50 GHz Broadband SMT Package for Microwave Applications
Katsuyuki YOSHIDA, Takayuki SHIRASAKI, Seigo MATSUZONO, Chihiro MAKIHARA
Kyocera Corporation
Semiconductor Components Group, R & D
Address: 1-1, Yamashita-cho, Kokubu, Kagoshima, 899-4396, Japan
Tel: (81) 995 46 1100, Fax: (81) 995 47 1290
E-mail: k.yoshida@scpd.kyocera.co.jp , c.makihara@scpd.kyocera.co.jp
Abstract
Due to the rapid growth in the use of Internet and mobile
communications, the high speed transmission capacity
requirements have been increasing at a high rate. As a result
of this, the semiconductor and packaging technologies have
seen
significant
advances.
Especially,
wireless
communication systems in the microwave/millimeter wave
bands, such as LMDS (Local Multipoint Distribution System),
Point to Point radio, and satellite communications are
expected to see significant growth in a consumer market.
High Frequency Integrated circuits to be used in broadband
transmission would be implemented in materials such as
GaAs, SiGe, and InP. Some effort is seen to develop
passivation films for these devices so that they can be used
without additional packaging, but so far these attempts have
not met with much success. The requirements for ceramic
packaging have remained very strong and stable. As the
microwave/millimeter wave applications are gradually
transferring to the consumer market, the packaging must meet
the demands for high reliability, miniaturization, lower cost,
and low electrical loss. Although several surface mount
package types for high frequency have been proposed to meet
those requirements, the return loss starts dropping drastically
around 30GHz level. Then, there are no clear reports on the
work done to reduce these losses.
This paper shows and explains the key points of the
simulation and design technologies for high frequency SMT
package. Then, we introduce practical examples of packages
designed for high frequency by using simulation technology in
order to improve the transmission properties of the package.
We have manufactured prototype samples of such packages
and confirmed the results of simulation by actual
measurements. The measurement results show S11<-15dB
and S21<-1dB per port up to 50 GHz (including the board
interface). Therefore we introduce a surface mount BGA
package suitable for up to 50 GHz. As for package material,
we used low loss LTCC(Low Temperature Cofired Ceramic
[GL560]), (Er=6.0, tangent delta= 0.0023 at 10 GHz) in order
to lower the capacitance at the ball portion.
1. Introduction
The requirements for high frequency packages in the
LMDS and Point-to-Point radio programs are as diverse as
they are in number. Listed below are some of the
requirements:
1. Low transmission loss
2. Transmission from DC to broadbands
3. Hermeticity
4. Surface mount configuration type
0-7803-7038-4/01/$10.00 (C)2001 IEEE
Several SMT (surface mount type) packages have been
introduced into the market to lower the cost of package, as
well as that of board assembly. For example, in cellular
handsets, chip components such as SAW filter, crystal
oscillators, and PA module packages are mounted on the PWB
(Printed Wiring Board) using a single solder reflow cycle.
Likewise, this cost effectiveness is expected in packages used
at microwave frequencies by also adopting surface mount
configuration type packages, and there have been several
reports of such packages. However, at frequencies over
30GHz, the return loss (S11) in an SMT package is degraded,
and, therefore, a package that achieves DC signal I/O
transmission of 50Ghz with a low loss is still a goal that has,
until now, not been reached.
This paper describes a solution for an SMT package for
high frequency applications. In the course of developing this
solution, we have considered hermetic package sealing, as
well as first and second level package reliability. This solution
has been achieved by using novel approaches in investigating
the causes of performance degradation in package
transmission lines. In optimizing the package design, we have
extensively used computer simulations to thoroughly
investigate the mechanisms of reflection at all transmission
lines that compose the package.
2. Background
Figure 1 shows a photograph of an SMT package
developed for microwave bandwidths in the past. The
construction of this package is 11mm square OD, composed of
multilayer alumina ceramic. Interconnect board used for
package mounting is also alumina ceramic, and solder material
is used for package mounting. The mounting pad diameter is
0.3 mm. The bonding finger area of package and pattern of
interconnect board are executed as a coplanar structure with
ground pattern to enable measuring of the electrical
performance of the package by using a wafer probe.
Fig.1
The example of conventional SMT package
The measurement method of the package and interconnect
board is illustrated in Figure 2, and the actual measurement
results are shown in Figure 3. Wafer probe ACP40-GSG200
2001 Electronic Components and Technology Conference
has been used for measurement, and LRM method is adopted
for calibration.
Fig.2
Measurement Method
signal-ground pads of 0.10mm, and impedance matching
controlled at 50 Ohm.
(b) In the VIA area, two port measurement has been
designed to have location of probe contacts in the same plane
(or X axis) – see Figure 5.
(c) Similarly, in the Interconnection area, two port
measurement has been designed to have location of probe
contacts in the same plane (or X axis).
As shown in Figure 6, each probe interconnects the pads of
the substrate, and measurement of the horizontal transitions
through the microstrip line area between the two ports is
made.
Fig.5
Fig.3
Test samples for VIA
Measurement results of conventional package
At frequencies over 20 GHz, the return loss is over -10dB
and package performance is significantly degraded. The cause
of the degradation is hypothesized as follows.
To develop an SMT package with low transmission loss
for microwave and millimeter wave applications, it is
important to accurately understand the qualitative and
quantitative behavior of transmission loss that occurs in each
of the areas composing the package. These areas are depicted
in Figure 4: GCPW (Grounded Coplanar Waveguide), VIA,
and Interconnection area (including the area of solder ball
attach). Therefore, all of these areas have been carefully
considered when optimizing the package design. To evaluate
the characteristics of each of these areas, a test vehicle,
composed of multilayer alumina ceramic, has been prepared
for measuring electrical performance of the package at high
frequencies:
Fig.4
RF Surface mount package
(a) GCPW area has been designed to have substrate
thickness of 0.4 mm, line width of 0.15mm, pitch of ground-
0-7803-7038-4/01/$10.00 (C)2001 IEEE
Fig.6
Test samples for Ball
The results of these measurements are shown in Figure 7:
(a) GCPW area and (b) VIA area show return loss to be lower
than –20dB at 40GHz. These results are acceptable for most
high frequency applications.
To improve transmission line performance, optimization in
the GCPW and VIA areas has been performed. Not only has
the impedance matching been considered, but the location of
the ground conductors has also been optimized to avoid
leakage of electromagnetic energy. However, the return loss in
the interconnection area is worse than –10dB at 20GHz range.
This is confirmed to be the cause of transmission line
performance degradation for the SMT package / interconnect
board assembly. Therefore, in order to develop an SMT
package with a low transmission loss for microwave and
millimeter wave applications, it is necessary to optimize the
solder ball attach area of the package.
2001 Electronic Components and Technology Conference
Fig.8
(a) GCPW
Simulation & measurement results of ball
Next, we have analyzed the E-magnitude distribution at 30
GHz – see Figure 9. It is confirmed that E-magnitude is
concentrated mainly at the mounting pad. Moreover, the
transmission mode is not orderly.
Fig.9
(b) VIA
E-magnitude distribution
Typically, to achieve a hermetic seal, a metal lid is used in
SMT packages for high frequency applications. Therefore,
metallization is required on the top side of the substrate for
sealing. This creates a capacitor between the solder ball
mounting pad and seal metallization. This capacitor greatly
influences the package, and accelerates the degradation of
package characteristics at high frequencies.
To reduce the capacitance value at the ball pad area, we
have prepared a test vehicle with a smaller mounting pad size
(with a diameter of 0.15 mm vs. 0.3 mm originally). Once
again, we have conducted 3D simulations and actual
measurements – see Figure10.
(c) Interconnection area
Fig.7
Measurement ｒesults of package elements
3. Improved package design
This section describes our effort in optimizing the
interconnection area.
We have conducted 3D simulations using HFSS. The
simulation results are shown in Figure 8. These results are
similar to these obtained in actual measurements. The return
loss is drastically degraded starting at 20GHz.
Fig.10
0-7803-7038-4/01/$10.00 (C)2001 IEEE
Measurement results of improved ball
2001 Electronic Components and Technology Conference
The results of both, simulations and actual measurements
show that return loss is better than -10dB in the
interconnection area. The E-magnitude distribution is
illustrated in Figure 11.
However, by reducing pad diameter to 0.15mm, it may be
caused to have difficulties at assembly.
Futhermore,
Reliability at solder joint level will be weaken when a package
would be mounted onto the board which has a large
mismatching of T.C.E.
4. Further investigation
Fig.11
E-magnitude distribution
From these results, we have concluded that reducing the
size of the mounting pad from 0.3 mm to 0.15 mm in diameter
will alleviate the concentrated E-vector distribution in the
solder ball attach area.
Next, using BEM(Boundary Element Method), we have
conducted simulations of impedance values by reviewing the
2D cross section at the solder ball attach area. The impedance
is ~38 Ohm when the diameter is 0.3mm, and ~46 Ohm when
diameter is 0.15mm. These results clearly show that, by
reducing the capacitance value (smaller diameter of mounting
pad), high frequency transmission characteristics in the
interconnection area are improved.
Using the gained knowledge, a test package has been
prepared to measure the package characteristics after second
level mounting. The test vehicle is 11mm square OD with 0.15
mm diameter RF mounting pad and pad pitch of 0.8 mm,
mounted on alumina ceramic interconnect board. The test
results are presented in Figure 12.
Fig.12
Measurement results of improved package
The return loss (S11) between DC and 40Ghz is better
than –10dB, and this proves that return loss is improved at
second level mounting. These results are acceptable for an
SMT package for high frequency applications.
We have established that the interconnection of an SMT
package has wide influence on the electrical performance of
the package. Package characteristics are drastically improved
by reducing the capacitance value in this area.
0-7803-7038-4/01/$10.00 (C)2001 IEEE
(a) RF characteristics
One of the methods to reduce the capacitance value in the
solder ball attach area is to use a package material with low
dielectric value. Measurements of S parameters have been
performed on a test vehicle made from Low Temperature
Cofired Ceramic (Er=6.0, tangent delta=0.0023 at 10GHz
[GL560]). Figure 13 shows the measurement method,
including the interconnect board. The actual measurement
results are shown in Figure 14.
Fig.13
Measurement method of SMT package
Fig.14
Measurement results of SMT package
The construction of this package is 7mm square OD with
0.3 mm diameter RF mounting pad and pad pitch of 0.8 mm,
mounted on RO4003 (Duroid) interconnect board, and solder
material is used for mounting. The bonding finger area of
package and pattern of interconnect board are executed as a
coplanar structure with ground pattern to enable measuring of
the electrical performance of the package by using a wafer
probe. The measurement results, including the second level
mounting, are: S11<-15dB, S21<-1dB per port (up to 50
GHz). The simulation results are very similar.
(b) 2nd level reliability test
As we have demonstrated, by reducing the mounting pad
diameter the electrical performance of the package is
significantly improved. However, this reduces the solder joint
2001 Electronic Components and Technology Conference
reliability. We have, therefore, evaluated the second level
reliability of the package. The construction of this package is
Low Temperature Cofired Ceramic 7mm square OD with 0.3
mm diameter RF mounting pad and pad pitch of 0.8 mm,
mounted on RO4003 (Duroid) interconnect board. A
temperature cycle test with test conditions of -40 to 85 degrees
has been used for this purpose. No failures have been detected
for 1,000 cycles. The results (see Figure 15) show that the
developed package has adequate reliability.
Fig.15
Results of 2nd level reliability test
(c) Package level reliability
Furthermore, we confirmed the package reliabilities
(Fig.16).
Results show that requirements for TCT
(Temperature Cycle Test),PCT (Pressure Cooker Test) and
HTS (High Temperature Strage) and they were satisfied.
We have also tested the integrity of package sealing. The
requirements for 5 x 10-8(Pamm3/s) and below are satisfied.
Fig.16
Results of package reliability test
5. Conclusion
(1) The interconnection area of SMT package for microwave
and millimeter wave applications is found to be the main
cause of increase in return loss at frequencies over 30GHz.
(2) The capacitance value in the solder ball attach area is the
cause of degradation in the transmission line characteristics of
the package. Reducing the diameter of mounting pad improves
those characteristics.
(3) The newly developed SMT package composed of Low
Temperature Cofired Ceramics (Er 6.0; tangent delta 0.0023 at
10GHz[GL560]) mounted on RO4003 (Duroid) interconnect
board has been measured. Both, measurement and simulation
results after second level assembly show that this
0-7803-7038-4/01/$10.00 (C)2001 IEEE
configuration achieves S11<-15dB, and S21<-1dB per port
(up to 50GHz).
(4) Solder joint reliability testing shows that the developed
package has adequate reliability (no failures at 1,000 cycles
with test conditions of -40 to 85 degrees).
(5) The testing of package level reliabilities (TCT,PCT,HTS
and hermeticity) were confirmed to be satisfied.
We have found that, in order to achieve satisfactory
electrical performance of SMT package for high frequency
applications, it is necessary to optimize not only the
characteristics of each area that composes the package
structure, such as GCPW and VIA areas, but also the
characteristics package interface to the interconnect board.
Computer simulations have been used to investigate the
mechanism of performance degradation in the solder ball
attach area. As a result of these studies, we have developed a
package with no resonances from DC to 50GHz and low
transmission loss. Low Temperature Cofired Ceramic has
been proven to satisfy the requirements for high frequency
characteristics, package level reliability, and solder joint
reliability.
Further work is planned to establish technology for
measurement and simulation that is capable of the quantitative
analysis of S parameters in each of the package areas.
To respond to the needs for an even higher frequency
packaging, we are planning to develop a surface mount
package for 60GHz taking into consideration the second level
mounting reliability.
References
[1]Reinmut K.Hoffmann ; Handbook of Microwave Integrated
Circuits (1987)
[2]Report of millimeterwave device, material and system;
Technical report electrical society c-492 (1994)
[3]K.Miyauchi et al; ”Introduction to Microwave and Optical
Wave Engineering ”; CORONA PUBLISHING
CO.,LTD;pp.77-81;1989
[4]S.Tomie, S.Morioka, M.Maetani and T.Okumichi ; ”Design
Technology for Millimeter-wave Package ” ;
Dec.1997;MWE’97 Workshop Digest (in Japan);pp.226229
[5]T.Okumichi, S.Tomie, M.Fujii ; ”Development of
Millimeter-wave Package for W-band”;Oct.1998 ;
European Microwave Conference ’98
[6]Seigo Matsuzono , Shoji Uegaki , Shigo Sato , Shin
Matsuda , Mitsuo Yanagisawa , Hisayoshi Wada , Kota
Ikeda and Katsuyuki Yoshida ; ”Snap Array CSP-Ceramic
CSPs for High Performance and High Reliability
Applications” ;
May.2000;50th Electronic Components
and Technology Conf.
2001 Electronic Components and Technology Conference