Manufacturing Embedded Resistors
Increasing component density and the requirements of higher performance electronic devices are driving the development of embedded passive devices in the printed circuit board (PCB). The benefits of embedded passives are that they free surface space for active devices,improve performance and signal quality by lowering inductance and reduce overall system cost. Embedded passives also yield a more reliable printed circuit board by reducing the number of solder joints. A resistor is an important passive device in an electric circuit. To enable high performance devices,an embedded resistor must achieve a tolerance that allows the PCB design to meet electrical timing and circuit signal quality requirements. The tolerance of embedded resistors is not only determined by the uniformity of the resistor material but also by the PCB manufacturing process which forms them,especially when the sizes of the embedded resistors are small. The stability of the material when subjected to typical printed circuit board processes will also affect the final tolerance of embedded resistors. Gould has developed a thin-film NiCr alloy resistive layer sputtered onto rolls of copper foil for embedded resistor applications. Nickel-chromium alloys possess high electrical resistivity,good electric performance and high thermal stability. The thin film is very uniform,and is capable of forming resistors with tolerances that meet the requirements of high performance PCBs. Merix Corporation is involved in the NIST Advanced Embedded Passives Technology (AEPT) consortium,and has built test vehicles for the consortium and customer prototypes using Gould's thin-film alloy resistor foil. This paper reviews data from the material manufacturer on the effects of specific processing factors on the overall tolerance of the resistor,and describes the board fabricator's process development for reducing resistor variation and improving yields.