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Nickel-Phosphorous (NiP) embedded thin-film resistive material has been used since the early 1970s as terminating resistors in a variety of electronic systems. Through generations of digital designs, the shape, size, and layout of the terminating resistors has evolved to meet the demands of increased IC density and the changing packaging architectures. This paper will review some of the earlier applications of NiP embedded thin-film terminating resistors and describe the improvements made in the material to meet today's demanding requirements for high density PCB termination.
Proper signal-line termination reduces signal reflection increases speed and performance and improves signal integrity. This is critical for high speed digital signals as it reduces trace, pad and via impedance and via stubs.(1)
NiP embedded thin-film terminating resistors can be placed very close to the end of the driveline, improving impedance matching and reducing line delay. Figure 1 and Figure 2 show typical design layouts for both series and parallel termination using NiP embedded thinfi lm as embedded resistors in multilayer circuit boards. Unterminated drivelines and stubs are also reduced or eliminated and the inductive reactance is lowered when compared to discrete resistors. This is critical as the inductance of chip resistors combined with via inductance can seriously impact cross-talk as well as the leading edge of the high speed signal.(2)
Design and Use of NiP Embedded Thin-Film Termination Resistors
NiP embedded thin-film was used for terminating resistors in 1973 in the Tektronix 21 and Tektronix 31 desk top programmable calculators. The terminating resistor network was used at the end of a connecting cable that tied peripherals to the calculator. The resistor network consisted of forty-four 120 ohm value resistors. Twenty-two resistors were on each side of a rigid doublesided FR4 substrate. NiP embedded thinfi lm 25 ohm per square sheet resistivity product was used and the resistor dimensions were approximately 60 mils wide and 290 mils long.(3)
A more sophisticated use of NiP embedded thin-film for terminating resistors occurred a year later in 1974. Control Data's Aerospace (Military) group designed 100 ohm per square NiP embedded thin-film into a multilayer board as parallel terminating resistors for ECL logic devices (Figure 3). The resistor design (Figure 4) had two elements in parallel extending from a common via. A 50 ohm...