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Fixed resistors − developments meeting market needs
Though all performing the same basic function,
today's fixed resistors come in a wide range of
technologies and executions to meet the equally wide
ranging requirements of modern industry. A clear
understanding of their characteristics, strengths and
limitations is therefore a useful aid to designers
wishing to achieve optimum performance and
reliability from their circuits.
Another important element stimulating resistor
development has been cost, especially in the consumer industries. However, the drive for ever lower
costs in the consumer sectors has always been in
conflict with the drive for higher performance in the
industrial sectors, and as each new technology was
introduced to further reduce costs, the industrial
sectors would look closely at it to determine if it
would yield some special features useful for specific
applications. Inevitably, of course, new technologies
also introduced drawbacks of their own resulting, in
many instances, in continued use of the older
technologies.
From 'standard' to 'application specific'
resistors
When the first resistors were introduced last century,
principally to meet the needs of the then rapidly
growing radio telegraphy industry, reliability and ease
of manufacture were the main requirements. Those
early products were constructed of carbon compositions compressed into tubular containers and fitted
with end-caps. Another early resistor was the wirewound resistor in which a resistive wire was spiralled
around a tubular body.
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Both the carbon-composite and wire-wound resistors
were manufactured individually and it wasn't until
the introduction of bulk processing that real breakthroughs came in manufacturability, reliability and
cost reductions. The first resistor to be manufactured
in a bulk process was the carbon-film resistor.
Though the process brought significant cost advantages, it also led to higher temperature coefficients
than those exhibited by wire-wound products. For
some applications, of course, low temperature
coefficient is important and for these applications
wire-wound resistors continues to be used. This was
the first application-specific requirement of a resistor.
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So from a standard product performing a standard
function, today we see a broad range of fixed-resistor
technologies and executions, all tailored to specific
application areas (Fig.1).
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"Standard"
resistors
Fig.1 The standard resistor developed into a broad range of
application-specific types
From 'leaded' to 'surface-mount' resistors
By the late 1970s, the reliability of resistors was so
high that the major cause of failure was the mounting. This was chiefly because the flexible leads of the
components caused stresses on the solder joints
which subsequently led to cracks in the solder.
Later, new resistor materials were developed such as
metal film to further improve reliability and manufacturability.
Later still, the development of radar and multichannel communication stimulated additional requirements such as high precision, and with the
arrival of portable equipment, size too became
important.
The introduction of surface-mount technology in the
1980s by companies such as Philips Components
greatly reduced this problem. By eliminating the
flexible leads, surface-mount technology allowed for
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much stronger solder joints and greatly enhanced
reliability. Surface-mount technology brought other
advantages too such as easier, cheaper mounting. It
also offered greater possibilities for miniaturization
since not only could the surface-mount devices be
more densely packed onto the PC board, but the
components themselves, without the need for external leads, could be smaller.
available in surface-mount, the initial penetration of
surface-mount technology was accomplished relatively
easily. A problem arises, however, when industrial
equipment manufacturers wish to move over to full
surface mount since many of the functions in their
equipment require application-specific resistors and
these are currently available only in leaded versions.
The future will, however, see this gap being filled as
companies such as Philips Components address this
problem.
The result of all this is that today almost 70% of all
resistors are in surface-mount execution, and this
figure is still rising.
Resistor technologies and their principal applications
The technology gap − application-specific surfacemount resistors
The first large-scale users of surface-mount resistors
were manufacturers of consumer electronics equipment. These use mostly standard resistors, and low
price being an important driver in this area, they
naturally went for the most cost-effective solution.
During the development of resistors, various manufacturing methods have been adopted, each offering
specific advantages and limitations. Moreover, each
specific part of a resistor such as its resistive element
and terminations will influence its performance in a
given application. Here we give an overview of the
advantages and limitations of the most commonly
used technologies.
So to protect their market position, manufacturers of
leaded resistors targeted their products at the application-specific industrial and professional sectors. This
helped to advance leaded-resistor technology but also
meant that standard leaded resistors were almost
entirely supplanted by surface-mount versions.
Let's consider each in turn.
General industrial
driven trend
Application
specific
Consumer
driven trend
Leaded
Every resistor comprises the following elements:
− resistive element
− resistor body
− contacts
− protective means (encapsulation etc.).
Resistive element
The resistive element can be a solid material such as
a metal strip, metal wire or compressed carbon
composite. It can also be in the form of a metal
film, metal-glaze or carbon film on a carrier (usually
a ceramic body). All these materials offer advantages
and disadvantages.
General
purpose
SMD
Fig.2 The trend toward application specific surface-mount
resistors is being driven by general industrial market segments
that have consumer characteristics, i.e. telecom, computers,
automotive
Resistors with solid elements can generally absorb
high pulse loadings. Film resistors, on the other
hand, are less expensive to manufacture and metal
resistors generally have more predictable and controllable temperature coefficients. These days we see
high-ohmic carbon-composite resistors increasingly
being replaced by metal-glaze resistors as the performance of metal glaze approaches that of carbon
composite.
Today, however, improved capabilities together with
increasing pressures on price in the industrial sectors
has compelled manufactures of industrial equipment
to look seriously at surface-mount components. Since
industrial equipment uses mostly standard resistors
which, thanks to the consumer market, were readily
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Tolerances are strongly influenced by the manufacturing method though material properties are also
important. Metal-film generally offers the narrowest
tolerances though metal-strip and wire-wound
resistors can also offer narrow tolerances. With these
latter two technologies, however, narrow tolerances
are more difficult to achieve. Moreover, with wirewound resistors the material is again a critical factor.
Resistor contacts
For resistors with rectangular ceramic bodies or
metal-strip resistive elements, contact with the
resistive element is made via terminations fixed
directly to the body. For all other resistor types,
leaded contacts are used. A major disadvantage of
the termination method is that heat generated in the
resistive element is conducted directly to the solder
joint, limiting the maximum power that can be
applied to the resistor. Leaded contacts, on the other
hand, must be attached to the resistor body via caps
which can be relatively expensive.
The (current) noise is also strongly influenced by the
material of the resistive element: the more homogeneous the material, the lower the noise. Trimming,
in particular, degrades noise behaviour as it leads to
cracks in the material that can be a source of noise.
Lacquering/encapsulation
The resistive element is generally lacquered to
protect it from the environment. Lacquer is,
however, not used when a leaded resistor is adapted
for surface-mount and with resistors for high-power
applications. In the first example, standard leaded
resistors are removed from the production cycle
before lacquering. They are then covered in a
moulded plastic case after which the leads are
flattened and bent. In the second example, the
resistor is fitted into a ceramic case to improve heat
transfer to the environment. Where high power
dissipation and small size are important, the resistors
are fitted into metal cases attached to heatsinks.
The table compares performance criteria of the main
resistor technologies.
Fixed- resistor performance criteria compared
resistive
element
resistance
pulse-load TC
capability
noise price
metal strip
very low
+++
5 − 15
++
−
wire-wound low/medium +++
100
+
0
carbon
composite
medium
+++
5000
−
0
metal-film
medium
+
50 − 200 +
0
++
> 300
−
−
200
0
+/−
carbon-film medium
metal-glaze
medium/high +/0
Expanding possibilities
Each new development within each of the basic
resistor functions described above resulted in a series
of new possibilities. For example, a solution to the
problem of measuring low-ohmic values during
production also led to an important breakthrough
enabling the production of surface-mount (1206)
low-ohmic resistors. Later the same technological
breakthrough also led to the development of lowohmic surface-mount (1218) power resistors. So
breakthroughs in one area can lead to important
developments in several related areas.
Resistor body
The body of the resistor may be the resistive element
itself, or a separate body supporting the element, for
example, a rod around which the element is spiralled
or a rectangular block on which the element is
deposited. The body is usually of ceramic but other
materials such as glass fibre, glass or silicon are
sometimes used. Rods are very popular with leaded
resistors and with the exception of carbon composite
resistors, all leaded resistors currently use rods. A
major disadvantage is poor high-frequency behaviour
due to inductive effects caused by the spiral shape of
the resistive element.
In this respect, Philips has been actively working on
metal-film technology for surface-mount resistors.
The most important result of this work so far has
been its successful MPC01 high-precision surfacemount (1206) resistor series. As this technology
matures, other products will also benefit, such as the
MPC01's little brother due to be introduced later
this year and offering the same capabilities as the
The size of the body strongly influences the resistor's
power handling capacity since, as may be expected,
the larger the body the more heat it can dissipate
and hence the lower the body temperature. So
resistors intended for high power applications need
larger bodies than standard or low-power resistors.
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MCP01 but in size 0805. Besides its precision
properties, other positive features of this new
technology are being explored such as its pulse
handling capabilities leading to highly pulse-resistant
surface-mount resistors and its well-defined fusing
characteristics which could open the way to the
development of low-ohmic fusible resistors.
Power
Power resistors are usually larger than standard
resistors and are often distinguished from them by
their coating, which can withstand higher temperatures to accommodate greater power dissipation.
General-purpose power resistors come in leaded
metal-film technology allowing for dissipation up to
3 W, and in surface-mount thick-film technology
with dissipations up to 1 W. Other technologies are
used for higher power dissipation or to satisfy other
power-related requirements − very low ohmic values
for current sensing, for instance, or good pulse-load
behaviour for transient-sensitive circuitry. For
surface-mount resistors, the major factor limiting
power handling is not the resistor itself or its coating
but the temperature of the solder joint.
Principal application areas
For each application, a proper choice of resistive
element, body, contact and lacquering/encapsulation
needs to be made. There are, however, some general
guidelines that can be followed.
The principal applications can be roughly divided
into 6 categories: standard or general-purpose, power,
precision, grounding, fusible and trimmable.
Standard resistors
Carbon composite
Standard resistors are available in both leaded and
surface-mount executions though 80% of the market
is currently in surface mount. Leaded resistors
employ carbon- or metal-film technology while
surface-mount resistors employ metal-glaze
technology. The resistors normally exhibit tolerances
from 2% to 5%, dissipation up to 0.5 W and TCs
of between 100 to 200. The TC can be even higher
for carbon-film resistors and their noise behaviour is
inferior to metal-film. So for leaded resistors, metalfilm technology is often preferred.
Pulse
resistant
Leaded
Standard
Thick film
Wirewound
Thick film
Application
specific
Power
Thick film
Metal film
Leaded
General
purpose
SMD
Fig.4 Power resistors require different technologies for specific
power-related applications
With its 1218-sized PRC201 power series, Philips
has made an important contribution to the field of
surface-mount power resistors and is now planning
to introduce metal-film surface-mount products
offering high pulse-load capability.
Application
specific
Carbon film
Metal film
Current
sensing
Precision
Precision resistors are normally in leaded metal-film
technology. Surface-mount versions are currently
available with TCs as low as 25 and tolerances down
to 0.1%, but new developments are leading to even
narrower tolerances and better TC values. Philips is
currently expanding its range of surface-mount highprecision resistors with the introduction of its 0805
sized MPC11. Metal-film is not ideal for low-ohmic
precision resistors for current sensing, however, and
metal-strips (for very low-ohmic values) or wirewound leaded resistors (for power applications) are
often used instead.
General
purpose
SMD
Fig.3 Standard resistors are widely available in surface mount
execution
Philips is highly active in this area and currently
offers a broad range of standard resistors in both
leaded and surface-mount execution.
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Wirewound
Current
sensing
Metal strip
Fusible resistors
Coated with a special non-flammable lacquer, fusible
resistors are produced in metal-film technology since
this allows for the introduction of well-defined
fusing characteristics. Not only do they provide the
protection of a true fuse, they also function as a
standard resistor under normal operation. Formerly
available only as leaded products, surface-mount
(thick-film) versions have recently become available
with the development of new resistive pastes and
improved laser-trimming techniques. Philips offers
the most complete range of fusible resistors in the
industry and is currently developing low-ohmic
surface-mount types.
Application
specific
Precision
Metal film
General
purpose
Leaded
SMD
Fig.5 Precision is still the domain of leaded resistors but surfacemount technology is developing fast in this direction
Grounding
Though standard high-ohmic (> 100 kΩ) resistors
can provide circuit coupling with low leakage
current, application-specific types are normally
needed. Since it's commonly high-voltage
environments that require grounding, special highvoltage types are necessary, usually leaded metal-glass
resistors. In this respect, Philips' metal-glass VR
series offers good stability and excellent high-voltage
behaviour.
Metal film
Leaded
Carbon film
Metal film
Leaded
Standard
Thick film
SMD
Fig.7 Fusible resistors have recently becoome available in surfacemount execution
Trimmable resistors
Available only in surface-mount, trimmable resistors
perform the basic function of a potentiometer with a
limited adjustment range but with the important
advantages of being small and relatively inexpensive.
Though laser trimming is necessary, for large-scale
production this is not a disadvantage since the
purchase of a laser system by the circuit manufacturer will then prove cost-effective. Incidentally,
used untrimmed, trimmable resistors can offer advantages such as significantly improved pulse-load
capability and noise.
Application
specific
Circuit
Grounding
Application
specific
General
purpose
Surface-mount types are not available for these
applications since the very close terminal distances
would result in sparking across the terminals.
Metal glaze
Thick film
Fusible
General
purpose
SMD
Fig.6 High-voltage resistors are not available in surface-mount
execution since the contact distance needs to be greater than the
open air spark voltage (1 kV/mm)
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Thick film
Trimmable
Application
specific
Cost
Trimmable
Potentiometer
General
purpose
Leaded
SMD
Volume
Fig.8 Trimmable resistors are an attractive alternative to
potentiometers
Fig.8 Adjustment costs per unit. Initial costs of laser-trimming
equipment is high but is paid back when trimmable resistors are
used in large volume
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