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Industry News

A 60 W RF Transistor in a Cost-effective Plastic Package for 2 GHz Cellular Applications

May 14, 2005
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Cellular radio systems such as GSM Edge, N-CDMA or W-CDMA are seeing increasing demand in today’s wireless infrastructure. As volume production increases, equipment manufacturers require low cost solutions with good performance from their component suppliers.

Freescale Semiconductor has pioneered the use of high power plastic packages for high frequency RF transistors and integrated circuits used for wireless infrastructure applications. In doing so, the company has been able to offer an alternative to conventional metal ceramic packages and a cost-effective approach to automated RFpower amplifier assembly.


This article describes the design of the MRF5S19060N, a 60 W 2 GHz LDMOS transistor designed for N-CDMA1900 cellular applications in a cost-effective plastic package.

A Cost-effective Packaging Technology

The MRF5S19060N, high power LDMOS transistor is using a well characterized over-molded plastic process, commonly used in most semiconductor power packages for automotive and industrial applications with TMOS device structures.1

The TO270WB-4 package (standard JEDEC designation TO-270) offers a copper heat sink for excellent thermal performance, tight mechanical tolerances (50 ?m or less) and a small footprint. Examples of conventional RF packaging and alternative RF plastic packaging for 60 W 2 GHz products are presented in Figure 1.

Fig. 1 Footprint comparison of the MRF19060 ceramic and MRF5S19060N plastic devices.

The split lead design of this single-ended plastic device contributes to reliability, ensuring minimal stress during temperature cycling and eases the soldering process inspection. By working closely with encapsulant suppliers, thermo-physical properties of plastic such as glass transition temperature and adhesive characteristics have been optimized as well as RF properties like dielectric constant and RF loss tangent.

The selected plastic mold compound is capable of continuous operation at 200°C. This, combined with the Al wire and Al die metallization, allows actual device junction temperatures up to 200°C without the reliability risks associated with using typical plastic packages with Au wire on Al die metallization above 150°C (Kirkendall voiding). Such high temperature plastic RF devices also enable automated assembly of RF power amplifiers (JEDEC MSL3 260°C reflow capability and multiple mounting options including SMT2). Finally, this plastic package is compatible with Pb-free manufacturing process and compliant with RoHS standards. Tight mechanical tolerances and SMT mounting, in addition to a low component price result in a lower system cost over conventional RF packaging for the equipment manufacturer.

LDMOS IC Die Technology

The active device of this 60 W LDMOS plastic transistor is a single die with integrated passive shunt capacitors on the same die for effective input and output pre-match at 2 GHz. The internal configuration of both the plastic and conventional devices is represented in Figure 2. Compared to a standard discrete ceramic transistor composed of multiple die and passive components (six total for the MRF19060, i.e. 2 × 30 W active die, 2× input MOScap and 2× output MOScap), a single IC die brings assembly cost reduction and assembly consistency.

Fig. 2 MRF5S19060N plastic (a) and MRF19060 conventional RF package (b) internal configurations.

Design Methodology

The single die nature of the MRF5S19060N device requires an accurate prediction of the optimum pre-match capacitor values and locations for a first pass design success. A change of the capacitor value or distance from active area would necessitate a mask change and a new silicon fabrication. Design cycle time and time to market would be impacted.

At this step, a simulation tool is needed to accurately determine the optimum wire loop/shunt capacitor topology. The simulation of bond wires and IC shunt capacitor has enabled a first pass layout for this single die discrete transistor.

The needed flexibility to internally fine tune the device without sacrificing performance has been found on the wire bond side. The wire length being set by the die layout, wire loop height and wire-to-wire pitch DOEs (Design Of Experiments) have been run successfully to achieve optimum RF performances. It has been found that the BONDW model from the ADS commercial simulator is helpful to optimize the output wire array configuration.3

Impedances

Impedances achieved for the plastic part are slightly higher than for the conventional RF package part. Due to the drain-to-source capacitance and on resistance of the new technology being lower than the one used in the conventional design, the output real part of the impedance increases from 1.9 to 2.5 ? (see Table 1).

Video Bandwidth

Attention has been paid to the video bandwidth (VBW) to reach 30 MHz. The VBW of this traditional output pre-matched transistor is mainly limited by the resonance frequency of the drain feeder of the application circuit and the output shunt capacitance of the device (see Figure 3).

Fig. 3 The device's internal shunt capacitor to resonate with the printed drain feeder.

The output shunt decoupling capacitor value of the device has been divided by two compared to the equivalent conventional transistor. At the printed circuit board level, a second drain feeder has been added to reduce the associated low frequency inductance resonating with the internal output shunt capacitance of the device, as represented in Figure 4. Doubling the drain feeder pushes the resonance towards high frequencies by 1.4 times, as shown in Figure 5. Cumulative effects of both actions show an IM3 resonance above 40 MHz.

Fig. 4 The double drain feeder circuit layout.

Fig. 5 Effect of doubling the drain feeder on video bandwidth.

Thermal Resistance

The thermal resistance is better for the plastic device at 0.8°C/W versus 0.97°C/W for the conventional packaged device.4 Part of this reduction is coming from the use of copper for the heat sink. The other part is coming from the thermal optimization done at die level.

RF Performance

Typical gain for the MRF5S19060N plastic device is 14 dB compared to 12.5 dB typical for the conventional device MRF19060.5 This 14 dB gain level is considered a good figure, compared to the state-of-the-art 16 dB achieved in a conventional package on the latest technology generation (HV6).

Attention has also been paid to peak saturated power (100 W). Along with good VBW performance, this device is easy to operate in a linearized system, such as a digital pre-distorted (DPD) amplifier, for example. Also, the good linearity level in back-off conditions allows this device to operate as a driver, as shown in Figure 6.

Fig. 6 Typical two-carrier N-CDMA drive-up performance at 1960 MHz.

Conclusion

Several aspects of the design of the MRF5S19060N device have been covered. The inherent plastic design constraints have been explained as well as the methodology used to design a low cost discrete power transistor with good performance. Design for video bandwidth has been introduced and the main RF performances have been presented.

This device is a new step into RF power transistors. The MRF5S19060N is the first 60 W plastic device at 2 GHz and is an answer to lower cost high power RF transistors for cellular applications. Moreover, this high power plastic device now makes available full plastic line-ups for 2 GHz cellular applications, when associated with a three-stage IC plastic driver such as MW4IC2020M or equivalent.

References

1. D. Abdo, F. Danaher, A. Elliot and M. Mahalingam, “Continuous Operation at 200°C Device Junction Temperature: The Final Frontier for RF Power Semiconductor Plastic Packaging,” ECTC2004.

2. Freescale Application Note AN1907, “Surface-mount Solder Attach Method for the MRF9045MR2 in the TO-270 Plastic RF Package.”

3. Advanced Design System (ADS) software from Agilent Technologies – bond wire model (BONDW).

4. Freescale Application Note AN1955, “Thermal Measurement Methodology of RF Power Amplifiers.”

5. Freescale MRF5S19060N Datasheet.

Freescale Semiconducteurs S.A.S.,
Toulouse, France

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