NTC thermistors, which are used as temperature sensors in home appliances such as air conditioners, refrigerators and electric rice cookers, are indispensable components for temperature compensation, since they maintain the stability of circuit operation against the influence of temperature. Although the bead type and the disk type used to be common, the surface-mount chip type is now the main type. Further mini-aturization is being achieved to keep up with the evolution of smaller and lighter electronic devices.
The changing uses of chip NTC thermistors
Chip NTC thermistors first went into commercial production for temperature compensation of portable television LCD panels, and were later applied to auto focus motors in camcorders and to circuits in view finders.
With the increasing prevalence of mobile telecommunications devices, and a succession of small portable digital devices like PDA (personal digital assistants) released to the market, plus with further developments in the surface-mounting of devices and circuits, as well as smaller and lighter products, there is a rapid growth in demand for smaller chip NTC thermistors, starting with the 1608 type, then the 1005 type and now the 0603 type.
Limitations of conventional chip structure
There has been a particularly rapid increase in demand for temperature compensation of TCXO (temperature compensated crystal oscillators), RF modules and LCD panels, central to cellular phones and Japanese PHS (Personal Handyphone System), and for temperature monitoring of lithium battery packs and nickel-hydrogen battery packs.
However, conventional single plate chip NTC thermistors (the model shown above), in which electrodes are on the NTC thermistor element, fail to satisfy the simultaneous requirements of physical properties and miniaturization, because of physical limitations closely related to their geometry.
Use of multilayering techniques to overcome the problem
To respond to these needs, a new chip NTC thermistor model using laminated construction method shown in the next page were developed. The multilayer structure, with alternating layers of internal electrodes, overcame the limitations on miniaturization experienced by the single plate type, and massively expanded the range of resistance that can be produced by the same shape. Here’s a table of differences in terms of controlling the characteristics and shapes differentiated from the structures of the both.
The emergence of this multilayer type also led to remarkable growth in the application of chip NTC thermistors in such areas as temperature compensation and high accuracy temperature detection in state-of-the-art microdevices and circuitry.
Technological background to the development of the multilayer chip type
In this way, chip NTC thermistor technology has evolved from the single plate structure into the multilayer structure. Our achievements regarding multilayer chip NTC thermistors are outlined below, with a focus on the superiority of and problems with the multilayer type, and with comparisons made to the single plate type.
Control of resistance and B constant
NTC thermistors are semiconductors made primarily from oxides of transition metals such as manganese (Mn), nickel (Ni) and cobalt (Co). Although a variety of materials consisting of two or more oxides have been developed, limitations in the combination of resistivity and B constant (temperature coefficient) unique to materials make it extremely difficult for us to develop a material with low resistivity and high B constant, or high resistivity and low B constant.
For example, to make a low-resistance chip thermistor using a material with a high B constant value (and therefore a high resistivity), the resistance R of the element can be expressed as
When the value of ρ is fixed as in this example, the resistance R of the element can be controlled by shortening the distance between electrodes t or enlarging the area of electrode overlap s.
Similar to the model below, this operation can be applied to single plate chip thermistors by shortening the distance t between electrodes of the element (a), as well as by enlarging the area of electrode overlap (in other words, enlarging the area of connection terminal =b). However, as you probably know, chip products must be compliant with EIAJ standards in configuration: L2.0xW1.25mm (2012 type), L1.6xW0.8mm (1608 type) and L1.0xW0.5mm (1005 type).
To satisfy demand for products smaller than the 1005 type but still in compliance with the standards above, the electrode design and shape is limited, thus it resembles a flip-type multilayer ceramic chip capacitor with the connection terminal moved to the longitu-dinal side of the element, such as the models (a, b) shown below.
In other words, to enable the development of an ultra-miniature chip thermistor with low resistance and high B constant, it will be extremely difficult to overcome the limitations of structure and physical properties of the single plate type.
Temperature compensated crystal oscillators (TCXOs) are a good example. For the purpose of increasing design efficiency and freedom, chip NTC thermistors that compensate for the low temperature range (-30 to +25°C) require a resistance of as low as 30 to 150Ω. However, the results of the above example suggest that in order to provide a lower chip resistance to absorb the rise in resistance value resulting from size reduction, the semiconductor ceramics of a conventional single plate chip must have a much lower resistivity than normal.
In addition, because B constant, another key characteristic, fluctuates in proportion to the resistivity of the material, B constant for a single plate type chip thermistor with a resistance level of 30 to 150Ω is typically at 2750K.
In the case of multilayer structure, the distance between electrodes t and the area of electrode over-lap s in the equation above does not mean the distance between terminal electrodes (the length of the chip) and the area of the terminal electrode, as it does in the case of the single plate type. Instead, t is the length between internal electrodes (the thickness of ceramics layers between electrodes) and s is the area of their overlap.
Consequently, even with the size of the 1005 type fixed by EIAJ standards, we are able to control the values of t and s in various ways by using the number of electrode layers and the pattern of overlap in the internal design of the chip. This means that our use of materials with high B constant does not prevent us from lowering the resistance of the chip itself or from minimizing the size of the chip. The multilayer structure, in which semiconductor ceramics are divided into cells in internal electrode layers, is equivalent to parallel connections of tiny thermistor elements, so by controlling the resistance of every single element (or the overlap area and number of layers in the internal electrode), and by combining a B constant that meets individual circuit requirements with various resistance values, product selection can be expanded with chips of exactly the same size.
Overcoming the capacitance component problem
Ideally, NTC thermistors that are embedded in circuits as semiconductor resistors for temperature compensation, responding to changes in temperature and to heat, should have no capacitance component. Capacitance component can be a serious problem in high frequency ranges, especially for temperature compensation of crystal oscillators.
Below is an example of impedance-frequency characteristics of NTC thermistors for direct current circuits (for general use).
At around 100kHz, the influence of capacitance component becomes apparent. At a range of 10 to 40kHz, the range in which crystal oscillators oscillate, there is a sharp drop in impedance. The temperature compensation of crystal oscillators requires measures to avoid this sudden change in property. In other words, a technique to minimize the capacitance component of the chip is required.