Stock material or miscellaneous articles – Structurally defined web or sheet – Discontinuous or differential coating – impregnation or bond
Reexamination Certificate
2001-09-20
2003-09-23
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Structurally defined web or sheet
Discontinuous or differential coating, impregnation or bond
C428S689000, C428S699000, C428S672000, C428S673000, C428S680000, C310S31300R, C310S31300R
Reexamination Certificate
active
06623842
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a surface electrode structure on a ceramic multi-layer substrate and a process for producing surface electrodes on the substrate, which can be applied to the production of high-frequency modular parts having soldered surface-mounted parts and flip chip mounted surface-acoustic-wave devices on a ceramic multi-layer substrate and which enable consistent mounting of individual parts to provide higher reliability.
The market always demands smaller electronic equipment and it also demands reduction in the size and weight of the parts used. This tendency is noticeable in high-frequency equipment typified by cellular phones and is particularly pronounced in the parts used. In high-frequency equipment, there has been an increase in the parts mounting density to meet the demand for smaller size and lighter weight. Device mounting substrates are not an exception and in order to meet the demand for smaller size, the substrate having a single conductor layer is mostly replaced by the multi-layer substrate.
The ceramic multi-layer substrate has the insulating layers formed of electrically insulating ceramics and has the conductor layers formed of silver or the like. Compared to the conventional resin multi-layer substrate, the ceramic multi-layer substrate has many advantages including low loss at high frequencies, good heat conduction, as well as high dimensional precision and reliability.
The ceramic multi-layer substrate has a further advantage in that by forming a coil of the internal conductor or parallel plates of it, an inductance or a capacitance can be provided internally. What is more, the low loss and high dimensional precision features enable internal formation of high-Q and small-tolerance devices.
These features are deliberately utilized in cellular phones and other high-frequency circuits as a module or a device assembly of various parts that are mounted on the surface and which also have high characteristics, as well as meet the demand for smaller size.
In the high-frequency module, circuits are grouped by function, so compared to the conventional technique of forming circuits by mounting discrete parts individually, it allows for a simpler equipment structure and can provide equipment of better reliability and characteristics. Speaking further of the conventional discrete parts, their characteristics are combined to perform the intended functions and this results in a complicated design. In modularization, the specifications for the characteristics of each module are predetermined, so equipment design can be structurized and completed in a shorter period with reduced labor.
FIG. 6
is a block diagram for the GSM dual-band cellular phone which is used at the largest number of terminals in the world. In the figure, ANT designates an antenna for transmitting and receiving radio waves, DPX a diplexer (two-frequency switching filter) as a filter to separate two frequencies, T/R SW a transmission/receiving conversion switch as a means of switching between transmission and receiving of radio waves, LPF a low-pass filter as a filter to suppress harmonics at the transmission stage, and BPF a bandpass filter at the receiving stage.
In the illustrated circuit of cellular phone, modularization is realized for several functions and, in a typical case, devices are actually mounted on a multi-layer substrate in the antenna/switch section.
FIG. 7
shows an exemplary module for the antenna/switch section. In the figure, numeral
10
designates a ceramic multi-layer substrate having an inductor portion
11
and a capacitor portion
12
in the interior, as well as an external electrode
13
. Chip components
15
such as diodes working as switching elements and resistors are mounted on the ceramic multi-layer substrate
10
and a shield case
16
is provided to cover up the top of the ceramic multi-layer substrate. Note that the module shown in
FIG. 7
does not include surface-acoustic-wave devices (hereunder referred to as SAW devices) or they are mounted as packaged parts.
As of today, modularization has been realized in mono-functional devices such as power amplifiers and antenna/switch modules. If a broader range of functions are modularized, the advantages of modularization will be further obtained. Of course, modularization of devices including SAW devices is also important.
Conventional SAW devices have used so-called “packaged” parts. Modularization can of course be realized by mounting packaged parts; however, as will be described later in the present invention, direct mounting of device chips on a substrate is believed to be more effective in realizing smaller and lower profiles, as well as lower cost.
The ceramic multi-layer substrate is characterized by its ability to contain inductance and capacitance as built-in parts to thereby allow for size reduction. On the other hand, lower profiles are difficult to realize. Therefore, common modules having packages mounted on a substrate cannot fully meet the ever increasing demand for lower profiles. In addition, packaged devices will occupy larger areas than the initial bare chips. Among the parts used, SAW devices are of the highest profile and occupy the wider area. Under these circumstances, it is desired that SAW chips be somehow mounted directly on the ceramic multi-layer substrate without using packages.
The production of SAW devices consists of two steps, one for fabricating SAW chips and the other for mounting and sealing them in package, and each step requires similar amounts of cost. If direct mounting on the ceramic multi-layer substrate is possible, inexpensive equipment can also be made in the absence of the step of mounting and sealing SAW devices in package.
As described above, it is desired for high-frequency modules that SAW devices be directly mounted as chips on the ceramic multi-layer substrate and that other parts be mounted by soldering.
To this end, the ceramic multi-layer substrate must be compatible with both the step of flip mounting SAW devices and the step of soldering other parts.
SAW devices are commonly bonded by gold-gold bump bonding with gold (Au) forming the topmost layer of the surface electrodes on the ceramic multi-layer substrate. In bonding by solder, the surfaces of lands on the substrate are commonly made of a tin or solder film, each of which is usually formed by plating.
The soldering process commonly comprises the steps of applying a paste of solder to the lands on the substrate surface, then placing the parts to be soldered, and performing a heat treatment such as reflowing to fix the parts. In this case, the flux in the paste of solder evaporates and the interface with the surface electrodes is activated to secure the wettability by the solder.
In the present invention, it is presupposed that SAW devices are mounted as exposed, so if they are first mounted, their characteristics will be greatly affected by the flux deposited in the subsequent step of soldering other parts. Hence, no method has yet been established that enables both the mounting of SAW devices in a bare state and the soldering of other parts as in the present invention.
The currently available small-size SAW device, as typically disclosed in Unexamined Published Japanese Patent Application (kokai) No. 10-79638/(1998), is fixed to a ceramic substrate or a resin substrate by a method called “flip chip” mounting. This is shown in
FIG. 8
, in which
20
designates the substrate and
30
a flip chip as the SAW device. Formed on the substrate
20
are electrodes
21
whose surface is made of gold (Au), and the flip chip
30
has gold stud bumps
31
formed on the principal surface having an SAW ladder-shaped electrode. With the SAW ladder-shaped electrode carrying principal surface facing down, the flip chip
30
is flip mounted by gold-gold bonding (face-down bonding).
This method would be effectively adopted in the present invention to mount SAW devices but it must satisfy the condition that no problem occurs if the SAW devices are mounted together with soldered part
Goi Tomoyuki
Uchikoba Fumio
Blackwell-Rudasill G. A.
Jones Deborah
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
TDK Corporation
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