High Frequency + High Reliability = Thoughtful Component Consideration
By Jerry Seams, Applications Engineering Manager, IRC Advanced Film Division

The trend in today’s military electronics is to higher frequency operation. By the same token, defense applications have typically required high reliability components. This has led to the need for electronic components that fit both high reliability and high frequency specifications. Nowhere is this need greater than in high frequency passive components providing the attenuation and line termination functions that preserve signal integrity of high frequency microwave and high speed digital military electronic systems.

A variety of factors determine the success or failure of components fitting the bill for both high frequency and high reliability performance. The design and layout of the component as well as the materials used in the construction of the device may affect the high frequency performance of a device. In addition, the materials used in the component may determine the survivability of the device in harsh environmental conditions such as high moisture or humidity.

Thin film materials are often used for high frequency circuits. The ability to pattern very small and very accurate geometries as well as low loss at RF and microwave frequencies make thin film technologies attractive for these designs. Figure 1 shows the return loss of a thin film chip specifically designed for broadband termination applications to 40GHz.

A wide variety of materials are available to pattern thin film conductors. Titanium, palladium, tungsten, gold and aluminum are among the typical elements used to pattern thin film conductors in microwave circuits.

But when the time comes to select a resistive material for a high frequency circuit, the choices shrink dramatically. Tantalum Nitride and nichrome (nickel-chrome) are the only two popular thin film resistive materials available. Though these materials are based on metals like the conductor elements, they make superior resistor elements due to their stability over time and temperature. Nichrome and Tantalum Nitride have similar electrical and environmental performance characteristics except when designed into applications where moisture or humidity may be present. This, of course, includes almost all outdoor applications and many indoor applications as well.

Nichrome films dissolve when exposed to moisture in the presence of an electric potential. Tantalum Nitride (also known as TaNFilm®) films do not dissolve when exposed to moisture in the presence of an electric potential.

Tantalum Nitride is one of a small number of metals referred to as “valve metals.” A naturally occurring passivating layer of oxide that forms on the surface of the resistor exhibits a rectifying characteristic and opposes corrosion of the film in an electric field, as shown in Figure 2. The formation of this oxide layer occurs naturally in a Tantalum Nitride resistor and is referred to as self-passivation.

On the other hand, nichrome resistors are not self-passivating. The packaging, sealing and mechanical protection of nichrome resistor films is critical to prevent failures in a nichrome chip when subjected to moisture. If moisture permeates this barrier, then the resistor film will be corroded. Nichrome thin film resistor elements are dependent on the mechanical integrity of the package and sealing materials to prevent failure -- Tantalum Nitride resistive elements are not.

Nichrome resistors were known to have moisture performance problems since their development by IRC in the 1950s. As late as 1991, General Electric® ASCD, Binghamton, NY, discovered “open” failures during a powered moisture test of MIL-R-55342 chip resistors manufactured with nichrome films. General Electric published a GIDEP alert (EE-P-92-01) identifying the problem. General Electric identified the failure mechanism as follows:

“The failure mechanism is an electro-chemical interaction which dissolves the resistive film. In extreme conditions, open resistor element failures can occur in seconds. In less severe cases, a gradual increase in resistance value will occur. The resistance increase will continue until failure if the nichrome element is further exposed to moisture with voltage applied.”

General Electric engineers concluded that the moisture performance of nichrome resistors is dependent on the integrity of the package, commenting, “As a consequence, nichrome chip resistors will not survive even short-term exposure to moisture unless protected or placed in a hermetic enclosure.”

The dissolution of nichrome resistor film in water is demonstrated in Figures 3, 4 and 5. Figures 3a and 3b show a nichrome resistor and a Tantalum Nitride resistor with no encapsulation. Figure 4 shows the same resistors with a drop of de-ionized water on the surface and connected to a 9-volt battery to simulate circuit conditions. Figures 5a and 5b show the resistors after 60 seconds’ exposure to water and the applied voltage. The large grayish area in Figure 5a is the area where the nichrome film actually dissolved into the water drop! The device open circuited in approximately 30 seconds.

The Tantalum Nitride resistor, on the other hand, is still intact. A small area of discoloration can be seen near one of the terminals. This is the area where the self passivating properties of the Tantalum Nitride element allowed the naturally occurring oxide layer to grow thicker during the test, protecting the resistive element beneath from damage. The Tantalum Nitride resistor shifted by +0.16% compared to the catastrophic failure of the open nichrome device.

Let’s take a more detailed look at the most popular resistor package today -- the chip resistor. There are many variations of the standard chip resistor used in military RF and microwave applications for line terminators, loads and attenuator pads, but they are similar in construction. Chip resistors normally possess encapsulation on only one side of the device, as shown in Figure 6. Instead of sealing the entire resistor on all sides, the chip is coated only on the side containing the thin film element. Even though the resistor element is sealed inside a protective coating at the time of manufacture, it may not remain sealed in the demanding outdoor environments of many military and defense applications.

For example, a chip resistor in an outdoor environment is subject to the temperature cycles and humidity present in the outdoor environment. For an airborne application, ambient temperature extremes can range from -70 degrees centigrade to +60 degrees centigrade -- or even more inside of an enclosed space such as a cabinet or box. Rapid thermal cycling or thermal shock is an issue on aircraft that can change altitudes in terms of miles in seconds.

As the PC board in the aircraft is exposed to these temperature variations, mechanical stresses appear at the chip solder connections to the printed circuit board. Organic PC board materials expand and contract at a different rate than ceramic chip resistors. The temperature coefficient of expansion (Tce) of FR-4 PC board material is about 16ppm per degree C, whereas the Tce of the ceramic substrate used to construct a chip resistor is about 7ppm per degree C. Different expansion and contraction rates result in stresses on the chip solder joints and terminations, producing flexing of the chip when exposed to thermal variations.

The surface roughness of the ceramic provides excellent adhesion for the protective overcoat applied to the surface of the chip. At the ends of the chip, however, the encapsulant adheres to the much smoother termination metals -- not the ceramic. It is at this point that the seal to the resistor film can be breached due to delamination of the encapsulant from the termination metals. If the delamination continues to progress so that the resistor film is exposed to a humid or moist atmosphere, dissolution of a nichrome resistor film can occur, resulting in positive resistor shifts, and ultimately, an open circuit.

A compromise between the high frequency performance advantages of thin film and the inherent reliability risks of nichrome thin film resistive elements no longer need be made. High frequency Tantalum Nitride line terminators and attenuators are now available with the intrinsic reliability of Tantalum Nitride thin film resistor elements. The blend of high reliability self passivating Tantalum Nitride film resistive elements with computer-aided high frequency design bring together the best of both worlds to produce passive components for high reliability RF, microwave and high speed digital design.

IRC AFD offers Tantalum Nitride based high frequency devices for RF/microwave and high speed digital applications. The PFC-HF series chips are standard 0603, 0805 and 1206 size chip packages specially designed for high frequency performance to 6GHz. These devices are available with MIL-PRF-55342 approval. The MWR series flip chip provides microwave terminations characterized to 40GHz. In addition to chip products for use in RF and microwave applications, IRC offers BGA (ball grid array) flip chip terminators characterized to 40GHz in the frequency domain and to 25pS rise times in the time domain.

IRC Advanced Film Division
www.irctt.com
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