire
spacing.


## 3.1. Inductors

HFSS simulation generates a set ofS
parameters from the inductor structure which are then converted to a set of Y
parameters. The effective quality factor Q and inductance L can be then extracted
from the Y parameters by using the following equations: (1)Leff=−1ωIm(y11),Q=−Im(y11)Re(y11).Figure3 shows the modeled inductance value of bondwire inductors with a ferrite epoxy
coating thickness of 0 (bare wire), 10, 20, 30, and 40 mils over a frequency
range from 1 to 500 MHz. The bondwire is 17.5 mm long and 20 mil (0.51 mm) in
diameter. A permeability of 16 is assumed for the ferrite epoxy material in the
simulation work. The inductance value of
the bondwire increases from 14 nH without ferrite coating to over 70 nH with a
ferrite coating of 40 mil in thickness. A five times increase in inductance
value is therefore observed.Figure 3
Effect of ferrite epoxy coating thickness on the
inductance value of a bondwire based on HFSS simulation.
Measurement data is also included for comparison. The
bondwire is 17.5 mm long and 20 mil in diameter. Aμ value of
16 is assumed.A set
of measurement data is included in Figure3 for comparison with the modeling
result. A reasonably good agreement can be seen between the modeling and
measurement results. Key bondwire inductor design parameters include length and
diameter of the bondwire, and thickness and permeability of the ferrite epoxy
coating. It is observed in Figure 3 that the inductance of the bondwire
inductor increases with ferrite epoxy coating thickness since a larger ferrite
thickness leads to a smaller magnetic reluctance, greater magnetic flux, and
greater inductance. However, there are practical limitations on how much
ferrite epoxy coating can be applied into the IC package which has a limited
space.Figure4 shows the modeled inductance
value of bondwire inductors with a ferrite epoxy permeability of 1 (bare wire),
9, 16, 25, 36, and 49 over a frequency range from 1 to 500 MHz. Naturally, the
inductance value increases with increasing permeability of the ferrite epoxy
material. Iron oxide-based ferrite ceramic materials usually demonstrate a
relative permeability up to a few thousands. Ferrite epoxy materials with mixed
ferrite powders and organic binder tend to have a much lower effective
permeability in a range of 9 to 20 depending on the loading percentage of the
ferrite powders. For example, the permeability of the ferrite epoxy used in our
experimental study from Methode Development Company (Chicago, Ill, USA) is between 12 and 16. However, it is possible
that new high-μ ferrite epoxy materials
can be developed in the future. The simulation results indicate that the
inductance of a bare bondwire can be increased by a factor of 10 if a μ value of 50 can be used.Figure 4
Effect of ferrite epoxy coating permeability on the
inductance value of a bondwire based on HFSS simulation.
The bondwire is 17.5 mm long and 20 mil in diameter. A
thickness of 20 mils is assumed for the ferrite epoxy coating.The effect of bondwire diameter on its
inductance value is also studied. Figure5 shows the HFSS simulated trend of
inductance increase with decreasing wire diameter. However, it is observed that
the inductance increase is not significant. Furthermore, a small diameter of
the bondwire leads to a high dc series resistance and a small Q factor. Therefore,
it may not be an effective solution to increase the inductance by using very
thin bondwires in power SOC's.Figure 5
Inductance of bondwires with a diameter of 5, 10, and
20 mils based on HFSS simulation. The bondwire is 17.5 mm
long. There is no ferrite epoxy coating.In short, the EM simulation work indicates
that the bondwire inductance can be increased by increasing the thickness and
permeability of the ferrite epoxy coating, and to a less extent, by using
thinner bondwires.


## 3.2. Transformers

Self inductance, mutual coupling
coefficient, series dc resistance, andQ factor are among the most important
properties of a power transformer. In general, a high mutual coupling
coefficient and a high self inductance along with low dc series resistance are
desirable. We first study those parameters with a modeling and analysis approach.
Figure 6 shows a two-port transformer model that we use in our study for
analysis and parameter extraction. Lp,Ls, and M are the
primary self-inductance, secondary self-inductance, and mutual inductance,
respectively. The T-section model in Figure 6 uses three inductors to model the
mutual coupling between the primary and secondary windings. This model considerably
simplifies analysis of circuits containing transformers [20].Figure 6
A two-port transformerT-model.Figure 7
Simulated self inductance, mutual inductance, and quality
factor of a 15 mil spacing bondwire transformer.The turn ratioT, mutual inductance M,
primary inductance Lp, and secondary inductance Ls are
given by (2)T=LpLs,Lp(leakage)−=Lp−M=Lp⋅(1−k),Ls(leakage)=Ls−MT2=Ls⋅(1−k),M=kLpLs=kLp,ifLp=Ls, where k is the coupling coefficient (0≤k≤1) and can be extracted from modeling and measurement data.Transformer
parameters can be simply extracted from the two-port networkZ-parameters as the
following: (3)Lp=Im(Z11)ω,Ls=Im(Z22)ω,Qp=Im(Z11)Re(Z11),Qs=Im(Z22)Re(Z22),k=Im(Z12)⋅Im(Z21)Im(Z11)⋅Im(Z22).TheZ-parameters of the transformer
network can be converted from its S-parameters from electromagnetic field
simulation or network analyzer measurement. The S-parameters from HFSS
simulation of the two-port transformer can be described as the following.|S11|2 is the reflected
power from Port 1.|S12|2 is the transmitted
power from Port 1 to Port 2.|S21|2 is the transmitted power
from Port 2 to Port 1.|S22|2 is the reflected
power from Port 2.For two-port transformers, the
transmission coefficient is simplyS21 which indicates the capability of the
transformer to transfer power from the primary side to the secondary side.It is found from our modeling analysis
that the spacing between the bondwires has a great influence to the transformer
parameters. A small spacing improves the transformer's performance
significantly. Figure4 illustrates the simulated self-inductance, mutual
inductance, and quality factor of 1:1 bondwire transformer with spacing of 15 mil between
the two bondwires. A maximum Q factor of 138 is observed.Figures8 and 9 show the coupling coefficient k and power transmission parameter S21
extracted from HFSS simulation for a bondwire spacing ranging from 12 to 50 mils. It is observed that both parameters improve as the distance between the
bondwires decreases. This is indeed encouraging since the advanced wire bonding
machines used in today's microelectronics manufacturing can deliver a bondwire
spacing as small as 1 mil, which can guarantee excellent coupling between
bondwires.Figure 8
Coupling coefficientk for variable bonding spacing.Figure 9
Transmission coefficientS21 for various bondwire
spacing.


## 4. Experimental

We
have conducted experimental investigation of the proposed on-chip bondwire magnetics
concept using two types of ferrite epoxy composite materials. The first
material is a custom formulated magnetic epoxy comprised of manganese-zinc
(MgZn) ferrite powder with an average particle size of 10μm, thermaoplastic
resin, and solvent from Methode Development Corporation. The manganese-zinc ferrite loading powder is
commercially available (Steward 73300). The average surface area of the powder is
1.4 m2/g. The saturation moment of the
bulk powder is 79.4 emu/g.  The cured ferrite
composite (no solvent) consisted of 96% by mass ferrite with the balance
consisting of polymer. The effective permeability is between 12 and 16. The
second material is a ferrite nanocomposite from Inframat Corporation, Conn, USA, that is comprised of
very fine (NiZn)Fe2O4 nanoparticles with an average size
of 5–15 nm and a
commercial epoxy.The test reported herein was conducted on
standalone copper and aluminum wires initially and then on aluminum bondwires bonded
onto a PCB substrate. Copper wires with a fixed length of 20 mm but two
different diameters of 250μm (10 mil) and 500 μm (20 mil) were used to emulate
the bondwires in IC packages. Note that copper has a slightly lower resistivity
than more commonly used gold or aluminum. The ferrite epoxy materials were
manually brushed onto the copper bondwire to form a ferrite bead. Curing consisted
of a thermal treatment of the ferrite beads in an oven at 140C° for
30 minutes for both ferrite epoxy material options. The ferrite-polymer composites display
negligible conductivity and therefore are electrically self-isolated from the
bare copper bondwires. Figure 10 shows a
photo of aluminum bondwire transformer with and without the ferrite epoxy coating
on a PCB substrate. The wire bonding was conducted on an Orthodyne M20
wirebonder using 5 mil aluminum bondwires.Figure 10
Fabricated bondwire transformer(Np:Ns=9:1).


### 4.1. Inductors

The bondwire inductors were
characterized with an HP 4284. A high precision LCR meters in a low frequency
range up to 1 MHz. DC resistance was measured with an Instek 801 H milli-Ohm
meter. High frequency measurement was performed using an Agilent 8753S-parameter
network analyzer. The effective quality factor Q and inductance L can be then
extracted from the Y parameters by using (1).Figures11 and 12 show the measured inductance for the 10 mil and 20 mil bondwire
inductors with no ferrite epoxy coating, the ferrite polymer bead, and the
ferrite nanocomposite bead, respectively. The inductance of bare copper wires
was increased by a factor of 2.8 to 3.5 with the addition of the ferrite epoxy coating.
The dc resistance of the 10 mil and 20 mil bondwire inductors was measured as 7.1 mΩ and 1.7 mΩ respectively using an Instek 801H milli-Ohm meter. The 10 mil
bondwire inductor demonstrates an inductance of 38 nH and a dc resistance of
7.1 mΩ. Figure 13 compares the Q-factor of the state-of-the-art MEMS
microinductors, commercial wire-wound inductors, and the bondwire inductors in
this work. The bondwire inductors demonstrate a Q factor of 30–40 in a frequency
range of 2 to 20 MHz, similar to that of commercial wire-wound inductors, but
significantly higher than that of  the
MEMS microinductors. Figure 14 compares the state-of-the-art MEMS
microinductors, commercial wire-wound inductors, and the bondwire inductors in
this work in terms of inductance and dc winding resistance achieved. It is
clearly shown that the bondwire inductors offer a solution for high current
power SOC applications in which the MEMS microinductors fall short. Figure 15
shows the measured inductance of the bondwire inductors over a high frequency
range, indicating a self-resonant frequency between 700 and 800 MHz. The
bondwire inductors were also tested in a pulsed switching circuit for core saturation
characterization. Figure 16 shows measured switching waveforms of the inductor
current. No noticeable core saturation
phenomenon was observed for an inductor load current up to 47A, most likely due
to the distributed gaps between the ferrite particles in the ferrite epoxy
materials.Figure 11
Measured inductance versus frequency for a bare copper
wire and wires coated with two different ferrite epoxy
materials. The wire is 20 mm long and 10 mil in diameter.Figure 12
Measured inductance versus frequency for a bare copper
wire and wires coated with two different ferrite epoxy
materials. The wire is 20 mm long and 20 mil in diameter.Figure 13
Comparison ofQ factors between the state-of-the-art
MEMS micro-inductors, commercial wire wound inductors,
and the bondwire inductors.Figure 14
Comparison of the state-of-the-art MEMS microinductors,
commercial wire wound inductors, and the bondwire
inductors in terms of inductance and dc resistance.Figure 15
Measured high-frequency inductance for the 10 mil
bondwire inductors with and without ferrite beads.Figure 16
Measured switching waveform of the 10 mil
bondwire inductors for core saturation characterization.


### 4.2. Transformers

We
have conducted experimental investigation of the proposed on-chip bondwire transformer
concept with and without the ferrite epoxy composite materials. Figures17 and
18 show the primary and secondary voltage waveforms of a 9:1 bondwire
transformer without and with ferrite epoxy core at 16 MHz. It is observed that
the bondwire transformer with ferrite epoxy core demonstrated less distortion
in the secondary voltage waveform than its counterpart without ferrite core.Figure 17
Measured primary and secondary voltage waveforms
without ferrite epoxy core(9:1).Figure 18
Measured primary and secondary voltage waveforms
with ferrite epoxy core(9:1).It
is very difficult to characterize transformers in time domain at high
frequencies due to the parasitic effects. We use aS-parameter network analyzer
to extract all transformer parameters such self- and mutual inductances,
k-factor, and S21 parameter. The PCB substrate was designed as 50 Ω transmission lines to minimize
the input reflection. A Cascade Microtech’s (Ore, USA) M150 probe station is used to test the
bondwire transformers on PCB substrates. Two FPC-1000 probes are used for two-port measurement. The
Fixed-Pitch Compliant (FPC)-Series fixtures are high frequency 50 Ω coaxial probe that offers a
signal line with two low-inductance fixed-pitch ground contacts (GSG). The PCB
designs are based on detailed HFSS simulation, and work well with the FPC-1000
probes. A PNA E8361A Network Analyzer receives input from the two FPC-1000
probes and is used to measure the S-parameters of the transformers.Figure19 shows the extracted primary
self-inductance Lp, secondary self-inductance Ls and mutual inductance L with
and without the ferrite epoxy core for a 2:1 transformers. The dc resistance of
each bondwire is around 7 mΩ. It is observed that all inductances increase
after ferrite epoxy glob is applied.Figure 19
Self inductance and mutual inductance measurement
data for bondwire2:1 transformers with and without ferrite core.Figure20 shows the extracted k-factor with and without the ferrite epoxy core for the
2:1 transformers. It is observed that the k-factor increased from 0.46 to 0.6
(30% improvement) for 2:1 transformer. This shows the effectiveness of ferrite
epoxy core. Figure 21 illustrates the transmission coefficient S21 of the 2:1 transformer.
S21 is a critical parameter which determines the transformer’s power
transferring capability. In very high frequency span, since the coupling
coefficient k is no longer meaningful, S21 is the only performance parameter
for the transformer.Figure 20
Measuredk-factor of a 2:1 bondwire transformer
before and after applying ferrite epoxy.Figure 21
Measured transmission coefficientS21 of a 2:1 bondwire
transformer before and after applying ferrite epoxy.The
most important finding from this experiment is that the coupling effect of the
bondwire transformers can be significantly improved with the application of
ferrite epoxy core glob. Not only can we increase thek-factor, but also can we
boost the transmission coefficient S21. We experimentally demonstrated that the
performance of the on-chip bondwire transformer can be improved by ferrite
polymer epoxy globe in a frequency range of 10 MHz to 300 MHz, which is of
interest for future-generation, high frequency, high density switching power
converters.


## 4.1. Inductors

The bondwire inductors were
characterized with an HP 4284. A high precision LCR meters in a low frequency
range up to 1 MHz. DC resistance was measured with an Instek 801 H milli-Ohm
meter. High frequency measurement was performed using an Agilent 8753S-parameter
network analyzer. The effective quality factor Q and inductance L can be then
extracted from the Y parameters by using (1).Figures11 and 12 show the measured inductance for the 10 mil and 20 mil bondwire
inductors with no ferrite epoxy coating, the ferrite polymer bead, and the
ferrite nanocomposite bead, respectively. The inductance of bare copper wires
was increased by a factor of 2.8 to 3.5 with the addition of the ferrite epoxy coating.
The dc resistance of the 10 mil and 20 mil bondwire inductors was measured as 7.1 mΩ and 1.7 mΩ respectively using an Instek 801H milli-Ohm meter. The 10 mil
bondwire inductor demonstrates an inductance of 38 nH and a dc resistance of
7.1 mΩ. Figure 13 compares the Q-factor of the state-of-the-art MEMS
microinductors, commercial wire-wound inductors, and the bondwire inductors in
this work. The bondwire inductors demonstrate a Q factor of 30–40 in a frequency
range of 2 to 20 MHz, similar to that of commercial wire-wound inductors, but
significantly higher than that of  the
MEMS microinductors. Figure 14 compares the state-of-the-art MEMS
microinductors, commercial wire-wound inductors, and the bondwire inductors in
this work in terms of inductance and dc winding resistance achieved. It is
clearly shown that the bondwire inductors offer a solution for high current
power SOC applications in which the MEMS microinductors fall short. Figure 15
shows the measured inductance of the bondwire inductors over a high frequency
range, indicating a self-resonant frequency between 700 and 800 MHz. The
bondwire inductors were also tested in a pulsed switching circuit for core saturation
characterization. Figure 16 shows measured switching waveforms of the inductor
current. No noticeable core saturation
phenomenon was observed for an inductor load current up to 47A, most likely due
to the distributed gaps between the ferrite particles in the ferrite epoxy
materials.Figure 11
Measured inductance versus frequency for a bare copper
wire and wires coated with two different ferrite epoxy
materials. The wire is 20 mm long and 10 mil in diameter.Figure 12
Measured inductance versus frequency for a bare copper
wire and wires coated with two different ferrite epoxy
materials. The wire is 20 mm long and 20 mil in diameter.Figure 13
Comparison ofQ factors between the state-of-the-art
MEMS micro-inductors, commercial wire wound inductors,
and the bondwire inductors.Figure 14
Comparison of the state-of-the-art MEMS microinductors,
commercial wire wound inductors, and the bondwire
inductors in terms of inductance and dc resistance.Figure 15
Measured high-frequency inductance for the 10 mil
bondwire inductors with and without ferrite beads.Figure 16
Measured switching waveform of the 10 mil
bondwire inductors for core saturation characterization.


## 4.2. Transformers

We
have conducted experimental investigation of the proposed on-chip bondwire transformer
concept with and without the ferrite epoxy composite materials. Figures17 and
18 show the primary and secondary voltage waveforms of a 9:1 bondwire
transformer without and with ferrite epoxy core at 16 MHz. It is observed that
the bondwire transformer with ferrite epoxy core demonstrated less distortion
in the secondary voltage waveform than its counterpart without ferrite core.Figure 17
Measured primary and secondary voltage waveforms
without ferrite epoxy core(9:1).Figure 18
Measured primary and secondary voltage waveforms
with ferrite epoxy core(9:1).It
is very difficult to characterize transformers in time domain at high
frequencies due to the parasitic effects. We use aS-parameter network analyzer
to extract all transformer parameters such self- and mutual inductances,
k-factor, and S21 parameter. The PCB substrate was designed as 50 Ω transmission lines to minimize
the input reflection. A Cascade Microtech’s (Ore, USA) M150 probe station is used to test the
bondwire transformers on PCB substrates. Two FPC-1000 probes are used for two-port measurement. The
Fixed-Pitch Compliant (FPC)-Series fixtures are high frequency 50 Ω coaxial probe that offers a
signal line with two low-inductance fixed-pitch ground contacts (GSG). The PCB
designs are based on detailed HFSS simulation, and work well with the FPC-1000
probes. A PNA E8361A Network Analyzer receives input from the two FPC-1000
probes and is used to measure the S-parameters of the transformers.Figure19 shows the extracted primary
self-inductance Lp, secondary self-inductance Ls and mutual inductance L with
and without the ferrite epoxy core for a 2:1 transformers. The dc resistance of
each bondwire is around 7 mΩ. It is observed that all inductances increase
after ferrite epoxy glob is applied.Figure 19
Self inductance and mutual inductance measurement
data for bondwire2:1 transformers with and without ferrite core.Figure20 shows the extracted k-factor with and without the ferrite epoxy core for the
2:1 transformers. It is observed that the k-factor increased from 0.46 to 0.6
(30% improvement) for 2:1 transformer. This shows the effectiveness of ferrite
epoxy core. Figure 21 illustrates the transmission coefficient S21 of the 2:1 transformer.
S21 is a critical parameter which determines the transformer’s power
transferring capability. In very high frequency span, since the coupling
coefficient k is no longer meaningful, S21 is the only performance parameter
for the transformer.Figure 20
Measuredk-factor of a 2:1 bondwire transformer
before and after applying ferrite epoxy.Figure 21
Measured transmission coefficientS21 of a 2:1 bondwire
transformer before and after applying ferrite epoxy.The
most important finding from this experiment is that the coupling effect of the
bondwire transformers can be significantly improved with the application of
ferrite epoxy core glob. Not only can we increase thek-factor, but also can we
boost the transmission coefficient S21. We experimentally demonstrated that the
performance of the on-chip bondwire transformer can be improved by ferrite
polymer epoxy globe in a frequency range of 10 MHz to 300 MHz, which is of
interest for future-generation, high frequency, high density switching power
converters.


## 5. Conclusions

We have proposed and investigated the
feasibility of a new concept of realizing on-chip inductors and transformers in
power SOC's using existing bondwires with additional ferrite epoxy glob coating.
We have investigated the concept both experimentally and with finite element
modeling. AQ factor of 30–40 is
experimentally demonstrated for the bondwire inductors which represents an
improvement by a factor of 3–30 over the
state-of-the-art MEMS micromachined inductors. Transformer parameters including
self- and mutual inductance, and coupling factors are extracted from both
modeled and measured S-parameters. It is demonstrated that the performance of
the bondwire transformers can be improved using ferrite polymer epoxy globe. More
importantly, the bondwire magnetic components can be easily integrated into SOC
manufacturing processes with minimal changes, and open enormous possibilities
for realizing cost-effective, high current, high efficiency power SOC's. Future
work includes studies on the selection of ferrite materials, the influence of
the shape and volume of the ferrite beads, the effect of the height and length
of the bondwire loop, and possible implementation methods in mass production.



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*Source: 678415-2008-07-16.xml*