The continued success of the communications industry depends on the reliability of communications services. Whether it is data, voice or video, information must be transmitted in a timely manner. Service interruptions or unreliability often prompt customers to switch to other service providers. However, communication equipment manufacturers are still ignoring the reliability risks of passive devices, especially resistors.
Why is this so? Isn't resistance a simple device that only performs simple functions? What will happen? Design engineers or reliability engineers focus their time on the silicon integrated circuit in the center of the circuit board and pay less attention to the surrounding passive devices. What is wrong with this? In communications applications, as in any other electronic system, the reliability of the circuit depends on the weakest point. In fact, resistors do affect circuit reliability.
There are two materials used to make accurate chip resistors: nickel chromium and tantalum nitride ( TaNFilM) . The resistance of these two materials has similar performance characteristics, only one difference - that is moisture resistance. Nickel-chromium materials will dissolve in the presence of moisture, usually leading to catastrophic failure of resistors and circuits, while tantalum nitride resistors are resistant to water vapor.
Picture 1 Â Through-hole resistor package provides comprehensive sealing protection
Picture 2 Â Only one of the six sides of the surface mount chip resistor is sealed
In the 1980s and 1990s , before surface mount chip resistors were widely accepted, metallized hole-mounted molded resistors were commonly used in precision applications. Discrete resistors for through-hole mounting have always been fabricated by depositing thin film resistor materials on cylindrical ceramics. The leads are connected to both ends of the ceramic, and the columnar resistive device is molded or coated to produce a fully sealed product as shown in FIG. 1 . The chip resistor shown in Figure 2 is sealed only on one side of the device. Unlike full seals, chip resistors are coated and sealed only on the side that contains the thin film material. Despite the protective sealing of the resistance unit during manufacturing, this seal is not completely reliable in the harsh outdoor environment where many telecommunications applications are located.
For example, a chip resistor on a line card in an outdoor cabinet will be affected by temperature cycling and humidity in the outdoor environment. If the line card is located in the Midwestern United States, the annual temperature limit can range from -20oF until it exceeds + 100oF . When cold air comes or a storm, the short-term temperature fluctuation will reach 50o .
When the line card is exposed to this temperature environment, mechanical stress will occur where the resistance is soldered to the printed circuit board. The expansion and contraction coefficients of organic PC board materials and ceramic chip resistors are different. The temperature expansion coefficient (Tce) of the FR-4 PC board material is about 16 ppm / ° C, while the Tce of the ceramic material is only about 7 ppm / ° C. Different expansion and contraction speeds will cause stress at the resistance welding point, which will cause the resistance sheet to deflect during thermal cycling.
The surface roughness of the ceramic material allows the protective coating to be firmly attached to the surface of the chip resistor. However, at both ends of the resistor, the material to which the sealing material attaches is not ceramic, but a smoother metal material. Since the sealing material is detached from the metal lead, it will cause a gap in the sealing of the metal film. If the peeling is continued, the resistance film will be exposed to humid air. At this time, the nickel-chromium resistance film may be dissolved, which will increase the resistance value and eventually lead to a circuit break.
The dissolution of nickel-chromium resistance film in water is shown in Figure 3 . Figure 3a shows an unsealed nickel-chromium resistor. Figure 3b shows the nickel-chromium resistor surface immersed in a drop of deionized water and connected to a 9V battery (simulating the actual circuit ) . Figure 3c shows the same nickel-chromium thin-film resistor exposed to water for 60 seconds under voltage application . The relatively large gray area in Figure 3c shows that the nickel-chromium film has actually dissolved in the water droplets! It only takes about 30 seconds for the device to open.
In the field work, when the protective layer of the chip resistor is peeled off and the underlying resistance film is exposed, the same situation will happen, but the process at this time is relatively slow. In actual application, the resistance will not open after 30 seconds as shown in the photo , but as time goes by, the resistance value will become larger and larger and cause a final failure.
Unlike nickel-chromium resistor films, tantalum nitride chip resistors will not cause catastrophic device failure due to poor packaging or protective coating integrity. When exposed to air, an oxide layer will naturally form on the surface of the tantalum nitride film, thereby protecting the film 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. Nickel-chromium chip resistors must rely on the integrity and robustness of the package to prevent moisture from eroding the nickel-chromium film.
A chip resistor made of tantalum nitride film is self-passivating, and the protective oxide layer on the surface of the resistor unit protects it from failure, even when moisture is present. TaNFilm (Tantalum Nitride Film ) resistors do not depend on the integrity of the seal to protect the communication circuit from catastrophic failure due to moisture.
When nickel-chromium resistors are used in communication circuit design, outdoor applications such as line feed cards, remote DSLAMs, and cable TV cable amplifiers are threatened by moisture failure. In addition, if it is suddenly placed outside the usual office environment of 23 ℃ / 50% relative humidity, or is exposed to the hot summer weather, then the office exchange equipment will also be very dangerous.
In an office air-conditioning environment, the humidity is controlled and the thermal cycle is also very small. At this time, the nickel-chromium resistance is usually no problem. But when selecting resistive devices for non-air conditioning or outdoor environments, TaNFilm chip resistors can provide a robust connection for communication systems.
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