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A thick film chip resistor is a type of resistor manufactured by applying a thick layer of resistive material, typically metal oxides, onto a small ceramic substrate. This layer is applied through printing or spraying a paste, followed by drying and firing at high temperatures to form the resistive path. The term "thick film" refers to the thickness of this layer, which is greater than that in thin film technology. Thick Film Chip Resistors are known for their stability, reliability, and suitability for various electronic applications.
Advantages of Thick Film Chip Resistor
Size and Functionality
The size and functionality of this resistor type are small size, low power rating and low insulation voltage. Thick film resistors are designed to facilitate the use of heat sinks. This makes them only one-quarter the size of wire winding resistors, making them more suitable for systems with limited space.
Low Inductance
Low inductance is often an unnecessary characteristic of resistors, especially in filtering systems. All wires have inductance, and the inductance value depends on their geometry. Generally speaking, wide, flat, short geometries have lower inductance than long thin winding wires. Conductive inks here form the resistor element on a wide, flat ceramic insulation layer. This provides extremely low inductive resistance. Therefore, low inductance thick film resistors are the preferred resistor type for high-frequency applications such as PWM filters or current dividers.
Heat Dissipation
The hot key to heat dissipation of resistors is the surface and the interface with the cooler carriers. Thick film resistors have a large flat area, which means they are easy to connect to a cooler or cold plate. 2kW of power dissipation can be achieved on a square face of 60mm.
High Isolation Voltage
High voltage power transistors printed on ceramic substrates have appropriate terminal-to-substrate insulation. Coating the resistor element with materials such as polyimide can increase insulation. The ceramic substrate is then placed in a load-bearing mechanical housing filled with silicon to supplement the electrical insulation solution. This series of ultra-high power resistors can achieve substrate insulation levels of 12kVrms.
Rugged Construction
The mechanical strength of solid thick film transistors is higher than that of wire wound resistors, making them less susceptible to damage due to improper handling, packaging or roughness. This design allows the resistor to withstand high-power overvoltage better than wound wire resistors. The ceramic layer (with printed resistors) is placed in a specially designed plastic housing for protection, which can also include mounting equipment. The housing can also be equipped with springs to control the required pressure, providing reliable thermal contact without causing the ceramic to break.
Types of Thick Film Chip Resistor
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.
Applications of Thick Film Chip Resistor
Thick film resistors are mainly used in current limiting, voltage dividing and circuit calibration in electronic circuits. The following are common application scenarios for thick film resistors:
Current limiting and voltage dividing applications
Thick film resistors play the role of regulating voltage and current in circuits. They are mainly used to limit, balance and distribute the current to ensure the stability and reliability of the circuit. For example, the electronic components in the voltage-controlled oscillator require a stable resistance value. Sometimes multiple thick film resistors must be connected in series to form a voltage divider ratio to achieve the purpose.
Temperature sensor
Some thick film resistors can be used as temperature sensors to a certain extent, because although the resistance value is related to the current, its resistance value is also affected by temperature.
Timer and other applications
Thick film resistors are widely used in various electronic devices such as timers, frequency meters, and current measuring instruments. They can also be used in various applications such as filters, dampers, voltage dividers, voltage droppers, and calibration circuits.
Embedded systems
In embedded systems, thick film resistors are also widely used in circuit boards, chip components, etc., and are used in control circuits of various embedded systems, such as Internet of Things devices, sensors, etc.
In general, thick film resistors have a wide range of applications, and their advantages are particularly obvious in practical applications. Their use can effectively improve the performance of electronic circuits and achieve higher stability and reliability requirements.
How to Select the right Thick Film Chip Resistor for Your Business
Resistor Values
To determine the required resistance value for your circuit, consider voltage levels, current requirements, and desired functionality. This value is crucial as it affects the resistor's performance.
Tolerance Selection
Resistors come with tolerance ratings like ±1%, ±5%, and ±10%. Choose based on your application's precision needs.
Power Rating
Evaluate the power rating needed for the resistor to handle power dissipation without damage. Calculate using P = I^2 * R or P = V^2 / R and select a resistor with a higher power rating for safety.
Temperature Coefficient
Consider the temperature coefficient, which indicates how resistance changes with temperature. Choose a suitable resistor for your application's temperature range.
Resistor Type
Select from carbon film, metal film, wirewound, and thick film resistors based on precision, stability, power handling, and size constraints.
Environmental Factors
Account for humidity, moisture, vibration, and temperature extremes that may affect performance. Choose resistors with appropriate environmental ratings.
Cost Evaluation
Compare prices from different manufacturers to find the best balance between quality and affordability, considering your budget and quantity needs.
Datasheets Review
Consult datasheets for compatibility with application requirements, focusing on temperature range, voltage rating, and mounting options.
Specialized Requirements
Choose resistors with specialized characteristics optimized for high precision, low noise, or high-frequency operation if required by your circuit.
Noise Performance
In sensitive applications, select metal film resistors for lower noise levels compared to other types.
Voltage Rating
Ensure the resistor's voltage rating is sufficient for the maximum voltage in your circuit to prevent failure or breakdown.
Size and Mounting
Consider the physical size and mounting options, especially in space-limited applications, choosing between SMD and through-hole resistors.
Temperature Considerations
Choose resistors that can withstand the operating temperature range of your circuit without significant resistance changes.
Derating Practices
Operate resistors at lower power levels than their maximum rated power, typically derating to 70-80% for reliability and to prevent overheating.
Environmental Compliance
Ensure resistors meet relevant environmental regulations, such as RoHS compliance, for your industry or region.
Long-Term Stability
For long-term reliability, consider metal film or wirewound resistors known for stable characteristics over time.
Application-Specific Requirements
Choose resistors designed for high-frequency operation, high precision, or high-voltage handling as needed by your application.
Expert Consultation
Consult experienced engineers or technical experts for guidance if unsure about resistor selection or facing complex requirements.
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 resistors are made from a layer of metal oxide paste. Due to the special composition and processing of the thick film paste, it is ideal for many applications. The thick film paste is applied to a high-quality ceramic substrate using a screen printing process. This special type of ceramic substrate efficiently dissipates thermal energy and as a result ensures a high power density.Production takes place in several steps:
1. Prepare the raw materials, such as the thick film paste and the ceramic substrate. They are checked and cleaned to make sure they are free from contamination and damage. The resistance paste is selected in accordance with the target ohmic values.
2. The resistance layer, which can feature a specific shape and dimensions, is applied using a screen printing process. The layout, print thickness and type of resistance paste have a major influence on the ohmic value. The temperature coefficient and other technical parameters are primarily influenced by the thick film paste used and the process parameters.
3. In the next step, dry and bake the resistance layer to sinter it to the ceramic substrate. A protective layer containing glass is printed over the resistance layer and is also sintered.
4. Next comes the stabilization process, which fixes the final parameters and completes the basic manufacturing process.
5. During the quality check and testing phase, it is ensured that the thick film resistors meet the required quality and technical specifications.
The manufacturing process requires precision and time to ensure the quality of the final product. Stable production parameters are largely responsible for our high-quality standards.
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