内容

Elasticity of fuzz button brings about an effective contact with LTCC T/R ...In the future work, when the mini-SMP and lid described in this paper

 AN OVERALL LTCC PACKAGE SOLUTION FOR X- BAND TILE T/R MODULE

 Zhong-Jun Yu*, Zheng Xu, Yun-Kai Deng, and Zhi-Guang Zhang
Institute of Electronics, Chinese Academy of Sciences, Beijing 100190, China
 

Abstract|An overall Low-Temperature Co-¯red Ceramics (LTCC) package solution for X-band T/R module has been presented in this paper. This tile type package contributes to a dramatic reduction in size and weight of the T/R module. Moreover, an obvious merit of ceramic housing is better consistency of Coe±cient of Thermal Expansion (CTE), compared with the traditional combination of ceramic board and metal housing. The schematic diagram and 3-D structure of the T/R module have been presented and a novel vertical interconnection based on Ball Grid Array (BGA) has been proposed to connect vias in the lid and those in the stage of the main LTCC pan. The LTCC T/R module has been fabricated and measured. It
is compact in size (20 £ 20 £ 2:6mm3) and has a weight of 3.5 g. The measured transmit output power is 33§1 dBm in the frequency range from 8.8GHz to 10.4GHz, and the measured receive gain and Noise
Figure are 29{30.5 dB and 2.6{2.8 dB, respectively.

 1. INTRODUCTION
Phased array radar has versatile performance and has been widely  applied in civilian and military missions. In the extensive applications,  a phased array radar system consists of thousands of Transmit/Receive
(T/R) modules. The cost, weight, and volume of the phased array  radar are mainly related to T/R modules [1]. Therefore it is  indispensable to do research on low-cost, low-weight, and compact T/R  module. Low Temperature Co-¯red Ceramics (LTCC) provide a 3-D  circuit design method [2] and a great many microwave and millimeter  wave components and devices have been researched and realized [3].
     Tile type T/R module adopting overall LTCC package solution  shows an advantage in reducing weight and volume compared with  traditional metal-sheltered LTCC board [4]. To realize the compact  T/R module mentioned, it has to achieve speci¯c requirements, such  as vertical interconnection and e±cient integration. Seldom there are  reports on compact X-band LTCC T/R module. In a published case,  although the X-band MMIC receiver front-end is compact receive gain  needs to be improved [5].
In this paper, an overall LTCC package solution for X-band T/R  module is presented. The system and structure of the T/R module is  provided and a novel vertical interconnection taking advantage of Ball
Grid Array (BGA) is proposed to connect vias in the lid and those in  the stage of the main LTCC pan. Based on the prototype, a LTCC T/R  module of 8.8{10.4GHz has been fabricated and measured. Test result
reveals that the T/R module delivers a remarkable performance while  maintaining tiny and compact. It shows promise in diverse applications  in addition to phased array radar. The LTCC tile with vertical RF feed
is quite suitable for smart antenna use [6, 7], and the low weight feature  allows it to be mounted on small UAV for agile missions [8].

  2. SYSTEM OVERVIEW
The schematic diagram of the T/R module is shown in Figure 1.
The whole function circuits except for the control unit are realized by  ¯ve GaAs MMICs, including core chip, serial to parallel chip, Power  Ampli¯er (PA) chip, Low Noise Ampli¯er (LNA) chip and switch chip.
They are connected with microstrip as well as bond wires.
To reduce the size of the T/R module, both 6-bit phase shifter and  6-bit attenuator are integrated in the core chip, as shown in Figure 1.
Figure 1. Schematic diagram of the X-band T/R module.Progress In Electromagnetics Research Letters, Vol. 38, 2013 183

Switches inside the core chip and a switch next to the antenna operate  together to alter between transmit mode and receive mode.
In transmit mode, the switches inside core chip routes the signal  from the common port to PA, then from PA to antenna. While in  receive mode, signals received by antenna are routed from LNA to  core chip, then to the common port. As for the control unit, FPGA  generates control signals, and they are routed to the manifold on the  bottom of the T/R module.
All MMICs are mounted inside a hermetic box using LTCC  processes. The hermetic housing not only protects vulnerable and  friable MMICs from outer pollution and scratch, but also works as  a shielding cavity for a single T/R module. As shown in Figure 2, to  obtain a compact LTCC T/R module, LTCC is employed to design  both side walls and board of the housing, resulting into an overall  ceramic pan. And at the radiating end a stage is reserved in the cavity  for vertical interconnection of signals.
The lid of the housing is also fabricated by LTCC processes, the  same technique as that of the pan. There is bene¯t for a high level of  consistency and e±ciency on manufacturing.
Kovar provides a CTE compliant with ceramics. For this reason a  Kovar frame is fabricated as the seal ring, which conjoins ceramic pan  and lid mentioned above, shown in Figure 2. As for RF signal feeding  (a) radiating end  (b) common port end
Figure 2. Structure of the LTCC T/R module.184 Yu et al.  between the pan and the lid, BGA (Ball Grid Array) is adopted. The  central ball routes RF signal while outer balls serve as coax-like GND.
By the way, the height of Kovar seal ring is fabricated slightly less than  the diameter of BGA balls. Thus BGA balls are able to contact with  lid and pan e®ectively.
At the common port end, RF signal carried by microstrip is fed  down to bottom of the ceramic pan through a quasi-coaxial structure,  which is formed by a central RF via and six outer shield vias. Beneath  the ceramic pan there locates an aluminum carrier, in which a quasi-coaxial structure is embedded. This quasi-coaxial structure is formed  by fuzz buttons embedded in a FR4 cylinder. Beneath the aluminum  carrier there locates the PCB (Printed Circuit Board). Another quasi-coaxial structure is formed by vias in the PCB. These three quasi-coaxial structures interconnect together to let through RF signals. In  the end, a mini-SMP is mounted upside-down on the PCB to supply an  interface to the common port end. Actually there are many scattered  vias in the ceramic pan, aluminum carrier (embedded in sheltering FR4  cylinder) and PCB which serves to supply power and route control  signals vertically. They are not shown in the illustration for a clear
view.

  3. VERTICAL INTERCONNECTION DESIGN
The challenge of proposed tile package solution is mainly on vertical  interconnections. At the very end of vertical interconnection, mini-SMP is adopted to supply an interface while maintaining a compact  size. As for mini-SMP, detailed parameters have been provided by the  manufacturer [9]. Return loss is greater than 26 dB at X band, and  insertion loss is less than 0:1 £pf, which is about 0.3 dB in this case.
Such an insertion loss is quali¯ed in this work.
As for vertical interconnection at the radiating end, three quasi-coaxial structures are designed, i.e., in the lid (Figure 3(b)), in the stage  (Figure 3(a)) and the other constructed by BGA balls. Dimension  of the quasi-coaxial structure imitates that of coaxial structure, thus  50Ohm match impedance is achieved [10].
The LTCC green tape employed here is 0.094mm thick each layer  after sintering, which possesses a dielectric constant of 5.9. The lid is  made of 5 layers, and the ceramic pan is made of 22 layers with top
10 layers excavated to form a cavity and a stage. Top of the stage is  partially plated to form a ground plane with clearance around inner RF  path (Figure 3). At the same time 2 layers below the stage there locates
a ground plane. It serves as the GND of microstrip lines distributed  inside the cavity routing signals among MMICs.Progress In Electromagnetics Research Letters, Vol. 38, 2013 185
The quasi-coaxial structures in the lid and that in the stage share  the same dimension. Seven vias construct outer shield with a diameter  Do (1.8mm) and inner via possess a diameter Di (0.254mm). Then
BGA is adopted to connect vias in the lid and those in the stage of the  ceramic pan. Every single BGA ball possesses a diameter of 0.5mm.
When microstrip line routes into the stage, it changes into stripline  (Figure 3(a)). Width of the microstrip WMS is 0.28mm. The stripline  is actually o®set strip transmission line, since the distance between the
stripline and ground plane on top of the stage (10 layers) is not the  same as that between the stripline and internal ground plane (2 layers).
Width of the o®set strip transmission line WSL is initially calculated  according to [11]. Then it is moderately tuned and set to 0.13mm  since upper GND of the o®set stripline is not perfect. A taper is  located between stripline and microstrip for smooth transition thus  energy re°ection is reduced [12]. Length of the taper Ltp is set to be  0.5mm.
The whole RF path at the radiating end, i.e., from top of the  lid down to end of the microstrip in the cavity is simulated by HFSS  software. Simulation results are show in Figure 4. The insertion loss is  better than 0.1 dB and Standing Wave Ratio (SWR) is less than 1.14.
According to simulation results, the vertical interconnection at the  radiating end is adequate for use.
(a) (b)
Figure 3. Vertical interconnection at the radiating end.186 Yu et al.
(a) (b)
Figure 4. Simulation results of interconnection at the radiating end.
(a) VSWR. (b) Insertion Loss.
Figure 5. Vertical interconnection at the common port end.
Another interconnection requiring evaluation is that of the  common port end (Figure 5). Three quasi-coaxial structures exist here.
RF signal traveling along the microstrip is vertically fed down through  the ceramic pan. An inner via and six outer shield vias construct a  quasi-coaxial structure. All of these vias in the ceramic pan possess aProgress In Electromagnetics Research Letters, Vol. 38, 2013 187  diameter of 0.254mm. The diameter of outer shield ring is 2mm.
Clearance are made on GND planes where RF via pass through.
Diameter of clearance in the GND beneath the ceramic pan (D2)  is preset to be 1.8mm, thus diameter of clearance in the internal  GND (D1) has to be elaborately adjusted to get 50Ohm characteristic  impedance [13].
At the bottom of the ceramic pan, i.e., the bottom of the tile T/R  module, RF vias are pressed with fuzz buttons. The fuzz buttons are  embedded in a FR4 cylinder, the same material with that of the PCB
beneath. The quasi-coaxial structures in the FR4 cylinder and that in  the PCB share the same dimension. Elasticity of fuzz button brings  about an e®ective contact with LTCC T/R module and PCB [14].
Vertical interconnection at the common port end is simulated in  HFSS. All of the components illustrated in Figure 5 are included. In  Figure 6, simulation result shows the structure in Figure 5 has an  insertion loss less than 0.2 dB, while its SWR is about 1.2. From the  simulation results in Figures 4 and 6, a conclusion is drawn that vertical  transitions presented in this paper are capable in the design of LTCC  T/R module.
Figure 6. Simulation results of interconnection at the common port  end.

  4. FABRICATION AND MEASUREMENTS
The fabricated X-band LTCC T/R module is shown in Figure 7. The  T/R module has a compact size of 20 £ 20 £ 2:6mm3 (without mini-SMP connector), and has a weight of 3.5 g, including mounted MMICs  and mini-SMP connector. In the future work, after the present lid with  mini-SMP is replaced with smart antenna, the weight can be reduced  further.188 Yu et al.
(a) (b) (c)
Figure 7. Fabricated X-band LTCC T/R module. (a) Front view.
(b) Bottom view. (c) Weight measurement of the T/R module.
Figure 8. Aluminum carrier with fuzz buttons embedded.
As shown in Figure 7(b), bottom of the T/R module is etched to  distribute RF and DC pads. These pads are pressed with fuzz button  to test the fabricated LTCC T/R module. In this demonstrator, fuzz  buttons are embedded in FR4 bricks. After enclosed with thin metal  foil, FR4 bricks are then embedded in an aluminum carrier which  is plated with Ni in Figure 8. The bottom side of the carrier is  pressed with PCB. When performing tests with the carrier, an indium  slice on the carrier where PA locates is recommended for better heat  dissipation.
In addition, a test vehicle is specially designed and fabricated for  this X-band LTCC T/R module, as shown in Figure 9. The vehicle  provides the feed of RF signal, bias voltage and control logic. Tile T/R  module is ¯xed by test jigs. The test vehicle can test two tile modules  simultaneously.
Measurement of output power in transmit mode is performed by  connecting the common port end to Agilent E8257D signal generator  and connecting the radiating end to Agilent N1912A power meterProgress In Electromagnetics Research Letters, Vol. 38, 2013 189  Figure 9. Test vehicle with LTCC T/R module.
Figure 10. Output power in transmit mode.
through an attennuator. As shown in Figure 10, in the transmit  mode, a measured output power of 33 § 1 dBm is achieved in the  frequency range from 8.8GHz to 10.4GHz. And in this circumstance  the saturated input power is 12 dBm.
Measurements of gain and NF in receive mode are performed
with Rohde & Schwarz ZVT20 Vector Network Analyzer and Agilent
N8975A Noise Figure Analyzer, respectively. As shown in Figure 11,
receive gain is about 30 dB and the smoothness is acceptable for the
application. NF is 2.6{2.8 dB. According to the equation of cascade
receive Noise Figure refered as (1), the loss of vertical interconnection
(NF1) of BGA has an obvious impact on the receive NF. Since the
receive Noise Figure is less than 2.8 dB, it can be indicated that the
loss of the vertical interconnection of BGA is small. Therefore, the
presented T/R module prototype has been demonstrated and shows190 Yu et al.
Figure 11. Gain and Noise Figure in receive mode.
good performance.
NF = NF1 + NF2 ¡ 1
G1
+ NF3 ¡ 1
G1G2
+ : : : + NFn ¡ 1
G1G2 : : :Gn¡1
(1)
5. CONCLUSION
This paper has introduced an X-band tile T/R module prototype
which operates at a frequency range of 8.8{10.4GHz. Simulation of
vertical interconnection methods involved has been performed, and a
demonstrator has been realized and tested. An overall package solution
bene¯ting from LTCC processes is adopted, which contributes to the
reduction of size and weight of the module. The whole T/R module
has a dimension of 20 £ 20 £ 2:6mm3, bearing a weight of 3.5 g. The
tiny and compact module achieves high output power, receive gain
and low noise. Test result indicates that it is feasible to adopt vertical
interconnection with techniques detailed in this paper.
In the future work, when the mini-SMP and lid described in this
paper are replaced with a patch antenna integrated into the lid, weight
of the LTCC T/R module can be reduced further.
ACKNOWLEDGMENT
The authors would like to acknowledge the assistance and support of
colleagues in Department of Space Microwave Remote Sensing System,
IECAS.Progress In Electromagnetics Research Letters, Vol. 38, 2013 191
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