Thermistors (NTC and PTC) come in various packaging forms, with the main purpose of protecting their sensitive ceramic materials, providing reliable electrical connections, facilitating installation, and adapting to different working environments and heat dissipation requirements. The following are some of the main and common forms of encapsulation, logically summarized as follows:
1、 Lead type packaging (axial/radial)
This is the most traditional and basic form, with thermal core components equipped with metal leads.
Disc/Bare Chip:
Structure: The simplest form. Directly weld metal leads (usually tin plated copper wire) onto both ends of the sintered thermosensitive ceramic disc or square. Sometimes a thin layer of insulating paint (such as epoxy resin or silicone resin) is applied to the surface to provide basic protection.
Features: Lowest cost, fastest thermal response (due to small thermal mass, direct heat exchange with the outside world).
Application: Commonly used in cost sensitive, spacious, relatively mild environments that require quick response, such as embedded air temperature measurement and low-cost temperature compensation within devices. Commonly seen in NTC.
Epoxy resin coating/molded packaging:
Structure: On the basis of the disc type, the thermistor chip and some leads are completely covered with black or other colored epoxy resin, forming cylindrical, elliptical or flat pill shapes (similar to the DO-35 package of glass diodes, but usually black epoxy resin).
Features: Provides excellent mechanical protection, insulation, and moisture resistance. Moderate cost, slightly slower thermal response than bare chips but still within an acceptable range. The lead can be axial (led out at both ends) or radial (double lead at one end).
Application: Extremely versatile, suitable for temperature detection, compensation, surge suppression (NTC), and overcurrent protection (PTC) in most general circuits. It is one of the most common forms of packaging.
Glass encapsulation:
Structure: The thermosensitive chip is sealed in a miniature glass tube filled with inert gas, and metal leads (similar to glass diodes) are led out at both ends.
Features: Provides excellent sealing, long-term stability, high temperature resistance, and chemical corrosion resistance. The thermal response speed is between the disc and epoxy resin.
Application: Suitable for applications that require high reliability, long-term stability, and resistance to harsh environments such as high temperatures and corrosive gases, such as precision temperature measurement, automotive electronics, and aerospace. The cost is relatively high.
Bolt installation/high-power packaging:
Structure: Thermistors (especially PTC) designed specifically for handling high currents. Thermistor chips are usually larger and packaged in a metal casing (often aluminum or stainless steel) or high-strength plastic casing, with one or more mounting bolt holes (sometimes shared with electrical connections). The lead wire is usually a thick terminal block or copper sheet.
Features: Strong heat dissipation ability, able to withstand high steady-state current and surge current, high mechanical strength, easy to fix on heat sinks or equipment casings.
Application: Motor starting/protection, surge current limiting (NTC) for high-power power supplies, overcurrent/overheat protection components (PTC), demagnetization circuit (PTC).
Probe type/rod type:
Structure: Thermistor chips are usually placed at the tip of metal (such as stainless steel, copper) or plastic probes, with leads leading out from the other end. The probe tube provides mechanical protection and environmental isolation, while the metal tube can enhance thermal conductivity.
Features: Designed specifically for temperature measurement. The metal sheath provides good mechanical strength, corrosion resistance, and thermal conductivity, making it easy to insert into the tested medium (liquid, gas, solid surface). The response speed depends on the sheath material and size.
Application: Temperature sensors (mainly NTC) in industrial process control, household appliances (water heaters, air conditioners), medical equipment, food processing and other fields.
Package with connector/harness:
Structure: Encapsulate epoxy resin or other forms of thermistor, with its leads pre connected to wires and connectors (such as terminals, plugs, quick connectors).
Features: Easy to install and replace, improve assembly efficiency, and provide cable protection.
Applications: automotive sensors (such as water temperature, oil temperature, air conditioning temperature sensors NTC), household appliances (such as refrigerators, washing machines temperature sensors NTC), etc. that require quick plugging or long-distance wiring.
2、 Surface mount packaging
With the development of miniaturization and automated manufacturing (SMT) of electronic devices, SMD thermistors are becoming increasingly widely used.
Chip multilayer ceramics:
Structure: The most mainstream SMD form. Using multi-layer ceramic co firing process, the thermosensitive material is made into a thin layer and printed with internal electrodes in layers, forming solderable ends at both ends (usually tin plated or silver plated palladium).
Features: Compact size (such as standard sizes 0402060308051206), no leads, suitable for high-speed SMT mounting. The thermal response speed is usually fast (with a small thermal mass). There are thick film and thin film processes available.
Application: Extremely extensive, used for temperature detection, compensation, surge suppression (NTC) of almost all modern electronic devices such as mobile phones, computer motherboards, power modules, LED drivers, etc. PTC SMD is also commonly used for overcurrent protection.
SMD diode type:
Structure: Similar to a lead type epoxy resin cylinder (DO-35 style), but with both ends made into flat metal pads suitable for surface bonding.
Features: Slightly larger than the chip size, but usually with stronger power handling and surge capabilities. The thermal response speed is slightly slower than that of sheet metal.
Application: SMT applications that require higher power or surge capability than standard chips, such as surge suppression (NTC) at the input of switch mode power supplies.
Summarize key points
The core of packaging selection depends on application requirements - whether it is temperature measurement, compensation, surge suppression, or overcurrent protection? What is the required power/current size? Response speed requirements? Environmental conditions (temperature, humidity, corrosiveness)? Installation space and method? Cost?
Lead type vs SMD: Lead type is more suitable for situations where manual soldering, high power, physical isolation, or special probe design are required. SMD type is mainstream and suitable for modern electronic devices that are miniaturized and automated in production.
Thermal response speed: The general rule is: bare chip>glass ≈ SMD chip>epoxy resin ≈ SMD diode>sheathed probe>high-power bolt type. The thicker the packaging material, the greater the thermal mass, and the longer the thermal conduction path, the slower the response.
Understanding these packaging forms and their characteristics can help engineers choose the most suitable thermistor based on specific application scenarios.