BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a SAW (surface acoustic
wave) filter encapsulated in a ceramic package, and a ceramic package therefore.
Description of the Background Art
It is common practice with a ceramic package for encapsulating
a SAW filter to form pads on one of its opposite major surfaces. A SAW filter chip
or wafer chip is affixed to the package by adhesion or similar technology with its
one major surface carrying a pattern of electrodes facing upward. The pattern of
electrodes, i.e., electrode strips are connected to the pads of the package by bonding
wires. A lid is mounted on the top of the package and sealed by solder. The electrodes
are printed on a substrate of piezo-electric material, such as LiTaO3.
The above-mentioned conventional ceramic package has a
problem that the adjustment of impedance is extensively difficult partly because
the surface of the filter chip carrying the electrode pattern and facing upward
is hermetically sealed in the package and partly because the electrode pattern itself
is printed on LiTaO3 or similar fragile material.
discloses a SAW device comprising a ceramic package wherein the SAW filter
chip is mounted such that the pattern of electrodes face down towards the pads on
the surface of the ceramic package. Metallic bumps form the connections between
the SAW filter chip and the electrode pads. In one embodiment, the ceramic package
comprises a grounded counter electrode pattern formed on the surface of the ceramic
package and facing the surface of the SAW filter chip comprising the patter of electrodes.
The grounded counter electrode is connected in a manner that reduces the effect
of directly propagated electromagnetic waves between input and output electrodes
on the surface of the SAW filter.
JP 07 212180 A
discloses another SAW device wherein the SAW filter chip is mounted such
that the pattern of electrodes face down towards the electrode pads on the surface
of the ceramic package and wherein metallic bumps are used to connect the chip and
the ceramic package. The SAW filter chip is placed in a recess in the ceramic package
and is further attached with an epoxy adhesive to insulating glass elements positioned
in the periphery of the recess. The epoxy adhesive provides shock resistance to
JP 05 335 878 A
discloses another SAW device wherein the chip is mounted on bumps on the
plateau region of a shoulder formed in the recess.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a ceramic
package for a SAW filter capable of promoting easy adjustment of impedance.
According to the invention, there is provided a SAW filter
comprising a SAW filter made of piezoelectric material and having a major surface
on which a pattern of electrodes is formed to propagate surface acoustic waves.
The SAW filter further comprises a ceramic package having a recess formed for encapsulating
said SAW filter chip therein. The SAW filter chip is mounted to said ceramic package
such that at least part of the pattern of electrodes faces a bottom of the recess.
Moreover, bumps, formed on the bottom of the recess, are provided between the bottom
of the recess and the major surface of the SAW filter chip for connecting and supporting
said SAW filter chip to said ceramic package. The SAW filter, according to the invention,
is characterised by further comprising a conductive pad formed on the bottom of
the recess, the bumps being arranged to define an air gap between the part of the
pattern of electrodes and part of said conductive pad, and a shoulder with a plateau
region formed in the recess, said SAW filter chip and said shoulder being arranged
such that the plateau region of said shoulder faces and is spaced from end regions
of the major surface.
A capacitance is thereby established between the conductive
pad and the propagation paths on the major surface of the SAW filter chip. The shoulder
may be provided as part of a stepped structure that surrounds the SAW filter chip.
BRIEF DESCRIPTION OF THE DRAWINGS
The object and features of the present invention will become
more apparent from consideration of the following detailed description taken in
conjunction with the accompanying drawings in which:
DESCRIPTION OF THE PREFERRED EMBODIMENTS
- FIG. 1A is a plan view showing a ceramic package for encapsulating a SAW filter
and embodying the present invention;
- FIG. 1B is a front view of the package shown in FIG. 1A;
- FIG. 1C is a rear view of the package shown in FIG. 1A;
- FIG. 2 is a plan view, similar to FIG. 1, showing an alternative embodiment
of a ceramic package in accordance with the present invention;
- FIG. 3 shows a section of the package of the embodiment shown in FIGS. 1A, 1B
and 1C, cut along a dot-and-chain line III-III in FIG. 1A, together with a SAW filter
- FIG. 4 is a fragmentary, enlarged view of a portion designated with a circle
A included in the assembly shown in FIG. 3;
- FIG. 5 shows an equivalent circuit to a SAW filter encapsulated in a conventional
- FIGS. 6, 7 and 8 are equivalent circuit diagrams, similar to FIG. 5, each showing
part of a specific circuit configuration available with the SAW filter encapsulated
in the package of FIGS. 1A, 1B and 1C;
- FIGS. 9 and 10 are equivalent circuit diagrams, similar to FIG. 5, each showing
part of a specific configuration available with the SAW filter included in the package
shown in FIG. 2; and
- FIG. 11 is a section partially showing a conventional ceramic package and a
SAW filter mounted thereon.
To better understand the present invention, a brief reference
will be made to a conventional ceramic package for encapsulating a surface acoustic
wave (SAW) filter, shown in FIG. 11. As shown, the package, generally designated
with a reference numaral 1, has metal pads 2 formed on one of its opposite major
surfaces. A SAW filter chip or wafer chip 3 is affixed to the package 1 by, e.g.,
adhesive 5 with its surface 4 carrying a pattern of electrodes facing upward. The
electrode pattern, i.e., electrodes 6 are connected to the pads 2 of the package
1 by bonding wires 7. A lid, not shown, is mounted on the top of the package 1 and
sealed by solder. The electrodes 6 are printed on a substrate of piezoelectric substance
such as LiTaO3.
The problem with the afore-mentioned conventional package
1 is that the adjustment of impedance is extensively difficult partly because, on
one hand, the patterned surface 4 is hermetically sealed in the package 1 and, on
the other hand, partly because the electrode pattern itself is printed on LiTaO3
or similar fragile material, as stated earlier.
Referring now to FIGS. 1A, 1B and 1C, a ceramic package
for encapsulating a SAW filter in accordance with an embodiment of the present invention
will be described. As shown in FIG. 1A, the package, generally designated with a
reference numeral 10, is generally of a rectangular box. The package 10 includes
an input terminal 11, an output terminal 12, and four ground terminals 13-16 formed
in a recess 10C cut therein. The ground terminals are to be connected to ground.
As shown in FIG. 1B, the output terminal 12 and ground terminals 15 and 16 are respectively
connected to terminal pads 12A, 15A and 16A, which are formed on one or front side
face 10A of the package 10. Likewise, as seen from FIG. 1C, the input terminal 11
and ground terminals 13 and 14 are respectively connected to other terminal pads
11A, 13A and 14, which are also formed on another side, or rear face 10B of the
package 10 opposite to the front side face 10A.
A generally rectangular pad 17 is formed of metal and positioned
on the generally flat bottom 10D of the recess 10C formed in the package 10. The
metal pad 17 is surrounded by the input terminal 11, output terminal 12, and ground
terminals 13-16 as depicted. It is to be noted that the pad 17 may be provided with
any desired shape and may be located at any desired position on the bottom surface
10D of the package 10 shown in FIG. 1A. In FIG. 1A, a SAW filter chip 31 and bumps
32 to which the filter chip 31 is connected are indicated by phantom lines. How
to support the chip 31 by the bumps 32 will be described later.
FIG. 2 shows an alternative embodiment of the present invention.
In FIG. 2, the structural elements like those shown in FIGS. 1A, 1B and 1C are designated
by the same reference numerals, and a detailed description thereof will not be made
in order to avoid redundancy. As shown, this embodiment may be the same as what
is shown in and described with reference to FIGS. 1A, 1B and 1C except that with
the FIG. 3 embodiment the metal pad 17 is connected to the ground terminals 13-16
by connections, such as wires or metal strips, 18-21, respectively.
How the SAW filter chip 31 is mounted to the ceramic package
10 shown in FIGS. 1A-1C will be described with reference to FIGS. 3 and 4. As shown
in FIG. 3, the filter chip 31 is mounted to the package 10 face down, i.e., with
one of its primary surface 31A carrying an electrode pattern 33 facing downward
in the figure. The filter chip 31 is connected to the bottom surface 10D of the
package 10 by the bumps 32. The package 10 includes a shoulder 34 for forming an
appropriate air gap between the electrode pattern or propagation paths 33 of the
filter chip 31 and the pad 17 of the package 10. As shown in FIG. 3, the shoulder
has a steep slope 34A and a plateau region 34B. Moreover, when the SAW filter chip
is mounted with its primary surface 31A facing downward, as shown in FIG. 3, it
overlaps the plateau region 34B of the shoulder 34 such that the plateau region
34B of the shoulder faces and is spaced from the end regions 31B of the primary
surface. The shoulder 34 has a height, as measured from the bottom 10D of the recess
10C formed in the package 10, so selected as not to affect the connection using
the bumps 32. The electrode pattern or propagation paths 33 are printed on the primary
surface 31A of the chip 31 in the form of metal, comb-like strips, for example,
which are adapted to propagate surface acoustic waves over the surface of the piezoelectric
substrate 31 along the paths 33.
A lid, shown with a phantom line 50, FIG. 3, is fitted
on the top of the recess 10C of the package 10 in order to seal the package 10.
In this configuration, as imaginarily shown in FIG. 4, a capacitance C is produced
between the propagation paths 33 of the filter chip 31 and the pad 17 of the package
10 due to the above-mentioned gap. By adjusting the height of the bumps 32 to vary
the capacitance C, it is possible to control the impedance and phase of the propagation
paths 33. This is also true with the alternative embodiment shown in FIG. 2.
Assume that a three-stage SAW filter is mounted on a conventional
ceramic package by way of example. Then, the SAW filter encapsulated in the package
in the conventional way is usually represented by an equivalent circuit shown in
FIG. 5. It is noted that a capacitance is not formed in the circuitry. By contrast,
when such a SAW filter is now mounted on the ceramic package of the type shown in
FIGS. 1A, 1B and 1C, a capacitance or capacitances can be added, as shown in FIG.
6, 7 or 8. Specifically, a capacitance C, (FIG. 6), a capacitance C2
(FIG. 7) or capacitances C3-C11 (FIG. 8) can be added, as
desired, by appropriately providing, positioning and shaping the metal pads 17,
as well as adjusting the height of the bumps 32. The application shown in FIG. 8
is of a maximum number of capacitances available with this specific circuit configuration.
With the FIG. 8 application, the capacitances C3, C4, C5,
C7, C8, C9 and C11 are inserted in serial
to the propagation paths 33. The remaining capacitances C6 and C10
have one electrodes thereof grounded, established by grounding metal pads 17 by
connection wires 18, 19, 20 and 21 to the ground terminal 12, 14, 15 and 16, as
shown in FIG. 2.
Likewise, as shown in FIG. 9, when the above filter is
mounted on the ceramic package 10 of the type shown in FIG. 2, a capacitance C12
can be added in a parallel connection. Alternatively, as shown in FIG. 10, capacitances
C13, C14 and C15 may be added in a plurality of
parallel connections. In any case, the capacitance or capacitances allow the impedance
of the SAW filter encapsulated in the package to be varied and thereby allow the
frequence of the filter to appropriately be adjusted. Moreover, because the patterned
surface of the filter chip 31 faces downward, namely directed to the botom 10D,
it is protected from solder which may fly in the event of sealing of the package.
In summary, it will be seen that the present invention
provides a ceramic package for encapsulating a SAW filter and having various unprecedented
advantages, as follows. Because the package includes a metal pad, a capacitance
can be varied with ease on the bases of the shape of the pad. As a result, the impedance
and phase of propagation paths can be varied, as desired. Further, bumps allow a
SAW filter chip to be readily mounted to the package and reduce the cost of the
package. Moreover, a shoulder formed in the recess of the package and surrounding
the filter chip further promotes easy mounting of the chip and implements automated
production. In addition, the capacitance added to the packaged SAW filter varies
the impedance and thereby allows the frequency of the SAW filter to be adjusted.
While the present invention has been described with reference
to the particular illustrative embodiments, it is not to be restricted by the embodiments.
It is to be appreciated that those skilled in the art can change or modify the embodiments
without departing from the scope and spirit of the present invention. For example,
while the illustrative embodiments have concentrated on a ceramic package for a
SAW filter, they are similarly applicable to an RF (Radio Frequency) module using
bumps if the chip wafer is replaced with a monolithic module.