Nichrome Resistors Deliver
Reliability at Lower Cost
By Kory Schroeder, Product Engineering Manager,
Stackpole Electronics, Raleigh, N.C.
An alternative to expensive tantalum nitride
resistors, these precision, moisture-resistant
components can perform voltage division or
setting of gain in power-supply circuits.
M
etal exposed to moisture generally results
in undesirable corrosion. Just as it can
occur on the terminals of a car battery,
it also can occur on a precision thin-film
resistor in a critical portion of a circuit.
For a battery, the simple solution is regular maintenance
and a wire brush. However, until recently, the only proven
solution for nichrome resistors was to replace them with
thin-film resistors having tantalum nitride elements.
The self-passivating property of tantalum nitride elements made them necessary for applications where prolonged exposure to moisture is likely, such as automotive
or industrial electronics. However, the cost of these resis-
tors limited their use to only critical circuits where they
were absolutely necessary. Recently, new materials and
processing techniques have changed the thin-film resistor
landscape and allowed certain nichrome-resistor types to
serve as more cost-effective alternatives to tantalum nitride
resistors.
It is possible for any nichrome resistor element to experience corrosion in a humid atmosphere. However, there are
certain conditions that act as catalysts and greatly increase
the chance for excessive resistance shift or even open failure.
The presence of electrical potential is definitely a catalyst,
but it is not the only factor contributing to corrosion.
High resistance values appear to be more susceptible
to corrosion, which is logical because
they are more likely to be operated at
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or near their maximum voltage and not
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at maximum power. A resistor running
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at rated power usually generates enough
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heat to prevent the accumulation of surface water. Conversely, this same logic
implies that a part of any resistance
value running from 10% to 20% of its
rated power will have a higher susceptibility to corrosion.
But even with these factors present, the only failures due to nichrome
resistor corrosion at the customer or
manufacturer levels also involved a flaw,
misalignment or pinhole in the outer
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protective coating. The manufacture
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of these precision-resistor types, while
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not at the extreme speeds of commercial
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thick-film chips, are still done in such
a way that an imperfection as mentioned
previously is rare but still possible. If
Fig. 1. Adding silicon-based glasslike material under the final passivation coating as well as to
the resistive element itself significantly reduces susceptibility to corrosion in nichrome resistors. the previously mentioned conditions
Power Electronics Technology August 2006
34
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were present for a resistor with this type of passivation flaw,
it could result in severe electrical damage.
One might then suggest that a solution would be to
slow down the passivation processes to the point where no
misalignments or pinholes are possible. To a certain degree,
some manufacturers do this. But in the real world of today’s
manufacturing, to guarantee no passivation miscues would
be impractical because the manufacturing costs for this nichrome part would then increase to essentially the same cost
as the tantalum nitride equivalents that the design engineer
is trying to replace.
A better solution is to introduce materials that are more
glasslike in nature, both as an addition to the nichrome resistive element itself and as a secondary line of defense against
moisture under the final passivation coating. The addition
of small amounts of this silicon-based material to the nichrome element itself reduces its susceptibility to corrosion.
As long as the amounts are kept to a minimum, the excellent
electrical and stability properties that a nichrome resistive
element provides are unchanged. The physical structure of a
nichrome resistor with these features is shown in Fig. 1.
Additionally, the use of a silicon-based material as an
initial protective layer against moisture under the final passivation layer makes the chance of a pinhole through both
layers so remote that it is essentially irrelevant. Both of these
additions increase cost only in small increments. Another
property that can help minimize the effects of moisture
on this type of part is substrate smoothness. A smoother
surface will have fewer valleys where the condensation can
collect or pool up. Again, if one of these valleys were to line
up with a passivation flaw or hole, chances for corrosion
are increased.
All of these techniques are easily incorporated into the
manufacturing of nichrome resistors and lead to products
that show resistance shifts over humidity testing that are
equal to or even less than those for a tantalum nitride element. Examples of these new devices are shown in Fig. 2.
The typical humidity test parameters used in the industry are
done at 85°C, 85% or greater relative humidity, 10% rated
power, and at least 1000 hours in length.
Tables 1 and 2 show the results of a moisture performance
test comparing standard nichrome and tantalum nitride
resistors to SEI’s new special-passivation NiCrSi devices.
The procedure for both tests were as follows:
• Place a drop of deionized water on each resistive
element
• Apply 9 Vdc to each resistor for 4 minutes
• Visually inspect each resistor
• Electrically test each resistor.
As evident from the results of these tests, the processing
applied to SEI passivated nichrome resistors results in a
significant improvement over standard nichrome elements,
as well as superior performance over tantalum nitride
technology.
All market segments have price pressure, and power
supplies depending on the end application are no different.
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electronica Munich, Germany
November 14 to 17, 2006
Hall B5, Stand B5.506
Power Electronics Technology
just everywhere
…
NICHROME RESISTORS
SEI passivated (400 )
Fig 2. These precision nichrome resistors
were manufactured with easily incorporated techniques that provide moisture
resistance comparable to tantalum
nitride devices.
Power supplies use precision resistors
at or near rated power more often than
many other applications. There are still
many occurrences of precision resistors used in voltage division, or to set
gain or other types of feedback where
the need for reliable, cost-effective
precision resistors are critical.
One example would be automotive
power circuits. Automotive power design engineers must take into account
all reasonable weather conditions and
environmental effects. Given the challenges these engineers face, resistor cost
and moisture sensitivity would likely
not be the first issues considered. This
new technology offers a significant
advantage by reducing potential concern over resistor cost and moisture
sensitivity.
Given their controlled operating
environment, power circuitry for the
medical industry is presumably less
vulnerable to moisture than automotive circuitry. However, medical devices
typically have a much smaller margin
for error. Furthermore, there is a growing market trend to make medical instruments smaller and portable so they
can be included onboard ambulances.
The reliability of precision resistors
in these applications, with respect to
all environmental conditions, then
becomes paramount.
Instrumentation is another obvious application where there is very
little room for environmental variance,
especially in aerospace applications. Instrumentation aboard boats and other
marine vehicles also would fall under
this category. This would include both
the circuitry used for the control systems and also gauges that measure and
report vehicle dynamics. For aerospace
applications, those dynamics could
Power Electronics Technology August 2006
Standard nichrome (1 k)
R1
R2
R%
R1
R2
R%
400.584
400.615
0.008
1.000227
1.54421
54.386
400.77
400.831
0.015
1.000907
1.04957
4.862
400.606
400.649
0.011
1.000146
1.05658
5.643
Table 1. SEI’s RNCS series-NiCrSi with special passivation versus standard nichrome technology.
SEI passivated (400 )
Tantalum nitride (1 k)
R1
R2
R%
R1
R2
R%
400.584
400.615
0.008
1.000763
1.000928
0.016
400.77
400.831
0.015
1.000827
1.000997
0.017
400.606
400.649
0.011
1.000279
1.000469
0.019
Table 2. SEI’s RNCS series-NiCrSi with special passivation versus tantalum nitride technology.
include speed, altitude, pitch, banking,
fuel, landing-gear position and radar
data. For nautical vessels, the instrumentation applications may include
speed, weather conditions, depth-finder
data, and fuel and trim settings, to
name a few. Both of these application
segments may also use instrumentation
for direction or GPS positioning.
These applications represent the
most extreme examples within the
range of mobile electronic equipment
having to cope with uncertain operating environments. For any device
fitting these criteria, whether portable
such as a PDA or embedded in a mobile platform such as those previously
mentioned, the concerns are largely
similar. While the space constraints
for an automobile are obviously quite
different than those for a PDA, both
will be required to provide predictable,
reliable power for extended periods of
time, regardless of geographic location. In that sense, designers for both
may be forced into a requirement for
an environmentally robust precision
resistor.
The communications industry of
today is being pushed to faster and
more efficient levels than previously
thought possible. If the equipment for
this voice and data communication
is housed within an office building,
then there will obviously be fewer
concerns about environmental conditions. However, if this equipment is at
the base of a remote communications
tower, the potential for weather-related
issues certainly exists. This commu36
nications hub could be a microwave
or fiber-optic link, but in either case
this high-speed communications
equipment would definitely need the
reliability that a precision moisturetolerant resistor would provide. And
given increasing equipment speeds
and corporate downsizing, cost is also
becoming increasingly important to
this market segment.
Industrial power and controls are
also another application where operating conditions can be very unpredictable. Elevator and escalator controls
are typically inside a controlled atmosphere, but they also may be exposed to
the elements at places such as parking
lots. Other types of industrial power
or controls such as boiler controls,
machinery such as ovens, fans, and
ventilation, and motors of many different types must consider both the
allowable price point as well as the
long-term reliability of their design.
Finally, this new precision nichrome resistor is a good solution for
companies with insufficient technical
manpower to determine the long-term
reliability of their design. It provides
assurance that moisture corrosion
will not be an issue in the design. And
it accomplishes this for only a minimal
increase to the cost of conventional
nichrome resistors, as opposed to a
tantalum nitride solution, which being
typically 40% to 70% higher in cost
would require the design engineer to
make absolutely certain that the application needed such a part before
designing it in.
PETech
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