Chip resistors: 0603 size

Chip resistors are constructed without wire bonds or glass-covered silicon substrates, which would suggest that discrete surface-mount chip resistors are superior terminators to QSOP or axial leaded resistor at high frequencies — and they are. The construction of a typical chip resistor package is shown in Figure 3(a). The device is built on a high purity alumina ceramic substrate and has a thickness of 0.022 inches. The wraparound terminations form small series inductances and the chip terminals form a small shunt capacitance, as shown in Figure 3(a), resulting in the high-frequency model shown in Figure 3(b).

Since the entire substrate thickness is dielectric, the shunt capacitance is greatly reduced from the QSOP-type assembly. Also, the series inductance is reduced due to the elimination of the bond wires found in the QSOP. This results in the parasitic series inductance being reduced to about 850 pH, and the shunt capacitance across the resistor element shrinks to approximately 0.25 pF. The improved return loss performance of this chip resistor is shown in Figure 5. The 0603 chip resistor performs as a good terminator in applications with significant spectral content to 6 GHz and beyond.

BGA resistors

So, what would happen if we could eliminate most or all of the series inductance due to the wraparound terminals of the chip resistor? One might speculate that the frequency performance would improve yet again — and this is certainly the case. The construction of a ball grid array (BGA) resistor array is shown in Figure 4(a). This BGA design uses a “flip chip” configuration with the resistor element mounting down toward the PC board. This mounting style eliminates the need to route current flow in an arched path as it is for the chip resistor terminals.

Now the current flows in a more planar fashion and the series inductance is reduced to about 150 pH. Also, since the resistor element size is reduced (and element density is increased), the shunt capacitance present across the resistor element is reduced to about 0.1 pF, as shown in Figure 4(b). Return loss is shown in Figure 5. BGA resistor arrays have been tested for return loss up to 40 GHz and have shown good terminator performance up to 25 GHz.

Conclusion

Not every resistor is created equal when it comes to unwanted parasitics and performance at high frequencies. One might expect newer surface-mount terminator arrays such as the QSOP to perform better than older, through-hole resistors such as axial leaded resistors but this is not always the case. The surface-mount QSOP array shows no significant improvement over the axial leaded through-hole device at microwave frequencies even though it is a newer surface-mount device.

The surface-mount chip and the BGA show significantly better high-frequency performance due to the elimination of lead and laser trim inductance as well as a reduction of shunt capacitance across the resistor element. BGA packages reduce the reactive parasitics even more than chip resistors through the elimination of series inductance found in bond wires and wrap-around terminals. The high-frequency performance of the flip chip BGA shows the best terminator performance in applications with significant spectral content to 25 GHz and possesses one of the highest element densities available in a resistive terminator today.

Notes:

1. The axial discrete is an IRC part number BR-01-50R0F, the 0603 chip is IRC part number PFC-W0603HF-02-50R0F, the QSOP is IRC part number GUS-QSCA-01-47R0J, and the BGA is IRC part number CHC-CC0910B-01-50R0F

2. Models were generated from measured return loss data taken from an HP 8753 network analyzer configured with S parameter test ports and 30 kHz to 6 GHz operation.

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ABOUT THE AUTHOR

Jerry Seams is the applications manager for TT electronics at IRC Advanced Film Division, Corpus Christi, Texas.