NTC chip has a characteristic of large cold resistance and resistance decreases as the temperature rises. The principle of temperature measurement of medical temperature sensor is to carry out temperature detection according to the characteristic resistance value of NTC chip changes with its temperature, and the electrical bridge is used to detection circuit. Medical temperature sensor mainly through the detection of human body temperature to understand the important indicators of life state, so NTC chip is required to have characteristics of small size, good ability of heat cycle, wide measurement range, high measurement accuracy.
In the existing technology, the production method of NTC chip is generally: thermal material preparation - ultra-high pressure molding - sintering - slicing - cleaning and drying - electrode printing - chip dicing - test. However, the NTC chip produced by existing method has the following disadvantages:
(1) The existing NTC chip is mostly silver electrode, its stability is not as good as the gold electrode NTC chip, silver migration is easy to occur under the three conditions of high temperature, humidity and electric field, and the surface of silver layer in the air is easy to vulcanize. In addition, after printing the silver slurry, the volatilization of organic matter of slurry during the sintering, which is easily make the electrode layer produce pores. Long-term use will cause the toxic substances in epoxy material gradually invade the thermal material with the pores, thereby destroying electrical performance.
(2) The costs of existing gold electrode NTC chip is high, the process is to obtain two-sided electrode through screen printing. This makes it difficult to make films printing, so there is a lot of loss of gold.
(3) Low accuracy. The printing film of existing process, its thickness uniformity is poor and the surface is not dense, the sintering of silver layer is influenced by the temperature fluctuation of furnace body, the vitreous body in the slurry can not inconsistently permeate to the NTC ceramic substrate, resulting in scattered electrical performance, as well as cracking, deformation.
Therefore, the NTC chip in the existing technology can not meet the requirements of the chip such as medical temperature sensor. Today, EXSENSE Electronics introduce a high-precision, high-reliability titanium-tungsten gold electrode NTC chip, this chip is small in size, with characteristics of high reliability, high precision, and with good ability of heat cycle resistance. This titanium-tungsten gold electrode NTC chip, including thermal substrate, titanium-tungsten alloy layer and gold electrode, titanium-tungsten alloy layer and gold electrode are layered from the inside out on two surfaces of the thermal substrate. Among them, the thermal substrate is ceramic substrate; The thickness of the titanium-tungsten alloy layer is 0.05 to 0.1μm; The mass ratio of titanium and tungsten in the titanium-tungsten alloy is 8:2-7:3; The thickness of the gold electrode is 1 to 2μmm; Between the titanium-tungsten alloy layer and the gold electrode,which are set up with a nickel layer, the thickness of the nickel layer is 0.2 to 0.5μm, the nickel layer can enhance the binding force of the gold electrode and the titanium-tungsten alloy layer. The titanium-tungsten alloy layer is placed on the two surfaces of the thermal substrate by vacuum coating, and the titanium-tungsten alloy layer are placed on two surfaces of the gold electrode by vacuum coating.
The preparation method for this titanium-tungsten gold electrode NTC chip, including the following steps:
(1) Preparation of thermal substrates;
(2) Through the method of vacuum coating, the titanium-tungsten alloy is sputtered onto the two surfaces of the thermal substrate, forming the titanium-tungsten alloy layer;
(3) Through the method of vacuum coating, the gold slurry is sputtered onto the surface of the titanium-tungsten alloy layer, forming the gold electrode;
(4) The thermal substrate covered with titanium-tungsten alloy and gold electrode is diced, and the titanium-tungsten gold electrode NTC chip is obtained.
This titanium-tungsten gold electrode NTC chip has characteristics of high reliability, high precision, with good ability of heat cycle resistance. Compared with the existing technology, its thermal time constant is reduced by twice as much as the current chip; Its high temperature aging performance is more stable,the change rate of the long-term use is ≤ 0.1%, which is suitable for medical temperature sensors and other areas requiring high precision of NTC chip.