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Continued growth in wireless communications is placing tremendous pressure on component manufacturers to provide standard packaged products for large-scale printed circuit board (PCB) assembly flow. Point-to-point and satellite communications systems at microwave and millimeter-wave frequencies have traditionally used expensive custom packages or bare die devices to meet high linearity, high power and broad bandwidth requirements for these systems. The challenge facing component manufacturers is to achieve the same level of performance in standard plastic packages at higher frequencies of operation. Standard plastic, surface-mount packages provide a number of advantages including low cost, ease of handling and assembly, as well as suitability for mass production.

The challenge facing microwave and millimeter-wave circuit designers is to provide high performance, low cost and functionally integrated packaged devices for high volume production. Packaging at frequencies between 10 GHz and 40 GHz has been proven in research papers using expensive custom designs based on ceramic or organic-based materials. The custom design of the package has to take into account the thermal characteristic of the gallium arsenide (GaAs) device, to minimize the losses and reflections of the input and output transitions. And, it must ensure that the package is surface mountable. Even though this approach guarantees the performance, it is expensive and it doesn't address the high volume and cost of the applications.

A transformation from these expensive packages to the standard plastic packages, similar to that which took place in the RF front-ends used in the handset devices world, has to take place if these products are to address the requirements of the new high-frequency systems and manufacturing environment.

Although, plastic packages provide the ultimate solution for security of supply, volume production, efficient assembly and ease of handling, it has always been disregarded by microwave and millimeter-wave engineers due to its poor frequency response, power-handling limitation and the lack of sufficient electrical models.

Circuit designers at Mimix Broadband who have been designing multifunction millimeter-wave products with a broad range of surface-mount packages have introduced a series of products in standard plastic packages that work in the Ku and Ka band frequency ranges. By extracting detailed electrical models of the package using extensive electromagnetic (EM) simulation and combining it with unique design techniques of the GaAs monolithic microwave integrated circuit (MMIC), new designs were developed that extended the performance envelope of plastic packages. Consequently, multiple functions like transmitters, receivers, doublers and power amplifiers that work all the way up to 40 GHz and can put out in excess of 1.5 W of power have been housed in standard plastic packages.

For example, a series of high-power amplifiers (HPAs) that operate in the Ku band between 13.5 GHz and 17.5 GHz are capable of delivering up to 1.5 W of power in standard plastic packages (CMQ1631-QH and the CMQ1432-QH). The design topology basically realizes a fully matched four-stage MMIC fabricated with a 0.25 µm optical pHEMT technology and is packaged in a 4 × 4 mm quad flat no-lead (QFN) package. The linear gain of these amplifiers ranges between 22 dB and 33 dB with input and output return losses better than -10 dB. This extremely compact chip size for a four-stage HPA MMIC was achieved using key design approaches like compact matching topologies based on optimum power transfer and load line analysis for all stages. Rigorous design and layout methods were considered in the optimization of power, gain and bandwidth. The matching networks (input, interstages and output) were realized using metal-insulator-metal (MIM) capacitively loaded transmission lines that allow flexible impedance matching and size reduction. Other GaAs technologies like MIM capacitors over vias were employed to realize shunt elements in the matching network. The capacitor over via provides the advantage of size reduction and adds layout flexibility. Three-dimensional (3-D) EM simulation was applied at different phases of the design cycle. First, EM analysis of different test structures made it feasible to predict the response of the signal over frequency and to come up with more accurate models that can take into account curvature geometries. Second, since the physical size of the matching elements gets smaller with frequency and the traditional models stop to be accurate, EM analysis becomes handy in predicting the tolerance of these elements over process variations. The dielectric of the molding compound and its effect on transistor fingers and matching elements can also be quantified with 3-D modeling. The aggressive use of EM simulation to design spiral inductors and meander transmission lines was also crucial to reduce the overall size of the MMIC. Finally, due to the compact size of the MMIC and how close the different elements are laid out, EM simulation can predict if any signal coupling between close elements is taking place. Plus, it can show how a layout change will stop it from happening. A combination of large on-chip bypass capacitors and resistive loading was used to prevent any low frequency, parametric and odd-mode oscillations, keeping the design electrically stable under all conditions. Power at saturation is plotted in Figure 1.

The HPA MMIC is mounted on the 4 × 4 mm QFN lead frame, with a thermal sliver loaded adhesive epoxy. The package uses appropriate die flag dimensions with respect to the MMIC size and short double bond wires to minimize loss and parasitic effects.

Thermal analysis, using closed forms derived from process characterization over temperature, as well as finite element thermal simulations have been performed demonstrating that at worst-case conditions of 80 °C base plate temperature, with or without RF drive, the peak channel temperature is maintained below 175 °C. Source vias, the layout of the transistor cells and using three-mil-thick GaAs die assured the thermal efficiency. Infrared thermal measurement plots are shown in Figure 2.

The total integration of the matches with the good thermal characteristics and the cost-effective surface-mount plastic package give system engineers the flexibility to reduce the number of external components, use smaller board area and run the product through the standard reflow process without suffering any added assembly cost. Due to their enhanced thermal performance, these devices were suitable for mounting on 30 mil-thick RF boards.

An active doubler (XX1000-QT) with output frequencies from 15 GHz to 45 GHz has been developed in a 3 × 3 QFN surface-mount plastic package that is RoHS compliant. This device combines an active doubler with an output buffer amplifier that delivers up to +15 dBm output power from 15 GHz to 45 GHz and has excellent rejection of the fundamental and harmonic products. The device is designed using Mimix Broadband's 0.15 µm gallium arsenide (GaAs) pseudomorphic high electron mobility transistor (pHEMT) device model technology.

Figure 3 shows the active doubler MMIC bonded onto a 3 × 3 QFN package with bond wires attached and the lid removed. Two bond wires were used at the input and output to reduce the bond-wire inductance and increase the bandwidth to higher frequencies. As can be seen in the image, the doubler MMIC was designed to fit snugly into the standard 3 × 3 QFN package. In order to achieve this level of miniaturization, the design was made as compact as possible with a high level of functional integration and common on-chip decoupling and resonant structures for the dc bias requirements. At millimeter-wave frequencies, unwanted coupling between on-chip structures becomes a major concern due to the creation of unwanted resonance that can lead to instability and oscillations. Large components and multiple structures in close proximity required EM analysis to ensure that there were no unwanted coupling interactions that could lead to instability. Similarly, the transitions from MMIC to package required EM simulation and modeling to ensure the MMIC design was properly matched to the bond-wire and transition impedance mismatches through the die to package transitions.

Schematic of the active doubler (XX1000-QT) shown in Figure 4 provides a functional diagram of the device within the 3 × 3 QFN package outline.

The MMIC doubler was designed using a differential amplifier that feeds a balanced signal into two slotted devices with a common drain. The differential amplifier behaves as an active balun that was designed to have a flat gain response over the entire band. This was achieved by appropriate tuning of the resistive and reactive elements to flatten the gain over the band. Conduction cycles occurring at twice the input frequency at the common drain pair provide the output frequency, which is matched to 50 Ω. The doubler output is then fed into a distributed amplifier that acts as a gain and buffer stage that has been integrated onto the single chip.

Unlike passive doubler devices, the active doubler operates over a range of input and output powers with monotonic behavior and has a smooth output power saturation making it a robust part that can be used in a saturated mode or as a doubler plus gain stage block in the radio chain (Figures 5 and 6).

The saturated output power can be adjusted by setting different dc bias conditions to optimize output power or reduce the power or gain to a desired level of operation.

This active and broadband doubler is designed for applications, including microwave, point-to-point radio, VSAT, E-band radio, automotive and satellite communications.

The XX1000-QT is one of the first QFN plastic packaged devices at 45 GHz. Using a low-cost plastic QFN package and extending its use to 45 GHz gives designers of advanced radio systems the advantage of guaranteed performance specifications, ease of assembly and further cost savings.

The use of low-cost high-frequency packaging solution at millimeter-wave frequencies is inevitable due to the cost pressure exerted by the consumer nature of these high-frequency wireless applications. The solution is to design GaAs MMICs that will compensate for the losses and drawbacks of plastic packages.

Amer Droubi is product manager with Mimix Broadband. Prior to joining Mimix, Droubi worked as product engineer for Celeritek Inc. He received a BS in electrical engineering from the University of Texas, Arlington.

Paul Beasly is a product manager with Mimix Broadband. He joined the company as a MMIC designer specializing in integrated receiver and transmitter products. Beasly earned a BSc (Engineering Physics) and BA (English) from the University of New South Wales, Sydney, Australia.

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