Resistance – Thick Film Resistor Vulcanization
A group of field instruments used in a chemical plant a year later, the instrument has failed. The analysis found that the instrument used in the thick film patch resistance has become larger, and even become open. The failure of the resistance observed under a microscope, you can find the edges of the resistance electrode appeared black crystalline material, further analysis of the composition found that the black material is silver sulfide crystals. The original resistor is corroded by sulfur from the air.
So what happened to this process? The structure of the thick film resistor is usually as follows:
The top electrode is the internal electrode connecting the ruthenium dioxide resistor and the solder tip. This electrode is generally silver-palladium alloy. Due to the resistance of the surface of the secondary protective layer and the welding tip is not a perfect match. The surface electrode is partially exposed to the air. Therefore, when the air contains a large amount of sulfide gas, the silver is reacted with sulfide to form silver sulfide. Since silver sulfide is not conductive, the resistance gradually increases as the resistor is cured until it finally becomes an open circuit.
In fact, not only resistors used in chemical plants can be cured, resistance in mining and thermal power plants is also at risk of being vulcanized, and in some cases only because of the use of sulfur-containing rubbers in closed environments It also causes sulfur released at high temperatures to cause resistance to cure. Therefore, automotive electronics has also begun to pay attention to the resistance of the curing.
In order to prevent the resistance of the vulcanization of people began to develop resistance to sulfur resistance. In general, the thin film resistor is made of nichrome or tantalum nitride. This thin film resistor contains no silver, so it inherently has good anti-vulcanization ability. So in general, anti-sulfur resistance often refers to the thick film resistance. Anti-vulcanization design of thick film resistors are generally used to adjust the composition of the surface electrode and adjust the thick film resistor structure.
Surface electrode is silver palladium alloy, increase the content of palladium can enhance the anti-vulcanization performance. However, after the increase of palladium silver palladium alloy melting point will increase, will have a certain impact on the process. So now the main production of anti-vulcanization resistor manufacturers are adjusting the resistance structure up and down enough.
Preventing the surface electrode from being directly exposed to the air is the main method of anti-vulcanization design by adjusting the resistance structure at present. This method is to use a non-erosive material on the surface electrode as a protective interlayer. The middle layer fills the gap between the secondary protective film and the solder tip to avoid direct exposure of the surface electrode. One of the most common structures uses gold as the middle layer.
A gold conductive layer is used as a protective layer of the surface electrode outside the internal surface electrode. Due to the metal in the precious metal so the cost of this anti-vulcanization resistor is relatively high.
In order to reduce costs, resistance manufacturers in this layer of protective layer composition began to think of ways, such as Rohm’s anti-vulcanization resistor is a special resin material instead of gold, while others according to their own technology, process characteristics of the use of nickel-chromium Gold substitutes.
The Fenghua Hi-Tech in the structure not only the use of the middle layer to protect the surface electrode, but also improve the process, the welding tip wrapped around the edge of the secondary protective film to prevent the exposure of the internal electrodes.
At present, the price of anti-sulfur resistance is more expensive than ordinary thick-film resistors. Generally, anti-sulfur resistance is used in the chemical, mining, thermal power and automotive electronics mentioned above, but also in some high-end applications that require strict reliability such as Telecom and other industries.