The embed lamination procedure induces the high stress on the resistor and produces the high tolerance alteration. The embed lamination increases tolerance above 1%. The variation changes while the first lamination process may be caused by the surface stain or chemical and mechanical interaction between resistive layer and resin. In the separate tests a kiss lamination cycle was implemented while the embed lamination step. The kiss cycle ramps the press temperature to high priority to the applications of the final pressure. This cycle induced a smaller increase in the tolerance as compare to immediate simultaneous application of high pressure and temperature at the starting of the cycle.
The resistance values describe the change in the average values induced by the stress analysis. The 1 square, 100 ohm per square materials were laser trimmed to 120 ohm. The resistance of the Nickel-Chromium-aluminum-silicon alloy varies less than Nickel-chromium alloy. The resistance variation offered by nickel-chromium alloy to a lower value while the Nickel-Chromium-aluminum-silicon alloy resistance changes to a much higher value. These variations in the average value can be counted for through artwork recompense as stated by Wang et al. the variations in the tolerance as shown in the following table are not recompensed by the artwork modification.
|Stress test||Resistive layer alloy||Sheet resistance||Tolerance change|
|1000 thermal cycles per mil STD 883||NiCr||25||.12|
|10X solder reflow per EIA/JEDEC 22-A112-A||NiCr||25||.01|
|Humidity 40oC, 95 % RH, 240 hours||NiCr||25||.30|
Resistors are 1 sqaure, 100 ohm per square laser trimmed to better than 1%, 3 sigma tolerance. The analysis in every bunch of four are from left to right thermal cycles, solder reflow, relamination and humidity.
Etched resistors constituted of thin film nickel-chromium alloy layers are easily laser trimmed to a large tolerance. The laser trimming doesn’t alter the underlying glass/epoxy substrate. The Nichrome alloy with aluminum and silicon alters less in average resistance value and alteration in tolerance of laser trimmed resistos.
The laser trimmed thin film nichrome resistors embedded in FR4 epoxy substrate attain nominal variations by exposure to printed circuit board production procedure steps and to natural stresses. The minor changes state that laser trimmed embedded resistors offer the potential to be produced into a finished multilayer board with finished tolerance less than 1%.