LTCC technology validation for RF packaging applications 

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LTCC as a solution for RF packaging

The LTCC technology was originally developed in 1980s for military and medical applica-tions. With the explosion of wireless technologies in middle of 1990s, the LTCC has become very popular for wireless and telecommunication products [36, 37] such as cellular mobile phones (0.9 − 2 G H z), Global Positioning Systems (GPS) (1.6 G H z), Wireless Local Area Net-work (WLAN)/Bluetooth (2.4 G H z), broadband access connection systems (5.8 − 40 G H z), millimeter wave communication systems (60 G H z) [38, 39] and vehicle anti-collision radar (77 G H z) [40].
The common requirements of such devices are good electrical performances especially, at high frequencies, high thermal and mechanical stability and, of course, low fabrication cost. Today, LTCC technology has attracted the attention of microwave engineers for RF packaging applications due to the excellent high frequency performance in terms of dielectric and conductors loss (up to 100 G H z), environmental conditions in terms of thermal stability and hermeticity, and finally the capability of integrating passive elements and fine line patterning that enables miniaturization and high packaging density.
As discussed in section 1.1.4, the electrical, thermal and mechanical material features strongly affect – beyond the design – the performance, size and reliability of RF packages. This section shows why LTCC is a good choice for RF and MMIC packaging applications. The high frequency characteristics, as well as, the electrical properties of LTCC materials are firstly discussed, and then the thermo-mechanical properties. Finally the passive integration, which is the main feature of LTCC technology, is also presented.

High frequency characteristics

The electrical properties of substrate materials are very important parameters for high-frequency applications. The LTCC substrate offers excellent electrical properties, and stability up to millimeter wave frequencies. The electrical parameters of substrate material include dielectric constant, dissipation factor and conductor loss.

Dielectric constant

The dielectric constant also called relative permittivity (εr ) is the real part of the complex permittivity (ǫ = ǫ′ − j ǫ′′) of the dielectric substrate. In high frequency electronic modules, different values of dielectric constant are advantageous depending on the application. For transmission lines (e.g . microstrip), a low dielectric constant is desired at microwave and millimeter wave frequencies to reduce capacitive effects between elements, and to avoid higher-order mode excitation (see section 2.1.1). On the other hand, the wavelength λ of electromagnetic waves in the material (Equation1.1) is inversely proportional to the square root of the dielectric constant. c λ = f εr.
where c is the free-space velocity (c ≈ 3 × 108 m/s), λ is the wavelength in m, and f frequency in H z. (1.1)  is the In this case, a high dielectric constant is beneficial in order to minimize circuit size such as filters, and other components. Furthermore, it is very important to use a very high dielectric constant to make decoupling capacitors [27]. The LTCC material offers a wide range of dielectric constants, because it depends on the proportion of glass material. Currently, commercial LTCC tapes exhibit εr (generally measured at 1 M H z) from 4 to 10 or more (Table 1.4). In addition, capacitive tape layers with high dielectric constant up to 300 can be found.

Dielectric Loss

The high frequency transmission loss is due to both dielectric and conductor loss (α = αc +αd ). The dielectric loss is related to the imaginary part of complex permittivity and is generally expressed in terms of the dissipation factor or loss tangent (t an δ) [41]. We note that the con-ductor and dielectric loss are expressed according to the transmission line topology (section where ǫ′ and ǫ′′ are the real and imaginary parts of the complex permittivity respectively.
In RF applications, a low dissipation factor is desired to avoid excessive losses due to the dielectric substrate. The dissipation factor of LTCC materials varies from 0.001 to 0.006 (at 1 M H z) which is very low when compared to some organic materials.

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Conductor loss

The other part of the total loss is due to the conductor loss (αc ). The conductor loss depends on the skin depth and surface roughness of the substrate. The skin depth (δs ) represents the distance that the RF current penetrates into the conductor thickness. At low frequencies, the current is uniformly distributed within the conductor. As the frequency increases, the RF current concentrates on the surface part of conductor. This results in a conversion of RF energy into heat, and therefore increases the insertion loss. The relationship for skin depth is given by Equation 1.3 [41] δs = 1 (1.3) σπµf.
where σ is the metal conductivity in (S/m), µ is the permeability of the metal (µ = µr µ0, µ0 = 4π × 10−7) and f is the frequency in (H z).
As discussed in section 1.2.2, the low firing temperature of LTCC materials allows the use of noble conductors such as copper, silver and gold. These conductors have a higher conductivity than other metals. It is also important to take into account the roughness of the conductor and dielectric which can increase the conductor loss at millimeter wave frequencies. The correction factor of attenuation due to the roughness is plotted in Figure 1.16. The new value of conductor loss is given by multiplying this factor by the calculated value of conductor loss without roughness as given in the following relation [42]

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 
Introduction
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

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