[Design Application]
High-Power Thick-Film Resistor Designs Boost Power Density
With Careful Attention To Materials And Construction, Resistors Can Achieve Continuous Power Ratings Of 250 W/in.² In Hybrid Designs.
The electronics industry is continuously searching for ways to reduce the package size of circuits. As a result, parts density is increasing with every design. Hybrid devices are part of this ongoing effort to fit greater amounts of circuitry into smaller spaces.
Reducing the size of hybrid designs requires the development of smaller high-power components and the ability to place these power components alongside temperature-sensitive parts in densely packed designs. As the parts density rises, however, heat radiated by power components becomes unacceptable because of the negative impact it has on circuit performance and reliability (see "Temperature Versus Reliability,").
As a result, one of the major goals in new designs is to minimize heat inside the package through techniques such as heatsinks, heat pipes, forced-air cooling, and water cooling. Because power resistors are among the main components that contribute to heat generation, they are the focal point of much of these efforts.
Traditionally, power resistors have been available in a variety of styles, including wirewound, carbon composition, carbon film, and metal film. In their usual cylindrical construction, such resistors typically operate at a power density of 5 W/in.2 Another style of power resistor, the conventional thick-film resistor, can operate at 10 W/in.2 At the same time, thick-film chip resistors can attain a power density of about 30 W/in.2
Unfortunately, these devices transfer most of their heat through radiation. (Thick-film chip resistors are a special case, since they conduct heat through their end terminations directly to the circuit board.) And because these resistor types are difficult to heatsink, their use in high-density designs is limited.
But planar thick-film resistors offer a higher-power alternative. These components can be made with integral heat spreaders that attach to heatsink assemblies. Properly constructed, planar thick-film resistors can operate at significantly higher power densities than traditional power resistors.
Using refined processes and materials, it's possible to manufacture planar thick-film resistors capable of operation at 250 W/in.2 The performance of such resistors has been tested for samples operating at 40, 60, and 1000 W, constructed in various package styles using either air- or water-cooled heatsinks. The results obtained with these prototypes can be applied in the design of hybrid devices, as well as multichip modules.
Operation at very high power densities requires manufacturing techniques of the highest caliber. Thick-film resistors can tolerate minor defects in the resistor layer when operating at 10- to 50-W/in.2 But at 250 W/in.2, the resistors will fail as a result of minor defects, such as pinholes or airborne contaminants resting on the wet-screened surfaces during manufacture. Management of current densities, spatial thermal heat patterns, and voltage stress are all critical to success at the higher power densities.
Operation at 250 W/in.2 stresses components to much higher levels than customary. The resistor is operated at higher voltage, current, and power levels than are usually accepted in hybrid design. These higher levels also are considerably above the manufacturer's published specifications for the thick-film inks used to make the resistors. Tests performed on the experimental thick-film power resistors show the effects of this higher stress level.
Although all of the resistor samples tested were designed to function at 250 W/in.2, packaging and assembly varied according to each resistor's overall power rating. Each resistor element consisted of thick-film resistor material deposited on an alumina substrate.
In the case of the 40-W resistor, the substrate was housed in a TO-220 package, while the 60-W version was encased in a somewhat larger Z package. In both cases, solder was used to attach the package to the heat spreader.
For the 1000-W resistors, packaging consisted of an aluminum heatsink mounted to the resistor substrate. Detailed assembly instructions are given below.
There are several different material systems used to make thick-film inks. Ruthenium dioxide (RuO2) is the classic resistor material of choice in the thick-film industry. Its resistivity ranges from 1 Ω per square to 1 MΩ per square. (It is conventional practice in the thick-film industry to use a unit called "ohms per square." The basic equation for resistance is R = *l/A, where * is resistivity, l is length, and A is area, all in consistent units of length. A = thickness (t) * width (w) so that:
R = */t * l/w
The term */t is defined as the resistivity in "ohms per square.") These resistivities are 12.5 to 12.5 million times higher than those of wirewound alloys.
It's then possible to use these inks to make resistors with very high values in a very small area. Inks made with RuO2 have been optimized for resistance stability at fairly low power densities.
Another option, pure silver inks, were originally formulated for use as conductors. Their resistivity is comparable to that of wirewound alloys with a temperature coefficient of resistance that's very high, often in the range of 500 to 1000 ppm/°C.
Additionally, alloys of palladium and silver or of platinum and silver have recently become available. The temperature coefficient of resistance of these alloys is low, often in the 50-ppm/°C region, which is satisfactory for most resistor applications. Besides these alloys, many others have been made into thick-film inks to satisfy the need for a particular characteristic.
Conventional RuO2-based resistor inks perform poorly in surge applications. Their oxide and glass layers aren't good at removing heat from the printed resistor. To overcome this deficiency, thick-film manufacturers are developing materials specifically for surge protection, which is required in many telecom and power-supply applications. Surge materials must not only withstand large power spikes, they also must dissipate heat effectively. In repetitive surge situations, heat can build up quickly. Proper packaging is required to aid the heat flow from these materials.
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