Environmental resistance

This section introduces products with superior moisture resistance and sulfuration resistance compared to general-purpose products. They are suitable for use in polluted air or high humidity environments.

sulfuration prevention

Mechanism of sulfuration

Sulfuration is a phenomenon that occurs in flat chip resistors (Typically thick film chip resistors) that use silver for internal electrodes*. When flat chip resistors are used in an atmosphere containing sulfur, sulfur entering through gaps between protective film and the external electrodes react with silver in internal electrodes to form silver sulfide, which have no conductivity and lead to a disconnection of resistors due to sulfuration.

*"internal electrode" refers to an electrode that is not exposed to outside of a resistor, and is there for the purpose of electrically connecting resistive element and external electrode that is for soldering.

Sulfur is contained in sulfide gas generated near hot springs and volcanoes, and is also generated by combustion of heavy oil. In addition, some machine tool oils and rubber products such as cables and tires contain sulfur.


In electronic equipment used in such environments, disconnection of flat chip resistors due to sulfuration may occur. In recent years, complete sulfuration resistant type flat chip resistors using non-sulfidation materials for internal electrodes, and sulfuration delaying types with a structure that prevents sulfur from entering inside have been developed.

KOA offers a series of complete anti sulfuration resistors with internal electrodes made from special materials that do not sulfide.

  Countermeasure Effect

General type


Slow-sulfuration type


Complete anti sulfuration type


Anti sulfuration - SMD(Surface Mount Device) Type -

Item Series Product Name
Resistor RK73B/H/Z-RT Flat Chip Resistors (Anti Sulfuration)
Resistor RK73G-RT Flat Chip Resistors (Ultra Precision Grade, Anti Sulfuration)
Resistor RS73-RT High Reliability Chip Resistors (Anti Sulfuration)
Resistor WK73-RT Wide Terminal Type Flat Chip Resistors (Anti Sulfuration)
Resistor WK73-RT Wide Terminal Type Flat Chip Resistors (High Power/Anti Sulfuration)
Resistor SG73-RT Surge Resistant Flat Chip Resistors (Anti Sulfuration)
Resistor SG73P-RT Endured Pulse Power Flat Chip Resistors (Anti Sulfuration)
Resistor SG73S-RT Endured Surge Voltage Flat Chip Resistors (Anti Sulfuration)
Resistor HV73-RT Flat Chip Resistors For High Voltage (Anti Sulfuration)
Resistor HV73V-RT Flat Chip Resistors For High Voltage (For Automotive, Anti Sulfuration)
Resistor SR73-RT Low Resistance Flat Chip Resistors (Anti Sulfuration)

Electrolytic corrosion resistance of resistors

Electrolytic corrosions may occur when the thin film resistors made of the metal film are used in high humidity environment, resulting in disconnection. However, the metal film chip resistor RN73H , which has a special protective film inside, has improved moisture resistance compared to conventional products and is less susceptible to electrical damages. This product has superior moisture resistance compared to conventional products even under the condition of moisture resistance load life test for automotive applications which is considered to be severe.

Also, some general-purpose chip resistors such as RK73B use metal glaze for the resistive film, which has extremely high humidity resistance and does not cause electrolytic corrosion.

In addition, carbon film resistors such as CF, which are commonly used leaded resistors, may cause electrolytic corrosion on the carbon of the resistive element and lead to disconnection if used in a high humidity environment, as in the case of metal film resistors.

On the other hand, RCR, which uses metal glaze as a resistive element, has extremely high moisture resistance and is free from electrolytic corrosion.

Electrolytic corrosion resistant resistors

Item Series Product Name
SMD Type RN73H Metal Film Flat Chip Resistors (For Automotive, High reliability)
Leaded Type RCR Coat-Insulated Fixed Anti Surge Resistors