Updated July 8,
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film resistor fabrication
Thick film components are made
by screening and firing conductor and resistor "pastes"
onto hard substrates, usually alumina. In thick-film technology,
alumina of reduced purity is used (95%) compared to thin-film (99%).
Firing temperature is usually around 850 degrees C. The resistor
materiel is carbon-based (that's why the resistors appear black.)
Thick film resistors can be made into chip resistors of various
sizes for surface mount technology (SMT), or as part of custom thick-film
hybrid networks, or even as part of a low-temperature co-fired ceramic
(LTCC) part. When you buy inexpensive surface-mount chip resistors,
they are almost always made in a thick film process.
Thick film is an additive process,
as opposed to most thin-film processes which are subtractive (chemical
etching). In some thick film parts, before resistors are deposited,
a conductor pattern is printed and fired first. The conductor and
resistor materials are deposited using a screen-printing process
using a squeegee, similar to how Grateful Dead tee-shirts are made.
Because it is a "low-tech" process compared to thin-film
resistors, and the substrate material is lower cost, thick film
resistors are almost always less expensive than thin-film.
Sheet resistance of thick-film
resistors can be much higher than thin films, from 20 ohms per square
up to 500K ohms per square. On a custom job it is possible to use
multiple pastes with widely-varying sheet resistivity, so that almost
any resistance value can be accommodated, each in nearly the same
small area. Thick film resistor values can be
laser trimmed. Typical processes add an overcoat of glass, fired
at 600C, to protect the resistor.
Chip resistors are used by the
billions in surface-mount technology (SMT) assemblies. They come
in standard sizes, in the USA the sizes are have been standardized
to dimensions in mils (thousandths of an inch). Other countries
use sizes in millimeters. Sizes that are most common on microwave
surface-mount boards are 0603 (60 x 30 mils) and 0805 (80 x 50 mils).
Smaller resistors are available in 0402 and even 0201 formats, but
handling such small parts can make your surface-mount hardware more
expensive. Larger resistors such as 1205 will start to cause trouble
in microwave circuits at X-band and above, because they start to
be large enough so that they are no longer can be modeled as
The ends of thick-film chip resistors
are coated with conductor "wraparounds" as shown in light
blue the figure below. The metal that is used is most often something
that is easily solderable, and often a solder is wetted to the surface
to facilitate SMT assembly.
What are the wraparounds for?
Chip resistors with wraparounds can be mounted with the resistor
surface (shown in orange) either up or down. Although most "low-frequency"
boards mount the resistor face-up so that they can be easily inspected,
in microwave applications resistor-down mounting can mean higher
Standard sizes for thick film
chip resistors are given below, along with approximate power rating
when mounted on a printed circuit board such as FR-4.
||20 x 10
||40 x 20
||60 x 30
||80 x 50
||120 x 60
||200 x 100
||250 x 120
The rated power dissipation is
established by the maximum temperature that the resistor will see.
This is a function of the part, as well as how it is mounted. This
is a very complicated subject, and often consumes tens of thousands
of dollars of analysis. Sometime soon we will provide we you the
High-power resistors can employ
high-conductivity substrates, such as BeO or AlN, which spread heat
much better than alumina and reduce temperature rise. The downside
to BeO is that it is a known carcinogen, so you should avoid using
it in a design whenever possible. AlN is more inert, but doesn't
dissipate as well as BeO.
Temperature coefficient of resistivity
(TCR) for thick film resistors is usually 100 to 250 ppm. Thin-film
resistors behave better over temperature.
Resistors must be linearly derated
over temperature. We'll add a figure later that shows what this