High Power Density Thin Film Resistors

Stability characteristics of nickel-chromium thin film resistors have been measured for power densities ranging up to 4500 W/in
²
Resistance change was found to depend on resistor temperature, sheet resistance and substrate Wpe. For 220
\Omega
/sq films the time for a one percentresistance change, t
1
\approx
3.8 X 10
^-14
exp(16820/T), where t
1
is in h and T is mean resistor temperature in °K. Resistors with 4 X 10
^-5
in
²
area, deposited on glazed alumina, increase 0.04°C for each W/in
²
power density. Consequently, for room temperature ambient t
1
\approx
,1.7X 10
⁵
h (20 years)at 2000 W/in
²
(0.08W). Resistors on as-fired alumina run cooler (0.01°C/W/in
²
) and therefore have higher values of t
1
Top-hat trimmed resistors developed hot spots which caused failures at ~1000 W/in
²
. Modified trim configurations were tested and shown to avoid these failures.