ABSTRACT
Miniaturization
has been a key contributor to advances in electronic technology. Many
electronics applications have serious space considerations that are pressuring
manufacturers to reduce component size. Much of the motivation for this
Certainly, miniaturization has been made possible mostly through remarkable breakthroughs
in reducing the size of active components. But as integrated circuits get
smaller and more complex, there is an increasing need to also reduce the space
required for the supporting passive components.
Passive integration suggests that if the
passives were so small and flat, then they could be inserted between layers of
the circuit board itself, rather than taking space on top of it. The integrated
passives would be a part of the circuit board itself, formed when the board
was, so odds are good that their overall cost could eventually be less than
what manufacturers pay today to buy and solder on discrete devices. Advantages
of this technology point to a potentially huge shift for the electronics
industry. Passive component integration is and will continue to be an important
contribution in the development of electronic technology.
INTRODUCTION
Miniaturization
has been a key contributor to advances in electronic technology. Certainly,
miniaturization has been made possible mostly through remarkable breakthroughs
in reducing the size of active components. But as integrated circuits get
smaller and more complex, there is an increasing need to also reduce the space
required for the supporting passive components. If any of the electronic
devices such as cellphone, camcorder, computer, or other consumer electronics
system is opened, one or two circuit boards on which are mounted a few
integrated circuits and dozens and dozens of tiny discrete devices-resistors,
capacitors, inductors etc can be seen. It is those so-called passive devices
that dominate the board’s real estate.
Passive integration suggests that if the passives were so
small and flat, then they could be inserted between layers of the circuit board
itself, rather than taking space on top of it. The electronic devices could be
thinner and sleeker than they are today, or they could contain more
electronics, or if it is a phone can have much larger batteries and therefore
longer talk time and brighter color screens. The same goes for almost every device from PDAs
to portable DVD players.
The integrated passives would be a part of the circuit
board itself, formed when the board was, so odds are good that their overall
cost could eventually be less than what manufacturers pay today to buy and
solder on discrete devices. Speaking of solder, eliminating it is another
advantage of integration, because bad solder joints are one of the most common
reasons electronic gear fails. Less solder also means less harm from lead
waste.
Advantages like
these point to a potentially huge shift for the electronics industry. Over a
trillion passive components were bonded to boards last year, according to the
National Electronics Manufacturing Initiative's road map. These devices are
minuscule, and that makes putting them in place a chore. The smallest discrete
passives today measure 0.50 mm by 0.25 mm; spread on a sheet of paper, they'd
look like ground pepper. Such compact components are difficult to handle and
attach, even for automated assembly equipment. And though the total cost of
each part—including capital, assembly, and the prorated cost of the underlying
board—is less than two cents on average, collectively the impact of integrated
passives on system cost, reliability, and, most of all, size, could be
enormous.
In a
sense, the situation with passive components today is a lot like that of active
devices 40 years ago, when Intel, Fairchild, and others had just introduced ICs
that combined active devices like transistors and diodes on a single substrate.
But don't expect Moore
NEED FOR PASSIVE
INTEGRATION
Passive
components refer to such kind of electrical components that cannot generate
power. Typical components are resistors, capacitors and inductors. The primary
functions of passive components are to manage buses, bias, decouple Ics,
by-pass, filter, tune, convert, and sense and protect. It is a huge,
multi-billion business, supporting the various electronic products in
automotive, telecommunications, computer and consumer industries, both for
digital and analog-digital applications. There are a large number of passive
components that are used in consumer electronic products such as VCRs,
camcorders, television tuners, and other communication devices. Most of the
passive components nowadays are discrete surface mount passive components that
directly mount on the surface of the printed circuit board. It is called as
discrete passive component-a singular component enclosed in a single case that
must be mounted to an interconnecting substrate. Passive components are
commonly referred to as “glue components” since they “glue” integrated circuits
together to make the system.
Surface mount technology was starting to take
deep root in our industry in early 80’s and is fully developed till today. In
the early days, surface mount components were many times more expensive than
through hole components and new surface mount assembly equipment costs were off
the charts. As time went on, the cost of the components, assembly equipment and
all of the other infrastructure came down, today it is less expensive to build
a surface mount assembly than a through hole assembly. However, the faster bus
speeds required new technology. PCB traces have always had transmission line
characteristics and are more sensitive at subnano-second rise times. The
package lead inductance and line capacitance have greater impact on signal
integrity. The integrated circuit industry is achieving faster speeds by
shrinking technology; it follows that the passive solution must also shrink. In
addition to these, the need to drive out every cent of costs, miniaturization,
improved product reliability and the passive to active ratios have caused to
seriously consider much higher levels of passive integration than in the past.
Then comes the idea of integral passives.
Integral passives are noted as passive components
embedded within or on the surface of a substrate. These are distinguished from
discrete chips and also from integrated(multiple passive functionality within a
single package).It is a part of the printed circuit board using some type of
material to make resistors, capacitors or inductors. The requirement for
integral resistors, capacitors and inductors are:
Resistors:
A primary requirement for integral resistors is that they be size
competitive with the chip resistor. It dictates that the largest dimension be of
the order of 1.0mm.Cost considerations dictate that trimming should not be
required to obtain a 5%-10% tolerance. The
range of values used,from one ohm to one mega-ohm dictates that if that range was implemented there would be
insufficient numbers in the tails of the value distribution to justify
integrating the full range.
Capacitors:
Discrete capacitors are used in larger number and greater density
than any other discrete passive component. There are atleast two distinct application
potentials for integration, one in which the polymer or ceramic board itself
provides dielectric and capacitor plates within the interconnection; and the
second, wherein progressively higher dielectric constant materials make
increasingly larger capacitance feasible. There is a possibility to eliminate
about 40% of the discrete capacitors in a hand held product by simply designing
low value capacitors into one or more of the two level interconnection patterns
normally used.
Inductors:
Inductors are currently
used in such low quantities,that the equivalent per part cost will probably be
too high to incorporate any special processes or materials. High values are
best attained with conventional discrete parts.However,about 80% of the
inductors used in a hand held product are low enough in value, that they can be
incorporated directly into wiring of a suitable substrate.They require fine
line capability small vias and thin dielectrics.Careful attention to design
will be necessary due to coupling to nearby metal..
Integrated
passive components (RC circuits) and passive component arrays (MLC capacitors,
MLV transient suppressors, and thick-film resistors) used in medical
electronics
INTEGRATED PASSIVE
TECHNOLOGY
The
technologies available for the packaging of microelectronics at that time
were generally thick film and thin film circuits hermetically sealed in a
package made of ceramic or metal with glass to metal feed-throughs. The need
for a package, interconnect board plus discrete components complicated the
assembly of the hybrid microcircuit and increased volume and weight
requirement.A new technology that could integrate these three functions would
dramatically reduce size and assembly complexity with concurrent improvements
in cost and reliability.
None
of then existing technologies were suitable for all three functions. The
cofired ceramic could provide a durable hermetic package but was limited to
refractory metal systems due to the high firing temperatures. There are several
disadvantages: high trace resistances, a requirement of plating for all exposed
metal to provide for corrosion resistance and subsequent metallurgical
connections, and firing in a reducing atmosphere which limited the range of
cofirable film components which could be included.
Thick
and thin films use gold, silver or copper metallurgy which have excellent conductivity and do not require
plating while being, except for copper. Thick
films, however, were not in general dense or strong enough for use in building
hermetic packages and were expensive when used for high count
multiplayer interconnect structures.
Low
Temperature Cofired Ceramics(LTTC) was seen as a potential solution for achieving a new integrated
packaging technology from a combination
of thick film and low temperature cofired dielectrics .LTTC has many
advantages such as it allows high density of lines throughout the part, be able
to construct various geometries of
interconnects by layer cut outs, good
heat transfer ability, etc. In addition to offering competitive capabilities in
packaging and interconnection, LTTC has a clear advantage over other
technologies in the area of integral
passive components. They are
·
Reduction in the number of contacts and
transitions: traditional assembly
has the internal contacts of the components themselves, the transition to the
attachment material and then to the interconnect. By integrating
these transitions, the associated losses are reduced dramatically.
·
Increasing reliability : Failures occur
primarily at transitions or interfaces between materials. Reducing the number
of transitions increase the reliability.
·
Cost saving : Few additional steps are
required for component integration and a
large number of assembly steps are eliminated.
·
Density saving : Component size and
component count are the typical drivers for assembly size. The same components
can be effectively spread ”two dimensionally” within the package substrate or
the package itself in traditionally unused or waste area.
·
LTTC provides wider components value range
compare to other technologies.0.1W to 10 MW for resistors under the tolerance of 25%, 4pF to
0.04mF for
capacitors with the typical tolerance
from 5 to 10%, 15nH to over three order of magnitude inductors with the
tolerance around 5%.
Integrated passive
technologies are not exactly new. They have been used for decades in the
ceramic substrates that underlie circuits in military, microwave, and mainframe
computer systems. But those represent a specialty within the electronics
market. The vast majority of circuit boards today are made using FR4, the
ubiquitous green epoxy insulator reinforced with glass fiber. FR4 boards are
formed by sandwiching alternating layers of insulator with etched copper
circuit traces and laminating them under heat and pressure. Drilled holes, or vias, plated with copper, connect
conductor segments on different layers to form circuit interconnects.
A smaller but growing
portion of the circuit board market has been going to "flex," which
are laminated stacks of unreinforced polyimide (Kapton), polyester, or layers
of other polymer film, each 25 to 125 µm thick, with copper traces on one or
both sides. Because the polymer layers can be thinner, enabling smaller vias,
flex allows more interconnects to be crammed into a given area than is possible
with FR4. But flex costs more per square centimeter than FR4.
In both FR4 and flex, the
presence of organic material limits their processing temperatures to about 250
°C, far below the 800 to 1200 °C used in processing ceramic substrates. So to
put passives within the layers of FR4 and flex boards, engineers had to come up
with new techniques.
The components in these
boards can be no thicker than a single layer of the board, maybe only a few
micrometers. So for all intents and purposes, the devices are planar rather
than three-dimensional. Manufacturers are using several different techniques,
including sputtering, plating, chemical vapor deposition, screen-printing, and
anodization, to deposit various film materials to produce the passives. All of
those deposition methods are compatible with the 250 °C limit for FR4 and flex.
Depending on the process, technicians can add material just where it is needed,
or cover an entire board layer with it and then subtract material where it is
not wanted.
The average value of
cellphone capacitors is typically 1 to 10 nanofarads and there can be hundreds
of capacitors in each board; a manufacturer would have to pack hundreds or even
thousands of nanofarads of capacitance into the board. For contrast, most
current products for making integrated capacitors are limited to polymer-based
low-capacitance density materials good for only about 5 nF/cm2. A
new company, Xanodics is commercializing
a capacitor process,
called Stealth, that is based on tantalum (common in cellphone
capacitors).. We anodize it at room temperature to create tantalum pentoxide in
a solution that is benign to the board and its copper conductors. This forms
devices with capacitance to be sure that the integration would reduce the
board's size. DuPont and others are developing processes that should yield over
100 nF/cm2, a value good enough to replace many of a cellphone's
surface-mounted capacitors with integrated ones.
Size matters
Integrating
passives can drastically reduce the size of an ordinary circuit board.Here,
four capacitors and six resistors have been removed from the surface and put
into an extra layer of circuit board [bottom]. Resistors are copper connection
points bridged by a resistive film, and capacitors are conductive plates
separated by a thin film of dielectric material.
After the board is
laminated, holes are drilled and plated to form vias that connect the
integrated components to other board wiring. An integrated one can replace not
every value of passive; two remain on the surface. Some commercial processes
would require separate capacitor and resistor layers.
In contrast to capacitors,
integrated inductors are a snap to fabricate. They are nothing but spirals of
interconnect metal. The challenge is not in the materials or process technology
but in their design. The main problem is that any nearby metallic structures,
such as interconnects or other inductors, will interfere with their magnetic
fields and change their performance. The dielectric material is FR4 and the
conductor material is Aluminium. At low frequency, the reactive inductance is
smaller than the series resistance, therefore, the resistance dominates the
impedence. The resonant frequency is determined by the parasitic capacitance of
the inductors.
Inductors are angled away from
each other to avoid crosstalk in this low-pass filter that fits between the
layers of a circuit board. Designed by one of the authors, and built by
Integral Wave Technologies for NASA's Langley Research
Center
BARRIERS TO PASSIVE
INTEGRATION
Just
as the early time of the surface mount components,integrated passive components
is a fairly new technology and there are several inhibitors keep embedded
passives from reaching their market potential.
There
are three main barriers to bringing integrated passives into the market: too
few design tools, inadequate computer models for predicting costs, and
insufficient infrastructure. Better design tools are crucial because taking
passives off the surface of a board and burying them generally means that more
board layers are required, complicating circuit trace routing. A few design
tools can take this complication into account when producing automated layouts;
a couple is just now becoming available from Zuken Inc. and Ohmega Technologies
Inc.
The lack of software to
analyze costs is also a problem. Before board fabricators will get into the
business of manufacturing passive components, they will want to have a pretty
good idea of how it would affect their bottom lines. And cost calculations are
tricky: unlike discrete, integrated
passives cannot be sorted for yield and value precision; one bad
component may scrap the entire board. And although most analyses suggest that
passive integration can save money, the analyses are very application-specific.
For instance, while there is probably a cost advantage to integrating cell
phones and other small devices having a high density of components, it may not
be cheaper to integrate larger boards, such as those in desktop computers,
where size is not a concern. Complicating matters is the fact that the
surface-mount world is not standing still; discrete components are getting smaller,
cheaper, and more closely packed every year.
The blem of infrastructure
is the usual chicken-and-egg story. When plotted against time, technology
adoption typically takes the form of an S curve, meaning prothat little happens
at first but eventually everyone gets on board.We are at the bottom of the
curve now, but there is evidence that adoption is increasing. About a dozen
products for integrating capacitors are on the market now, which is double the
number a year ago. Still, board manufacturers may be squeamish until there are
enough vendors in the business to guarantee a second source for their materials
and processes should their first choice fail.
The other inhibitors are:
·
Need to
demonstrate the technical viability of integral substrates,including
materials,processes,design and test system.
·
Need to
demonstrate the value or economic justification for substituting discrete
capacitor and resistors with integral technology.
·
Potential
delay to the product development cycle. These passives are usually designed in
the final stages of a product.The economic impact of a product delay could
easily out way any cost saving in size reduction or conversion costs.
·
Integral
passives reduce engineering and manufacturing flexibility.The ability to apply
engineering changes to an integral substrate without delaying the schedule is
critical.
·
Qualification-most
of the processes, materials, vendors and products in this space are not
qualified.
·
Lack of
availability from multiple suppliers.
·
Industry
standards are required to capture the true market potential for this
technology.
DECOUPLING
Decoupling
may be considered as a killer application of integrated passives. Decoupling is
used in high-frequency digital logic circuits, such as in the motherboards of
laptop computers. These circuits place severe demands on power-distribution
systems to supply stable, noise-free power during the clock-driven simultaneous
switching of millions of transistor gates.
Decoupling capacitors help
supply these large current surges, ramping as fast as 500 A/ns, to high-power
microprocessor and logic ICs during the switching portions of clock cycles.
This technique ensures that the logic voltage levels do not drop unacceptably
as a result of the high current demands on the power supply, which may be many
centimeters away and connected by unavoidably resistive and inductive conductor
planes.Between cycles of current demand, the power-distribution system
recharges these capacitors in preparation for the next switching cycle. With
ever-increasing clock speeds, decreasing power supply voltage, and increasing
current demand, designers are finding it harder and harder to supply
low-impedance, noise-free power to ICs. The main problem is that decoupling
capacitors can't deliver charge quickly, because of their intrinsic inductance.
Decoupling is an obvious
first application for integrated capacitors for two reasons: they won't take up
valuable real estate near the power-hungry microprocessor, and their electrical
performance is superior in this application by virtue of their extremely low
parasitic inductance. Especially on digital circuit boards, surface-mounted
capacitors surround the big ICs, often on both sides of the board. Since the
speed of the system is often limited by memory access times, eliminating the
capacitors from the surface and moving memory closer to the microprocessors
would result in a smaller and faster system.
Though special discrete
capacitors are being built with fairly low inductance, none of them can compare
with an integrated parallel-plate capacitor using a thin dielectric located
between the power and ground planes (conductor coated layers of the board
dedicated to either the ground or power supply). For example, thin-film devices
that we built on flex at the University
of Arkansas Fayetteville
THE INTERMEDIATE STEP
Before true integrated
passives take hold, widespread use of passive arrays can be seen, in which
multiple similar components (capacitors, say) are formed on the surface of a
substrate and packaged into a single surface mounted device like an IC. We'll
also see more passive networks, which combine different kinds of passives in
one package. These networks include devices internally connected to form simple
circuits such as filters, terminators, or voltage dividers. In either case, one
mounting operation replaces many and the overall footprint of the circuit is
much smaller. These arrays and networks are a middle ground—not fully discrete
but not fully integrated within a circuit board. They bring some of the
advantages of full integration such as a reduced number of placement
operations, fewer solder joints, and less board space. Many configurations of
arrays and networks are now available in quantity from California Micro
Devices, AVX, and other companies, and custom arrangements are also possible.
Devices from these companies are typically fabricated on a silicon or other
substrate using tried-and-true chip-making processes so the yields are high and
the prices reasonable.
The technique raises some
interesting possibilities. If ICs or other active devices are mounted atop a
passive network, they may form so-called functional modules, such as Bluetooth
or GPS subsystems. For example, a GPS module would include passives and
antennas integrated on a substrate and one or more ICs bound to it, all in a
single chip-scale package. The manufacturer would not have to worry about
learning to design and manufacture GPS systems and could also easily upgrade or
switch vendors.
FUTURE SCOPE
Less than 5 percent of the
trillion-plus passive devices mounted on FR4 and flex boards this year will be
surface-mounted passive arrays and passive networks, and hardly any passives
will be fully integrated into the circuit board.
The circuit board business,
in the United States
Whether or not passive
integration becomes an industry standard will depend on its economic viability.
Certainly, it is viable for decoupling and, in fact, may be the only way to
handle the future generations of high-power, high-frequency microprocessors.
For discrete replacement in general, though, the best processes and materials
are still being identified. If we find suitable technologies, then passive
integration will probably show a long, steady climb in use the way surface
mounting supplanted through-hole mounting in the 1980s. As the infrastructure,
supply chain, and industry acceptance grow simultaneously, eventually
integration will gain some significant fraction of the total market and put
passives in their place: hidden, ubiquitous, and cheap.
CONCLUSION
The
need for increased product miniaturization and increased product function will
eventually drive the electronic product to increase their use of integral
passive components.Embedded passives offer increased component density beyond
the physical capability of discrete-like devices.They also offer high product
reliability and eventually lower overall system costs via decreased conversion
costs.
Although severely lagging
behind developments in active components, passive component integration is
allowing the development of an assortment of new product offerings. Some of
these items have been possible for several years, but lack of widespread
customer acceptance and high costs have slowed their introduction into the
general marketplace. Some items are yet to be developed. For example, because
several manufacturers can perform both thick- and thin-film manufacturing,
hybrid components combining both technologies may be forthcoming. Passive
component integration is and will continue to be an important contribution in
the development of increasingly smaller medical electronics.
Resistors, inductors, and
capacitors are disappearing from view, integrated into the circuit board
itself. Passive integration may be the only way to handle the future
generations of high-power, high-frequency microprocessors.
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