At present, the commonly used NTC thermistor chip on the market is silver electrode NTC thermistor chip. The conductive silver paste used in the silver electrode layer has become one of the earliest conductive paste materials used because of its high cost performance. As a kind of functional conductive material, conductive silver paste has been widely used in electronic products. Compared with conductive carbon paste and conductive copper paste, it has attracted much attention and favor because of its better conductive performance.
Due to the wide application of conductive silver paste, so far it is still the largest amount of use. When it is found that silver migration occurs during the use of conductive silver paste, the reason of silver migration and how to prevent it have been the issue of great concern. Silver migration refers to the presence of DC voltage gradient in the wet environment, water molecules into the surface of silver-containing conductor electrolysis to form hydrogen ions and hydroxide ions. Under the action of electric field, silver ions migrate from high potential to low potential, and form flocculent or dendritic extension, forming black silver oxide on the boundary where high and low potentials are connected. Silver migration can be more clearly observed by the water drop test and which is very simple to operate. The migration of silver ions is observed by placing water drop between silver-containing conductors close to each other and applying a DC bias voltage.
For example, the performance of silver electrode NTC thermistor chip is usually good during the factory inspection, all parameters are fully qualified, but after a period of use, customers may find that resistance value of some silver electrode thermistor chip becomes small, or there is even a short-circuit self-passing phenomenon. The reason is the silver migration. In order to better control the quality of silver electrode NTC thermistor chip, an experiment on silver migration of silver electrode NTC thermistor chip was done. The report is as follows:
Take the test sample silver electrode NTC thermistor chips, apply 5V voltage and 1mA current; Test temperature is 65±2℃, test humidity is 93±3%RH.
7/17/2021 to 7/27/2021 for 10 days
The product model of silver electrode NTC thermistor chip is DT104G3964F, after 240 hours of test time, the samples were taken out for detection.
The test results are as follows:
The following analysis can be obtained from the above test results:
1.The resistance value of the sample decreased significantly after the test, and the change rate of resistance value was -40% to -97%;
2.Silver-white material was observed on the side of the ceramic body through Olympus microscope;
3.The migration of the silver electrode edge to the ceramic body was observed by scanning electron microscope;
4.The component analysis on the side of the ceramic body shows that the side of the ceramic body contains 20%~40% silver element, thus the silver-white substance on the side of the ceramic body is silver element.
It can be found from the silver migration test that the silver electrode NTC thermistor chip occurs a drastic silver migration phenomenon in a short time under the condition of current, high temperature and high humidity. Silver ions migrate on the side of the ceramic body, leading to a significant decrease in the resistance value of the thermal sensitive chip. There are two solutions:
First, customers must take waterproof and moisture-proof measures in the process of use to avoid silver migration due to water vapor;
Second, look for alternative product. Such as the launched new product produced by EXSENSE Electronics Technology Co., Ltd. - gold electrode NTC thermistor chip.
The gold electrode NTC thermistor chip is a high temperature-resistant, high-precision NTC thermistor chip with a surface gold-plated treatment, which operating temperature range is -50℃~200℃, and its size is small (minimum up to 0.3* 0.3 mm). The welding methods for gold electrode thermistor chips are bonding or non-bonding tin welding.
Compared to the silver electrode thermistor chip, the gold electrode NTC thermistor chip also has the following characteristics:
1.High reliability: gold electrode is not easy to produce ion migration in wet working environment. In contrast, the silver electrode works in a wet environment for a long time will easy to cause a natural phenomenon of silver migration. (Corresponding to the above test)
2.High stability: The gold drift rate of the gold electrode NTC thermistor chip is low, and the dispersion of the resistance value can be kept within the specified range for a long operating time, which is suitable for the scenarios requiring long-term stable temperature monitoring.
3.Suitable for a variety of hybrid design multi-functional modules: infrared thermopile, IGBT module, thermal print head, integrated modules, semiconductor modules, power modules and so on.
4, a variety of packaging methods: blue taping, trap packaging, bulk packaging, etc..
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