Analysis of Key Parameters of Thick Film Low Resistance Resistors
Thick film resistor technology uses screen printing of resistor paste on an insulating substrate (such as alumina ceramic), and then high-temperature sintering to form a resistor body. When designed for low resistance range (usually in the milliohm range, such as 1 m Ω to 50 m Ω or higher), its performance is significantly influenced by material properties and processes. The following are its core parameters:
Resistance range:
Core feature: This is the core indicator for achieving low resistance of thick film resistors. The minimum achievable stable resistance value is limited by the formula of the resistance paste (usually containing a high proportion of conductive metal particles such as silver, palladium silver, ruthenium oxide, etc.) and the accuracy of the printing process.
Typical range: The minimum resistance value that can be stably achieved by thick film technology is usually in the range of 1 m Ω to 50 m Ω, and some advanced pastes and designs can be lower (such as 0.5 m Ω), but the cost and technical difficulty significantly increase. Compared to thin film processes, thick films are usually less precise in achieving ultra-low resistance values (<1 m Ω), but have cost and power advantages in the mid to low milliohm range.
Rated power:
Thick film advantage: One major advantage of thick film technology is its relatively high power density. The resistance slurry layer is thick, and the substrate (such as Al2O3 ceramic) has good thermal conductivity.
Low resistance correlation: Low resistance resistors are typically used in high current applications. Its rated power directly determines the maximum current it can safely carry. When designing, it is necessary to ensure that the power meets the application requirements and consider the heat dissipation conditions. The power range can range from a few watts to tens of watts or even higher, depending on the resistor size and design.
Resistance tolerance:
Accuracy challenge: The initial accuracy of thick film technology in achieving low resistance values is usually not as good as that of thin film resistors. The factors that affect accuracy include slurry uniformity, printing thickness control, sintering process, etc.
Typical value: The common tolerance range is between ± 1% and ± 10%. For high demand milliohm level measurement applications such as current sampling, ± 1% or ± 5% are common choices, but the cost increases accordingly. A tolerance of ± 10% or wider is used in situations where low current limiting is not required.
Temperature coefficient of resistance:
Key parameter: This is a crucial and challenging parameter for low resistance thick film resistors. TCR represents the ratio of resistance change with temperature (in ppm/° C).
Thick film characteristics: The TCR of thick film resistors is usually larger (absolute value higher) than that of thin film resistors, and the curve may be nonlinear. For low resistance thick film resistors, TCR may reach ± 200 ppm/° C, ± 300 ppm/° C, or even higher.
Importance: In applications such as current sampling, the heating of the resistor itself and changes in ambient temperature can introduce significant measurement errors. Choosing specialized low resistance thick film pastes with low TCR (such as ± 100 ppm/° C or lower) is crucial, but the cost will increase. Some advanced slurries can be optimized to a level of ± 50 ppm/° C through material composites.
Structural design:
Kelvin connection: For precise milliohm level measurements, a four terminal (Kelvin) structure is almost necessary. It eliminates the influence of measuring leads and contact resistance, significantly improving measurement accuracy.
Electrode material: The terminal electrode is usually made of silver or silver palladium material with good weldability. Low resistance requires the electrode itself to have extremely low resistance and be well connected to the resistor body.
Shape of resistor: In order to achieve low resistance, resistors are usually designed in a short and wide shape (low aspect ratio) to increase cross-sectional area and reduce length.
Stability/Long term Drift:
Influencing factors: Refers to the rate of change in resistance after long-term operation under rated power, ambient temperature, and other conditions. The stability of thick film resistors is affected by factors such as paste composition, sintering quality, substrate matching, and operating temperature.
Low resistance consideration: Low resistance resistors have a large operating current and may have a higher temperature rise, which requires higher stability. Good design and craftsmanship can ensure that the resistance variation during the life cycle (such as 1000 hours of full load) is within an acceptable range (such as<1%).
Pulse load capacity:
Advantages of thick film: The structure of thick film resistors is usually better able to withstand short-term overloads or pulse currents than wound or metal foil resistors, because their resistor body is a uniform ceramic metal composite material.
Application related: This is an important parameter to consider in applications with pulse currents such as power start-up and surge suppression. The specific ability depends on the size, material, and design of the resistor.
Size/Packaging:
Thick film low resistance resistors come in various packaging forms, ranging from surface mount (such as 1206, 2010, 2512, etc.) to power type plug-in packages with heat sinks. The size selection needs to balance resistance, power, heat dissipation, and space requirements.
Cost:
Core advantage: Compared to thin films, metal foils, or precision wound low resistance, thick film technology usually has significant cost advantages, especially when medium precision, high power, and large batch sizes are required.
Summary:
When choosing thick film low resistance resistors, the core considerations are the target resistance value, required power, accuracy (tolerance), temperature coefficient (TCR), and whether a four terminal structure is needed. It should be soberly recognized that thick film technology is more challenging than thin films or metal foils to achieve high precision (low tolerance) and low TCR at low resistance values, but it has comprehensive advantages in power carrying, pulse capability, and cost. Therefore, it is widely used in power supplies, motor drives, battery management, current detection (when precision requirements are not extreme), current limiting circuits, and other applications that require low-cost, reliable, and medium performance milliohm level resistors. Be sure to weigh these parameters according to specific application scenarios.