High power density thin film resistors

Untrimmed Ni-Cr resistors on glazed alumina substrates, hermetically sealed in a dry nitrogen atmosphere, survive and remain stable when subjected to power densities greater 2000 W/in². Large (>7 × 10⁻⁴in2) resistors with standard top-hat trimming fail at less than 1000 W/in² due to hot spots, however, a change in trim line configuration results in stable resistors at greater than 1200 W/in². Resistance-time characteristics depend on mean resistor temperature, which increases (at a given power density) as the square root of resistor area. Results predict, e.g., that all resistors in a 3/8 × 3/8 × 0.080 inch (0.5W) package can be powered at 2000 W/in² for several years before a 1 percent resistance change is experienced. The resistor area for such a configuration, 2.5 × 10⁻⁴in², is 50 times less than that required with the present power density design value of 40 W/in². Resistors deposited on unglazed alumina are even more stable than those deposited on glazed alumina because of their lower operating temperature.