What is a Thick Film Resistor : Working & Its Applications

Thick Film Resistor`this resistor cannot be changed similar to a variable resistor because its resistance value can be determined at manufacturing time itself. The classification of these resistors can be done based on the process of manufacturing & also materials utilized in their manufacturing like carbon, wire wound, thin-film, and thick film resistor. So this article discusses one of the types of fixed resistor namely thick film resistor – working and its applications.

What is a Thick Film Resistor?

Thick film resistor definition: It is  the resistor which is characterized by a thick film resistive layer over a ceramic base. As compared to the thin-film resistor, this resistor’s appearance is similar but their manufacturing procedure and properties are not the same. The thickness of the thick film resistor is 1000 times thicker than the thin-film resistor.

 

Properties

The properties of thick film resistors include the following.

• The thickness of the film is ±100.
• The process used for manufacturing is screen & stencil printing.
• Trimming used is Abrasive or Laser.
• The resistive material used is Ruthenium oxide paste
• Resistance values range from 1 to 100 M
• Tolerance is ±1 to ±5%.
• The temperature coefficient is ±50 – ±200 ppm/°C.
• Max operating temperature is 155°C.
• Max operating voltage range is 50 to 200V.
• Non-linearity is >50 dB.
• The current noise is <10 µV/V.
• Power Rating is 1/16 to 1/4W
• More moisture resistance.

Construction

The construction of a thick film resistor can be done by applying the silk-screening conductive paste on top of an insulating substrate. This conductive paste can be fired to make a permanent connection. The paste contains finely divided particles of an inorganic ceramic based material (the resistor element), glass frit and silver. The paste is applied to a ceramic base made from aluminium oxide powder, mixed with glass frit and a small amount of an organic binder to hold the powder together during firing.

These resistors are not expensive in high quantity and are available in small sizes which is especially significant in integrated & hybrid circuits as these resistors can be printed onto the substrate to eliminate board loading as well as soldering steps.

These resistors are used up to 300°C and are also non-magnetic absolutely. So these are used where strong magnetic fields are there like MRI & CT scanners. Nickel or tin-free thick film resistor models are appropriate for the silver epoxy attachment or lead-free soldering.

All these resistors show low VCR or voltage coefficient of resistance and it is a type of property that can be defined as the change within resistance with respect to the applied voltage on a specific range of voltage. Thick film resistors provide the highest resistance values up to 10 tera ohms, high voltage capacity & very high-temperature performance.

Working Principle:

The working principle of a thick film resistor is to allow high resistance values to be printed on a plane or cylindrical substrate either enclosed completely otherwise in different patterns which are used in applications through stable frequencies.

It is a special type of resistor that has added insulation to reduce the amount of heat generated by the electrical current flowing through it. Unlike a conventional resistor, which dissipates heat by conduction, a thick film resistor dissipates heat by convection.

Thick Film Resistor Types

There are three types of thick film resistors like Metal Oxide, Cermet Film & Fusible Resistors.

Metal Oxide Resistor

Metal oxide Resistor is made by oxidizing a Tin Chloride’s thick film on a substrate like a heated glass rod. These resistors come under the fixed form axial resistors family which is similar to Carbon film or Metal film resistors but these resistors use metal oxide instead of a metal film resistance material.

These types of resistors are accessible in an extensive range of resistance through high-temperature strength. Additionally, the operating noise level is extremely low & can be used at maximum voltages. The application fields of this resistor cover medical equipment and telecommunication.

Cermet Oxide Resistors

These resistors are the type of thick film resistors that uses a thick conducting paste. This thick conducting paste is a blend of both metal & ceramic. The internal area of this resistor includes ceramic insulation materials and a metal or carbon alloy layer can be covered around the resistor, arranged within a ceramic-metal or Cermet. These resistors are available in square or rectangular shapes including leads.

The features of these resistors are good temperature stability, decent voltage ratings & low noise. These resistors offer constant operation within high temperatures since their values do not vary when the temperature changes.

Fusible Resistors

Fusible resistors are made by using either thick film or thin film by depositing it onto a ceramic core. By cutting the film of this resistor in a spiral shape with laser cutting or mechanical cutting, different resistance values can be obtained. These resistors are widely used in amplifiers, TV sets, etc.

Thick Film Resistor Failure Modes

The failure modes in thick film resistor occur rarely by the failure of resistive element but generally, failure occurs because of exterior environmental factors like handling issues, electrical & mechanical stresses. So there are different failure modes in thick film resistors like mechanical stress, environmental effects, thermal issues, constant overload, surge conditions, and ESD.

Mechanical Stress

Mechanical stress is one of the failure modes of Thick film resistors and it mainly occurs during the manufacturing process. This failure mode doesn’t affect the resistor directly but these failures can be caused by either inappropriate mounting or vibration of the device. The resistor material’s Micro cracking can cause by compression, vibration, or extension of the resistor because of inappropriate mounting. So this can cause change within the resistance value.

Environmental Effects

Environmental effects mainly include moisture, chemical elements & ambient temperature. If environmental effects are considered throughout the design of the resistor then their potential impact can be decreased. The thick film resistor can be protected from chemical & moisture elements by providing an appropriate coating during the manufacturing process end.

Thermal Issues

The resistor’s most mechanical failure modes can be propagated through heat. So it is very essential to understand the heat dissipation properties of the resistor. The dissipation of heat in a low power resistor can be done through conduction using connections or terminals of components whereas the heat dissipated by a high power resistor can be done through radiation.

Once current is supplied throughout the thick film resistor then it generates heat and the different material’s differential thermal expansions used within the manufacturing process of the resistor induce stresses within the resistor.

The best-known parameter of this resistor is TCR or Temperature Coefficient of Resistance which is used to indicate the stability of a resistor & describes the sensitivity of resistive element to change in temperature.

Overload Conditions

Thick film resistors will stop working in overload pulse conditions as they are incapable to dissipate the heat produced within the resistor device through the electrical energy of the pulse so the pulse amplitude & duration should be understood.

The main element to determine the performance of a thick film resistor is the resistor element’s mass. This is relative to its width multiplied by its surface region. When a surface area is larger, then it results in a higher film mass. So the increased surface area can allow more heat dissipation.

Surge Conditions

The most significant element to determine the surge condition of this resistor is the resistor element’s mass, which is directly proportional to its width multiplied by its outside area. The resistor geometry will also affect its surge withstand capacity. If a surface area is larger, then it results in higher film mass & finally, it improves surge performance.

ESD

The ESD damage mainly occurs because of a direct electric charge transfer from either a charged material or a human body to the resistor device. This damage can be separated into 3 main categories, parametric failure, catastrophic damage & latent damage.

The damage through this failure mode is a latent defect that is not easy to find. The thick film resistor might be degraded partially through ESD but continue to achieve its intended function. But, the possibilities of catastrophic or premature failure of thick film resistor are increased, then the device is exposed particularly to one or more of the above failure modes.

Thick Film Resistor Fabrication Process

The steps involved in the thick film resistor fabrication process include the following.

Substrates Lasering

Thick film resistor substrates are made with Al2O3 (alumina), AlN (aluminum nitride), BeO (beryllium oxide), stainless steel, and even sometimes with polymers, and in rare cases, it is coated with silicon dioxide (SiO2). For thick film resistors, most of the time, 94% or 96% alumina is used as a substrate because it is a very hard material

In the thick-film process, normally one substrate includes many units and the lasering is possible to etch, shape & drill holes. Etching is a lasering method where laser pulses are fired up into the alumina material. Once the material is fired up, then 30 to 50% of the material can be removed otherwise the substrate will get damaged.

Once the etching process is done, the material is shaped into round tubes and after that holes are drilled at the substrate on two sides, usually the size of the holes ranges from 0.15 to 0.2 mm.

Preparation of Ink

Inks are normally prepared by combining the ceramic powders or powders necessary with a ceramic thick film or polymer pastes to create a paste for screen-printing of resistors.

Screen-printing

In the screen printing process, ink is transferred through a pattern using a squeegee otherwise a patterned woven mesh screen. Thick film technology is very helpful in accuracy improvement, integration density, etc.

Drying

After printing the ink, every deposited ink layer is dried at a high temperature like 50 to 200 °C to evaporate the fluid part of the ink & attach the layer momentarily to the substrate.

Firing

A high-temperature firing (>300 °C) is necessary for many of the ceramic, glass & metal inks utilized in thick film processes to attach the layers on the substrate permanently.

Trimming

Once resistors are firing, then they can be trimmed with a precision abrasive cutting technique first. This technique uses a fine abrasive media typically 0.027 mm aluminum oxide. So, the technique achieves extremely high tolerances through no heat & no breaking of the glass frit utilized within the formulation of ink.

Laser Trimming

After the firing process is done, the substrate of resistors is trimmed to the exact value which is known as laser trimming. Many chip resistors are designed with thick-film technology.

In laser trimming, two modes are used active and passive trimming. Active trimming is used to regulate a precise voltage, and frequency whereas passive trimming is used to trim a resistor to a precise value & tolerance.

Separation of Elements

This step is frequently required because several components are placed on a single substrate simultaneously. So, separating these components from each other is necessary for through-wafer dicing.

Integration of Components

At this phase, the components may need to integrate with other electronic components, on a printed circuit board through a soldering or wire bonding process.

Advantages

The advantages of a thick film resistor include the following.

• High resistance values.
• Temperature performance is very high.
• Voltage capability is high.
• Non-inductive inherently.
• High precision & reliability.
• Efficient Package.
• More Compact.
• A wider range of temperatures.
• Packaging is completely encapsulated.
• Economical.

Disadvantages

The disadvantages of a thick film resistor include the following.

• These resistors need to overcoat frequently with borosilicate glass to guard them against chemical attack, environmental effects, etc.
• These are not strong.
• These resistors are very sensitive to electrostatic discharge voltages.

Applications

The applications of thick film resistors include the following.

• These resistors are available almost in every electrical device which has an AC plug or a battery,
• These resistors are most frequently used in electronic & electrical devices.
• The normal PC includes above 1000 thick film resistors.

Thus, this is all about an overview of thick film resistor – working, advantages, disadvantages, and its applications. Here is a question for you, what is a thin-film resistor?