Comparing Full Lateral Metallization and Reference Plane Stitching in LTCC Boards for Satellite
Title
Author One° , Author Two+ , Author Three*
°Affiliation One, Dept of Electrical Engineering, University of L’Aquila, L’Aquila, Italy
e-mail:
+UAq EMC Laboratory, Dept of Electrical Engineering, University of L’Aquila, L’Aquila, Italy
e-mail:
*Affiliation Three, Dept of Electrical Engineering, University of L’Aquila, L’Aquila, Italy
e-mail:
Abstract – In this paper a methodology for taking into account the frequency dependence of a possible referencing (grounding) techniques in LTCC boards for satellite applications is presented. The aim of this work is to analyze and compare the input impedance and S-parameters values for different strategies of inter connection of the reference planes among them.
The numerical analysis presented has proven the importance of the lateral metallization set up in the LTCC board working into a metal housing. For board used stand alone the lateral metallization can be replaced by an adequate density of vias stitching among the planes. The modelling agrees well with real grounding solution.
I. INTRODUCTION
Due to the increasing of the bit rates and the decreasing of the voltage and noise margins, it is known that a correct design of the power delivery network of hybrid digital/RF systems is becoming a critical issue.
For all PCB a correct design of the voltage reference system (very often referred as Ground or GND) [1] is fundamental either to maintain a correct balance and stability of the supply voltage used to supply the active devices and to control the impedance of the passive interconnections.
A nowadays used technique to achieve these goals is to connect all reference planes to each other with the aim to make the reference system almost totally equipotential at least in a defined range of frequencies.
The way to do this changes from application to application. In satellite systems, due to the requirements of reliability, this is done by the combination of internal stitching vias between the reference (GND) planes and an external lateral metallization of the full edges of the LTCC substrate as represented in Fig. 1
The limit of this technology is the impossibility to automate it and this implies an increase of the associated costs and time of production.
Because of this there is the need to explore other solutions for the connections of reference planes in order to reduce their production costs ensuring adequate electrical performances.
In this paper is carried out an analysis of the electrical performances of cheaper technological solution: the replacement of the lateral metallization by a properly designed array of internal and external stitching vias among the reference planes. This solution allows one to save time, eliminating the straining and drying operations for the metallization gold around the LTCC substrate.
A three dimensional (3D) electromagnetic analysis, based on the Finite Integration Technique [3,4] is used to characterize the electrical performances of the structures considered.
The paper is organized as follows: in Section II the geometric details of the considered configurations are presented along with the figures of merits used to establish the differences between the proposed solutions. In Section III the performances’ analysis is carried out for different densities of the stitching vias. In this Section is also performed a comparison between the performances of the actual and proposed configuration. In section IV it is explored the impact of the metal housing of the LTCC board on the performances of the proposed configuration with stitching vias. Finally section V offers some concluding remarks.
Fig. 1- An LTCC board with lateral metallization.
II. THE CONSIDERED CONFIGURATIONS AND FIGURES OF MERIT
The figures of merit chosen to measure the goodness of the interconnection among reference planes by using the classic solution of lateral metallization or the proposed one with internal and external stitching vias are:
- the input impedance Zin looking into the board from a port localized between the first two planes for different configuration of the reference planes connections
- the scattering parameter (and in particular S21) of a microstripe for different configurations of the connections of the strip’s reference plane to the other planes.
(a)
(b)
Fig. 2 – SMA connectors: (a) lateral view and (b) perspective view with waveguide port [4].
After having developed the computational model as described above a new computation of the S-parameters between the IN and OUT connectors (see Fig. 14) is performed.
The results are shown in Fig. 17a and 17b for the usual three configurations defined in Section III as:
I. full metalized (see Fig. 8 a)
II. stitching (see Fig. 8b)
III. metalized only at the front ends (see Fig. 8c)
Frequency [GHz]
(a)
Frequency [GHz]
(b)
Fig. 3 –LTCC board inside the metal box: (a) frequency spectrum of |S11| and (b) frequency spectrum of |S21|.
As expected the presence of the housing, of the connectors and of the bonding changes the shape of the S-parameters and in particular of |S21|.
The full metalized and metalized only at the front end models are quite similar with a maximum difference of 0.5 dB at around 10 GHz.
Different consideration should be done for the stitching model: the partial inductance introduced by the stitching vias causes at certain frequencies (12.5 GHz in Fig. 17b for example) significant peaks due to the resonance with the distributed capacitance of the planes. Resonances peaks, as shown in Fig.17b.
For the case considered, below 10 GHz, there is an almost perfect overlapping among the three configurations considered.
III. Conclusions
A detailed analysis has been carried out in order to identify the better figures of merit to quantify the effects of the connection of multiple reference planes in real configurations of LTCC board used in satellite applications.
Input impedance of the planes’ cavities and the S-prameters of interconnections using the reference planes have been identified and computed by means of three dimensional full wave electromagnetic simulations.
Multiple conneting strategies have been considered and from the total amount of results obtained it turns out that an adequate density of internal vias, stitching the planes among them, is equivalent to the costly (in terms of time and money) solution of the full lateral golden metallization.
This result opens interesting technological scenarios in the production of these boards.
ACKNOWLEDGMENTS
This work was supported by the Italian Ministry of University (MIUR) under a Program for the Development of Research of National Interest (PRIN grant # 2006095890).
References
[1]B. Archambeault, PCB Design for Real-World EMI Control, Kluwer Academia Publisher, Boston, USA, 2002
[2]J.L.Drewniak at All., “An experimental procedure for characterizing interconnects to the DC power bus on a multilayer printed circuit board”, IEEE Trans. on Electromagn. Compatibility, vol. n. 39, n. 4, November 1997.
[3] T. Weiland, “A Discretization Method for the Solution of Maxwell’s Equation for Six Component Fields”, Electronics and communication, (AEÜ), Vol. .31 (1977), p. 116.
[4]CST Studio Suite 2008, available at