Prior to the widespread acceptance of surface mounted sheet resistors in the 1980s and 1990s, metallized hole mounted molded resistance was usually used in precision applications. Through hole mounted discrete resistors have always been made by depositing thin film resistive materials on cylindrical ceramics. The leads are connected to both ends of the ceramic and the columnar resistance devices are molded or coated to produce a fully sealed product as shown in figure 1. The sheet resistance shown in figure 2 is sealed only on one side of the device. Unlike a full seal, the sheet resistance is coated with a seal only on the side containing the thin film material. Although a protective seal is applied to the resistance units at manufacturing time, this seal is not completely secure in the harsh outdoor environment of many telecommunications applications.
For example, a chip resistor on a line card in an outdoor cabinet can be affected by temperature cycling and humidity in an outdoor environment. If the line card is located in the Midwest, the annual temperature limit can range from -20of to more than + 100oF. When cold air comes, or when there is a storm, the temperature fluctuation can reach 50o in a short period of time.
When the circuit card is exposed to this temperature, mechanical stress may occur where the resistance is welded to the printed circuit board. The expansion and shrinkage coefficients of organic PC board materials and ceramic sheet resistors are different. The temperature expansion coefficient (Tce) of fr-4 PC board material is about 16 PPM /℃, while the Tce of ceramic material is only about 7 PPM /℃. Different expansion and contraction rates will result in stress at the resistance welding point, which will cause the resistance sheet to flex during the thermal cycle.
The surface roughness of the ceramic material allows the protective coating to adhere firmly to the surface of the sheet resistance. At both ends of the resistance, however, the sealing material is attached not to ceramic but to a smoother metal material. Because the sealing material is separated from the metal lead, the sealing gap of the metal film will be caused. If the peeling continues, the resistance film is exposed to moist air, where the ni-cr resistance film may dissolve, increasing the resistance and eventually leading to circuit failure.
The dissolution of ni-cr resistance film in water is shown in figure 3. Figure 3a shows an unsealed nickel-chromium resistor. Figure 3b shows the surface of the ni-cr resistor immersed in a drop of deionized water and connected to a 9V battery (simulating a real circuit). Figure 3c shows the same ni-cr film resistor exposed to water for 60 seconds with applied voltage. The relatively large gray area in figure 3c shows that the ni-cr film has actually dissolved in water droplets! It only takes about 30 seconds for the device to open.
In field work, the same thing happens when the protective layer of the sheet resistance is stripped off and exposed to the underlying film of resistance, albeit slowly. In practice, the resistance will not open after 30 seconds, as shown in the picture, but over time the resistance will become larger and larger, leading to eventual failure.
Unlike ni-cr resistor films, tantalum nitride chip resistances do not cause catastrophic device failures due to poor integrity of the packaging or protective coating. When exposed to air, an oxide layer naturally forms on the surface of the tantalum nitride film to protect it from erosion in the presence of moisture and voltage. It is this self-passivating oxide layer that provides excellent moisture resistance for tantalum nitride chip resistors. The ni-cr sheet resistance must rely on the integrity and robustness of the package to prevent moisture from eroding the ni-cr film.
Sheet resistances made from tantalum nitride films are self-passivating, and the protective oxide layer on the surface of the resistance unit protects it from failure, even in the presence of moisture. The TaNFilm resistance does not depend on the integrity of the seal to protect the communication circuit from catastrophic failure due to moisture.
Outdoor applications such as line-feed CARDS, remote DSLAM, and cable amplifier are all threatened by moisture failure when nickel-chromium resistance is used in communication circuit design. In addition, office switching equipment can be very dangerous if it is suddenly placed outside the normal office environment of 23 ° c /50% relative humidity, or if it is exposed to hot summer weather.
In an office air-conditioned environment, where humidity is controlled and the heat cycle is small, there is usually no problem with the nickel-chromium resistance. But when choosing a resistor for a non-air conditioning or outdoor environment, the TaNFilm chip resistor provides a robust connection to the communication system. The continued success of the communications industry depends on the reliability of communications services. Whether it is data, voice or video, the information must be transmitted in a timely manner. Service outages or unreliability often prompt customers to switch to other service providers. However, communication equipment manufacturers are still ignoring the reliability risks of passive components, especially resistors.
Why is that? Aren’t resistors just simple devices that perform simple functions? What could go wrong? What’s wrong with a design engineer or reliability engineer focusing his time on the silicon IC at the center of the circuit board and less on the passive components around it? In communications applications, as in any other electronic system, the reliability of a circuit depends on where it is weakest. In fact, resistors do affect the reliability of the circuit.
Two types of materials are used to make precise sheet resistors: chromium nickel and tantalum nitride (TaNFilM). The resistances of the two materials have similar performance characteristics, except that they are resistant to moisture. Nickel-chromium dissolves in the presence of moisture, usually resulting in catastrophic failure of resistors and circuits, while tantalum nitride resistors resist moisture erosion.
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