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. Interruptions or unreliability of services often prompt customers to switch to other service providers. However, communication device manufacturers are still ignoring the reliability risks of passive devices, especially resistors.
Why is this so? Isn't resistance a simple device that simply performs simple functions? What will happen? Design engineers or reliability engineers focus their time on silicon integrated circuits in the center of the board and pay less attention to the surrounding passive components. What is wrong with this? In communication applications, as in any other electronic system, the reliability of the circuit depends on the weakest place. In fact, resistors do affect the reliability of the circuit.
There are two materials used to make precise sheet resistance: nickel chrome and tantalum nitride ( TaNFilM) . The resistors using these two materials have similar performance characteristics, with one difference - that is, moisture resistance. Nickel-chromium materials dissolve in the presence of moisture, often causing catastrophic failure of resistors and circuits, while tantalum nitride resistors are resistant to moisture.
Figure 1 Â Through-hole mounting resistor package provides comprehensive seal protection
Figure 2 Â Only one of the six faces of the surface mount chip resistor is sealed
Before the surface mount chip resistors were widely accepted in the 1980s and 1990s , metallized hole mounted die-cast resistors were typically used in precision applications. Discrete-mounted discrete resistors have always been fabricated by depositing thin film resistor materials on cylindrical ceramics. Leads are connected to both ends of the ceramic cylindrical molding or coating through the resistive device, thereby producing a fully sealed is illustrated in FIG. 1 product. The chip resistor shown in Figure 2 is sealed only on one side of the device. Unlike a full seal, the chip resistor is only coated on the side containing the film material. Despite the protective sealing of the resistor unit during manufacture, this seal is not completely robust in the harsh outdoor environment in which many telecommunications applications are located.
For example, a chip resistor on a line card in an outdoor cabinet can be affected by temperature cycling and humidity in the outdoor environment. If the line card is in the Midwest, the annual temperature limit can range from -20oF to over +100oF . When cold air comes or storms, the temperature fluctuations will reach 50o in a short time .
When the line card is exposed to such a temperature environment, mechanical stress is generated where the resistance is soldered to the printed circuit board. The organic PC board material differs from the ceramic sheet resistor in expansion and contraction coefficients. The FR-4 PC board material has a coefficient of thermal expansion (Tce) of approximately 16 ppm/ °C, while the ceramic material has a Tce of only approximately 7 ppm/ °C. Different rates of expansion and contraction can cause stress at the resistance solder joints, causing deflection of the resistive sheet during thermal cycling.
The surface roughness of the ceramic material allows the protective coating to adhere firmly to the surface of the chip resistor. However, at both ends of the resistor, the material to which the sealing material adheres is not a ceramic but a smoother metal material. The detachment of the sealing material from the metal lead causes a gap in the sealing of the metal film. If the stripping is continued, the resistive film is exposed to the moist air, and the nickel-chromium resistive film may be dissolved, which causes the resistance value to become large and eventually leads to an open circuit.
The dissolution of the nickel-chromium resistive 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 ) . Fig. 3c shows the same nickel-chromium thin film resistor after exposure to water for 60 seconds under application of a voltage . The relatively large gray area in Figure 3c shows that the nickel-chromium film is actually dissolved in water droplets! It takes only about 30 seconds for the device to open.
In the field work, the same situation will occur when the protective layer of the chip resistor peels off and exposes the underlying resistive film, but the process is slower at this time. In practical applications, the resistor does not open up after 30 seconds as shown in the photo , but as time passes, the resistance value becomes larger and larger and leads to a final failure.
Unlike nickel-chromium resistive films, tantalum nitride chip resistors do not cause catastrophic failure of the device due to poor integrity of the package or protective coating. When exposed to air, the surface of the tantalum nitride film naturally forms an oxide layer that protects the film from corrosion in the presence of moisture and voltage. It is this self-passivating oxide layer that provides excellent moisture barrier properties 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.
The chip resistor made of a tantalum nitride film is self-passivating, and the protective oxide layer on the surface of the resistor unit protects it from failure, even in the presence of moisture. TaNFilm (tantalum nitride film ) resistors protect the communication circuit from catastrophic failure due to moisture, independent of the integrity of the seal.
Outdoor applications such as line feed cards, remote DSLAMs, and cable TV cable amplifiers are subject to moisture failure when using nickel-chromium resistors in communication circuit design . In addition, office exchange equipment can be very dangerous if it is suddenly placed outside the usual 23 °C / 50% relative humidity office environment, or exposed to the sweltering weather in summer.
In an office air-conditioned environment, humidity is controlled and thermal cycling is small, and nickel-chromium resistors are usually not a problem. But when choosing a resistor device for a non-air-conditioned or outdoor environment, the TaNFilm chip resistor provides a robust connection to the communication system.
M11 A Code,Circular Waterproof Connectors
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