Tantalum
capacitor options weighed
By Gina Roos
EE
Times
(02/06/01,
1:17 p.m. EST)
Component manufacturers, handicapped by shortages of
tantalum-chip capacitors for the past 18 months, are continuing to
explore replacement alternatives, including the use of
high-capacitance ceramic capacitors and aluminum electrolytic
capacitors so they're not caught short again. New technologies
such as the capacitive circuit architecture from X2Y Attenuators
LLC (Erie, Pa.) and other materials such as niobium are also being
considered as viable replacements for tantalum-chip capacitors.
Customers are using alternatives in whatever form suits
their needs, said Sandy Beck, vice president of worldwide
marketing at Kemet Corp. (Greenville, S.C.). Engineers have been
using replacements for tantalum-chip capacitors for years but that
practice has accelerated with the scarcity of tantalum capacitors,
said Eric Pratt, director of information services at iSuppli Corp
(El Segundo, Calif.). "There's nothing that forces the issue
[of alternatives] quicker than when an engineer can't get a
tantalum-chip capacitor," he added.
Many board manufacturers and designers are now setting up
their pads to accommodate either a tantalum or ceramic-chip
capacitor, which eliminates the conversion costs of shifting from
one to the other, Pratt said. As tantalum-chip capacitors become
more available in the smaller case sizes, some of the suppliers
will convert back, he said.
Passive component manufacturers, meanwhile, are planning
ahead for possible long-term shortages by strengthening their
high-capacitance ceramic capacitor and surface-mount
aluminum-capacitor lines so they can replace tantalum capacitors
in some applications.
Before a conversion can take place, though, manufacturers
must make sure that the replacement component offers capacitance
that is high enough to meet the design engineer's requirements,
said Frank W. Guiney, group product manager at Murata Electronics
North America Inc. (Smyrna, Ga.). The recent shortage spurred
numerous redesigns that required moving to ceramic capacitors,
Guiney said. Although base-metal multilayer ceramic chip
capacitors have encroached on the 1- to 10-microfarad range of
tantalum capacitors, the technical specs of the base-metal
products are not as good as tantalum capacitors, particularly with
regard to stability, said Glyndwr Smith, assistant to the CEO and
senior vice president at Vishay Intertechnology Inc. (Malvern,
Pa.).
If a design engineer has an option, he or she will use the
product that is best suited for a particular circuit application,
Smith said. Designers are evaluating each circuit to determine if
using a replacement capacitor is appropriate. In general, though,
they find that based upon the specs, a tantalum capacitor still
offers the best functionality for its size, he said.
Still, ceramic capacitor manufacturers are moving ahead with
the development of high-capacitance ceramic capacitors as viable
alternatives to tantalum. Murata Electronics, for instance,
doubled production capacity last year in Japan for its new GRM
line of high-capacitance ceramic capacitors after realizing that
these replacements for tantalum-chip capacitors are increasingly
popular with cell phone and computer OEMs. These new capacitors
can be used in several applications including coupling,
decoupling, bypassing and signal smoothing.
Benefits of Murata's tantalum replacement devices include
nonpolarization and low ESR. The high-capacitance ceramic caps
offer a lower ESR value, typically 0.02 ohms compared with an
average 1 ohm for tantalums.
The lower ESR allows a ceramic capacitor with a lower
capacitance value to easily replace a tantalum capacitor without
any loss of functionality, according to Murata.
Average lead times for the standard GRM Series of ceramic
capacitors, which offers a capacitance range from 0.5 picofarad up
to 47 microfarads, is about 10 to 12 weeks. The devices are
available in various package sizes including 1210, 1206, 0805,
0603 and 0402.
The tantalum A-case size crosses to a ceramic cap in a
1206-size package and the B-case crosses to the 1210 ceramic
equivalent. The parts are nearly dimensionally equivalent but are
not necessarily drop-in replacements due to small dimensional
differences as well as differences in electrical performance,
Guiney said. C-case and D-case sizes do not have crossovers that
have similar dimensions to ceramic capacitors, Guiney said.
Kemet Corp. also offers ceramic capacitors. The company is
shipping ceramic-chip capacitors for the X5R dielectric up to 10
microfarads and up to 22 microfarads for the Y5V dielectric.
Ceramic capacitors are probably the most frequently used
substitute for tantalum-chip capacitors, VP Beck said.
Aluminum-electrolytic capacitors are also viable options in
some cases, according to Rich Schuster, president of NIC
Components Corp. (Melville, N.Y.). Schuster said he's noticing a
trend where low-ESR and low-leakage surface-mount aluminum
electrolytic capacitors are being used in applications where space
is not a critical factor. These devices typically have a much
higher profile than tantalum capacitors. For example, the height
difference may be as much as 2 millimeters. There are other
trade-offs, too, such as life cycle. Although tantalum caps may
offer a longer life, aluminum-electrolytic capacitors offer a
higher capacitance/voltage (CV) range.
Surface-mount aluminum-electrolytic capacitors have been
available for several years. But once manufacturers recognized the
opportunity to replace tantalum caps in some applications, they
expanded into higher values and smaller packages as well as
increased production capacity.
For more technical information on using surface-mount
aluminum-electrolytic caps, visit NIC Components' Web page at www.niccomp.com/help/tantalum-xref.asp
. You'll find two papers: "Alternates to SMT Ta Chip Caps"
and "Using
SMT Aluminum E-cap on Existing SMT Ta chip E-cap Land Pattern".
Another alternative to using tantalum capacitors is solid
polymer aluminum electrolytic capacitors. Although there are less
than a handful of suppliers for these capacitors and very limited
production capacity, some users nonetheless are designing in these
devices. Kemet's low ESR AO-Cap devices are available in a
capacitance range of 47 to 220 microfarads at 6 volts and less.
These polymer aluminum electrolytic capacitors can plug-in to
replace low-voltage and low-ESR tantalum-chip capacitors, in
applications where capacitance, ESR and voltages are suitable for
power-supply decoupling, Beck said.
One of the most attractive features of polymer aluminum
electrolytic capacitors is that they require less voltage
de-rating than tantalum caps, Beck said. The typical industry
de-rating factor for a tantalum chip is 50 percent. An engineer,
for instance, will typically use a 16-volt-rated tantalum
capacitor for a 6-volt app.
Some up-and-coming technologies could also work as
replacements for tantalum caps. [link
to cross reference]
A new capacitive circuit architecture from X2Y Attenuators
seems poised to offer higher performance than current decoupling
and filtering solutions. It offers low ESR and low ESL. The
ultralow ESR characteristic makes them suitable as tantalum cap
replacements, according to the company. For instance, a 100 nf x
2Y in an x7R dielectric has an ESR rating of 2.4 milliohms. The
3-terminal capacitive X2Y component has an "x"
decoupling capacitor and two "y" bypass capacitors,
which means it performs two functions in a single package. The
devices from X2Y reduce or eliminate supporting inductors,
capacitors, resistors and other circuitry, which reduces component
count and placement costs. The new architecture can be used to
manufacture a variety of devices including capacitors, decouplers,
transient voltage suppressors and filters.
The ceramic-chip version is available in EIA sizes from 0603
to 1206 and capacitance values ranging from less than 10
picofarads to 10 microfarads. The high-cap value chips can replace
Ta capacitors, according to X2Y president and CEO Tony Anthony.
Another option that has gained momentum as a result of the
tantalum-cap shortage is niobium. Using niobium as an alternative
to tantalum is not a brand new idea; but it has gained credibility
now that the capacitance of niobium powder is equivalent to 70,000
to 100,000 CV/gram. That matches the CV/gram values of
state-of-the-art tantalum powders, said Bill Serjak, niobium
product manager at materials supplier H.C. Starck Inc. (Newton,
Mass.). Two new developments in tantalum powder processing-the
reduction of powder oxygen, and treatments that add nitrogen to
the surface and bulk of the powder-have proved to be crucial
factors in reaching capacitor-grade niobium powder, Serjak said.
Niobium caps can be made in the form of a surface-mount,
solid-electrolyte chip capacitor that will provide better
volumetric efficiency than aluminum electrolytic capacitors,
Serjak said. Niobium caps could also be used as a substitute for
OS-CON-type aluminum capacitors, and large niobium chip caps could
replace some tantalum apps where capacitance is more critical than
the low dc leakage provided by Ta caps.
H.C. Starck has sent niobium powder samples to several
companies, and preliminary results indicate that manufacturers
will be able to produce a capacitor whose qualities approach those
of tantalum, Serjak said. However, leakage may not be as good and
it's not clear how it rates in terms of long-term stability, he
said.
While the jury is still out on niobium's technical specs,
availability and pricing look good. The worldwide supply of
niobium is almost unlimited, Serjak said, and it costs less than
half the price of tantalum.
COMPANY CONTACTS
NIC Components Corp.
(631) 396-7500
www.niccomp.com
EETInfo No. 633
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