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RF packaging: an evolution towards SiP technology
Today, microwave products invade our daily life with an increased use in many areas such as communication, scientific and commercial applications. As shown in Figure 1.1 from ITRS 1 [1], the development of new microwave systems is characterized by the need of higher performance, smaller size, lower power, lower cost and multi-functional systems. These characteristics cannot be met with conventional packaging and interconnect technologies because there are limitations in interconnect density, thermal management, bandwidth and signal integrity. System-in-Package (SiP) (section 1.1.6.2) is perhaps the most important technology to address these limitations. In this section, we will give some background on RF packaging. After defining packaging, we discuss challenges and issues of several factors that influence the electrical, thermal and mechanical behavior of the RF package. Next, evolution of integration technology from Single Chip Package (SCP) to SiP through Multi Chip Module (MCM) (section 1.1.6.1) is reviewed.
Finally, the current assembly techniques at the chip level are discussed.
Radio Frequency
The frequencies falling between 3 M H z and 300 G H z are called Radio Frequencies. The term Microwave refers to the frequencies between 300 M H z and 300 G H z that are electromagnetic waves with wavelengths ranging from 1 m to 1 mm. The microwave frequencies are divided in operational bands. Table 1.1 shows frequency range and free space wavelength with letter designation of each band from 1 to 300 G H z. We note also that the millimeter wave range start at about 30 G H z.
Interest of packaging
To understand the importance of RF packaging, it is necessary to think of the package in terms of a system. Normally, a RF system is composed of several elementary functions such as frequency generation, amplification, mixing, filtering, etc. These elementary functions may be integrated on one semiconductor chip thanks to MMIC technology [5, 6]. However, monolithic integration of these functions on a single chip implies that the integrated circuit becomes extremely complex and may be specific to one single application. Moreover, additional constraints appear particularly in the dimensions of the chip which is very thin, difficult to use and handle. Another approach, offering more flexibility consists of placing and interconnecting the IC (or MMIC) in a package; it is possible to speak of SCP [7, 8] for one IC (or Chip Scale Packaging (CSP)) and MCM for several ICs. This approach (Figure 1.2) offers more flexibility than integra-tion on chip described above because the level of functionality is achieved at the package and not only at the chip.
RF Packaging: challenges and issues
Packaging at microwave and millimeter wave frequencies comes with the same challenges as packaging at lower frequencies. Furthermore, the complexity of the package increases with frequency, because the circuit features have dimensions that are in the same order as the wavelength. In this case, the package type will be selected according to the performance and cost. For some applications, such as space, we promote the performance of the package such as the hermeticity, reliability, environmental endurance and redundancy. For others, usually consumer high-volume products, we will tend to minimize the cost. Beyond the cost, the package becomes part of the circuit and strongly affects reliability and electrical performance of the system [9]. In order to minimize the effect of the package on the MMIC performance, different factors such as electrical, material, mechanical and thermal properties must be considered during the design and fabrication process as shown in Figure 1.3.
The design of an RF package must address three parts: RF signal, Direct Current (DC) bias and cavities. RF signal is the most critical part that will affect the electrical performance. The electrical performance of a package is determined by the level of signal integrity that it maintains for signal transmission, from the chip to the board. For instance, a MMIC within a package must be connected to the module; the most frequently used method is the wire bonding method. However, at microwave frequencies, a wire bond behaves as an inductor and at millimeter-wave frequencies it can even act as an antenna or resonator. Other major difficulties, at high frequencies, are energy coupling between adjacent traces, attenuation due to losses, cavity resonances over the operating frequency range and radiation.
Substrate material selection is another important part of the packaging at high frequencies because material proprieties will affect the impedance and insertion losses of transmission lines. Normally, there are two main families of substrate products: organic and ceramic mate-rials. The choice of the substrate material is based on various criteria such as the packaging technology (Printed Circuit Board (PCB), thick film, thin film . . . ), the electrical constraints that are related to the application, mechanical and thermal constraints that are related to the environment and of course to the cost which is very important for the manufacturer.
RF packaging: an evolution towards SiP technology
The main features of a dielectric substrate are:
• Dielectric constant or relative permittivity, εr.
• Dissipation factor or loss tangent, t an δ.
• Thermal Coefficient of Expansion, T C E (p pm/◦C ).
• Thermal conductivity (W /m.◦K ).
The dielectric constant affects the dimensions and impedance of transmission lines. A sub-strate having a high dielectric constant will reduce radiation losses and decrease the size of circuit while a low dielectric constant will minimize coupling and provide better isolation between parts of the package. In addition to the material itself, the thickness of the substrate should also be taken into account. A thin substrate results in a suppression of higher order propagation modes, reduces radiation losses and minimizes the circuit size.
The insertion losses of a circuit are related to the dielectric substrate. Low loss circuit requires a dielectric substrate having a dissipation factor as low as possible. We also note that the insertion losses are related to the metallic conductor’s loss through the conductivity of the used metallization (Cu, Ag, Au . . . ).
Another essential concern that must to take into account in packaging is the thermal perfor-mance. Thermal conductivity is a significant factor in the choice of materials. This is especially a very important factor in the case of RF power amplifiers which can have high power densities. For reliable direct attachment of die, the T C E of the substrate materials must be matched to the semiconductor materials such as Silicon (Si) and Gallium Arsenide (GaAs).
Levels of packaging
RF packaging can be performed at several levels; the number of levels is related to the final product. Normally, MMICs can be packaged at three levels [6]; these levels that represent the manufacturing process of microwave systems are shown in Figure 1.4. Each level of packaging has an interconnection device associated with it.
Microwave packaging levels are summarized in the following points:
• Level 1: Active and passive component interconnections on a monolithic semiconductor chip, wire bonding and flip chip assembly techniques
• Level 2: Packaging of semiconductor chips into SCP, Dual-In-Line Packages (DIPs), Small Outline Integrated Circuits (SOICs), Ball Grid Array (BGA), Land Grid Array (LGA), Quad Flat Package (QFP) . . . .
• Level 3: MCM, SiP using surface mount techniques, coaxial connectors, waveguides . . .
• Level 4: Microwave module, PCB, Board to Board connections . . .
Table of contents :
Abstract (English/Français)
Acknowledgements
Contents
List of Figures
List of Tables
List of Acronyms
List of Symbols
Résumé
Introduction
1 Introduction to RF packaging technology
Introduction
1.1 RF packaging: an evolution towards SiP technology
1.1.1 Radio Frequency
1.1.2 Definition of packaging
1.1.3 Interest of packaging
1.1.4 RF Packaging: challenges and issues
1.1.5 Levels of packaging
1.1.6 Evolution
1.1.7 Assembly technologies
1.2 LTCC technology
1.2.1 Definition
1.2.2 Evolution
1.2.3 LTCC materials
1.2.4 Advantages of LTCC technology
1.3 LTCC as a solution for RF packaging
1.3.1 High frequency characteristics
1.3.2 Thermo-mechanical properties
1.3.3 High density integration
1.4 Recent applications
1.4.1 IMST
1.4.2 NEC
1.5 Objective of this thesis
2 Establishment of LTCC technology
2.1 Choice of LTCCmaterial
2.1.1 Maximumoperating frequency limitations
2.1.2 Manufacturing process consideration
2.2 Design and Layout
2.2.1 Software requirements for LTCC design
2.3 LTCC process
2.3.1 Tape preparation
2.3.2 Via and cavity formation
2.3.3 Via fill
2.3.4 Screen printing
2.3.5 Stacking and alignment
2.3.6 Lamination
2.3.7 Co-firing
2.3.8 Post-processing
2.4 Technological problems
2.4.1 Screen printing problems
2.4.2 Mechanical distortion
2.4.3 Substrate shrinkage problems
2.4.4 High permittivity material integration
2.4.5 Via fill difficulties
2.4.6 Cavity deformation
2.4.7 Fired LTCC circuit cut
2.4.8 Circuit fragility andmechanical constraints during RF measurements .
2.5 Design rules implementation
3 LTCC technology validation for RF packaging applications
3.1 Theoretical background
3.1.1 Transmission line theory
3.1.2 Planar microstrip resonators theory
3.2 LTCC circuit simulations
3.2.1 Circuit and EMsimulations
3.2.2 Transmission line simulations
3.2.3 Microstrip resonator simulations
3.3 LTCC circuit fabrication and measurement
3.3.1 Fabrication
3.3.2 Measurement
3.4 LTCC characterization in theWband
3.5 « mm-packaging » project
3.5.1 Package design
3.5.2 Package fabrication and measurement
Bibliography
