U.S. patent application number 14/465876 was filed with the patent office on 2014-12-11 for multilayer ceramic substrate and method for producing the same.
The applicant listed for this patent is Murata Manufacturing Co., Ltd.. Invention is credited to Yuichi IIDA.
Application Number | 20140361470 14/465876 |
Document ID | / |
Family ID | 41318645 |
Filed Date | 2014-12-11 |
United States Patent
Application |
20140361470 |
Kind Code |
A1 |
IIDA; Yuichi |
December 11, 2014 |
MULTILAYER CERAMIC SUBSTRATE AND METHOD FOR PRODUCING THE SAME
Abstract
When a multilayer ceramic substrate with a cavity is reduced in
thickness, a bottom wall portion defining the bottom of the cavity
is reduced in thickness, thereby leading to the problem that the
bottom wall portion is likely to be broken. A bottom wall portion
defining a cavity of a multilayer ceramic substrate has a stack
structure formed with a high thermal expansion coefficient layer
sandwiched between first and second low thermal expansion
coefficient layers. This configuration generates compression stress
in the low thermal expansion coefficient layers during a cooling
process after firing, thereby allowing the mechanical strength at
the bottom wall portion to be improved.
Inventors: |
IIDA; Yuichi;
(Nagaokakyo-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Murata Manufacturing Co., Ltd. |
Nagaokakyo-shi |
|
JP |
|
|
Family ID: |
41318645 |
Appl. No.: |
14/465876 |
Filed: |
August 22, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12940073 |
Nov 5, 2010 |
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14465876 |
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PCT/JP2009/057896 |
Apr 21, 2009 |
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12940073 |
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Current U.S.
Class: |
264/605 |
Current CPC
Class: |
C04B 2237/348 20130101;
Y10T 428/24331 20150115; B32B 18/00 20130101; C04B 2235/9607
20130101; C04B 2237/62 20130101; H05K 2201/068 20130101; C04B
2237/58 20130101; H05K 1/0271 20130101; C04B 2237/702 20130101;
Y10T 428/24942 20150115; H05K 3/4629 20130101; Y10T 428/24851
20150115; H05K 2203/061 20130101; B32B 2250/40 20130101; Y10T
428/24926 20150115; H05K 3/4697 20130101; B32B 7/02 20130101; C04B
2237/704 20130101; C04B 2237/562 20130101; H01L 23/13 20130101;
H05K 3/462 20130101; H01L 2924/19105 20130101; H01L 2924/09701
20130101; H01L 2924/15153 20130101; H05K 1/183 20130101; H05K
2203/063 20130101; Y10S 428/901 20130101; H05K 1/0306 20130101;
C04B 2237/343 20130101; H01L 2224/16225 20130101; B32B 2250/42
20130101; H05K 3/4688 20130101; C04B 2237/68 20130101; H01L 23/5385
20130101; B28B 11/243 20130101; B32B 3/266 20130101 |
Class at
Publication: |
264/605 |
International
Class: |
B28B 11/24 20060101
B28B011/24 |
Foreign Application Data
Date |
Code |
Application Number |
May 15, 2008 |
JP |
2008-128102 |
Dec 26, 2008 |
JP |
2008-332086 |
Claims
1. A method for producing a multilayer ceramic substrate including
a cavity, comprising a peripheral wall portion including a first
ceramic layer including a through hole arranged to define the
cavity, and a bottom wall portion including a plurality of second
ceramic layers not including a through hole, wherein the plurality
of second ceramic layers of the bottom wall portion include at
least two types of ceramic layers, the at least two types of
ceramic layers including a high thermal expansion coefficient layer
having a relatively high thermal expansion coefficient and a
plurality of low thermal expansion coefficient layers having a
relatively low thermal expansion coefficient, and at least a
portion of the high thermal expansion coefficient layer is
sandwiched between a first low thermal expansion coefficient layer
and a second low thermal expansion coefficient layer of the
plurality of low thermal expansion coefficient layers, the method
comprising the steps of: preparing a first ceramic green layer
including the through hole, the first ceramic green layer to be
subjected to firing to form the first ceramic layer, and the first
ceramic green layer including a low-temperature sintering ceramic
material; preparing, as a plurality of second ceramic green layers
to be subjected to firing to form the second ceramic layers, a high
thermal expansion coefficient green layer to form the high thermal
expansion coefficient layer, a first low thermal expansion
coefficient green layer to form the first low thermal expansion
coefficient layer, and a second low thermal expansion coefficient
green layer to form the second low thermal expansion coefficient
layer, each of the plurality of second ceramic green layers
including a low-temperature sintering ceramic material; producing a
stacked composite body including a raw stacked body formed by
stacking the first ceramic green layer and the second ceramic green
layer, and an outer constraining layer provided on both principal
surfaces of the raw stacked body, the outer constraining layer
including an inorganic material powder which is not substantially
sintered at a firing condition at which the low-temperature
sintering ceramic material is sintered; firing the stacked
composite body under a firing condition at which the
low-temperature sintering ceramic material is sintered; and then
removing the outer constraining layers from the stacked composite
body.
2. The method for producing a multilayer ceramic substrate
according to claim 1, wherein an outer surface of the bottom wall
portion is defined by the first low thermal expansion coefficient
layer, and a surface of the bottom wall portion arranged in contact
with the peripheral wall portion is defined by the second low
thermal expansion coefficient layer, the raw stacked body further
includes, as the second ceramic green layer, a first constraining
interlayer arranged in contact with the second low thermal
expansion coefficient green layer, the first constraining
interlayer includes an inorganic material powder which is not
substantially sintered at a firing condition at which the
low-temperature sintering ceramic material is sintered, and the
inorganic material powder is solidified by permeation of the
ceramic material included in the low thermal expansion coefficient
green layer as a result of the firing step.
3. The method for producing a multilayer ceramic substrate
according to claim 1, wherein the raw stacked body further
includes, as the first ceramic green layer, a second constraining
interlayer arranged along a surface of the peripheral wall portion
in contact with the bottom wall portion, the second constraining
interlayer includes an inorganic material powder which is not
substantially sintered at a firing condition at which a ceramic
material included in the low thermal expansion coefficient layer is
sintered, and the inorganic material powder is solidified by
permeation of the ceramic material included in the low thermal
expansion coefficient layer as a result of the firing step.
4. The method for producing a multilayer ceramic substrate
according to claim 3, wherein in the raw stacked body, the through
hole included in the second constraining interlayer is made smaller
than the through hole included in the first ceramic green layer of
the peripheral wall portion arranged in contact with the second
constraining interlayer.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a multilayer ceramic
substrate and a method for producing the multilayer ceramic
substrate, and more particularly, to an improvement to increase the
strength of a multilayer ceramic substrate including a cavity.
[0003] 2. Description of the Related Art
[0004] Known methods for producing a multilayer ceramic substrate
include, for example, a method described in Japanese Patent
Application Laid-Open No. 2003-273513. In Japanese Patent
Application Laid-Open No. 2003-273513, in order to solve the
problem that a relatively high degree of shrinkage may occur at a
location further away from an open end of a cavity, in which the
shrinkage suppression effect produced by an outer constraining
layer is weakened, to undesirably deform the cavity when a
non-shrinkable process is used to produce a multilayer ceramic
substrate including the cavity, a firing step is performed with a
raw stacked body sandwiched between outer constraining layers
including an inorganic material powder to suppress shrinkage while
forming a constraining interlayer including the inorganic material
powder to suppress shrinkage along a ceramic green layer of the raw
stacked body to define a multilayer ceramic substrate, which is
located at a location at which the cavity formed.
[0005] According to the production method described in Japanese
Patent Application Laid-Open No. 2003-273513, in the firing step,
the shrinkage suppression effect produced by the constraining
interlayer acts in addition to the shrinkage suppression effect
produced by the outer constraining layer, thereby substantially
preventing shrinkage in the direction of the principal surface of
the ceramic green layer, and enabling a multilayer ceramic
substrate to be obtained without undesirable deformations of the
cavity.
[0006] However, the multilayer ceramic substrate including a cavity
has a problem in that a bottom wall portion defining the bottom of
the cavity is likely to crack or break.
[0007] As the size of electronic devices including a multilayer
ceramic substrate is reduced, the thickness of the multilayer
ceramic substrate is required to be reduced. Therefore,
particularly in the case of a multilayer ceramic substrate
including a cavity, the thickness of the bottom wall portion must
be reduced to achieve the reduction in the thickness of the
multilayer ceramic substrate when the size of a mounted component
to be disposed in the cavity is determined. Alternatively, when a
peripheral portion defining the periphery of the cavity must be
increased in height in order to accommodate mounted components
having various sizes and shapes in the cavity, the bottom wall
portion must be made thinner due to the increase in the height of
the peripheral wall portion. As a result of these circumstances,
the bottom wall portion is likely to be broken, and preventing such
a break is a big issue.
[0008] In addition, the multilayer ceramic substrate including a
cavity does not have a uniform thickness, i.e., the multilayer
ceramic substrate has a relatively thin bottom wall portion
defining the bottom of the cavity and a relatively thick peripheral
wall portion defining the periphery of the cavity, makes it more
likely to have undesirable deformations, such as warpage caused by
firing. In this case, depending on the relationship between the
thickness of the bottom wall portion and the height of the
peripheral wall portion, deformations, such as warpage, may be more
significantly produced in some cases. Therefore, when deformations,
such as warpage, are to be prevented, the degree of design freedom
of the multilayer ceramic substrate may be severely limited.
SUMMARY OF THE INVENTION
[0009] To overcome the problems described above, preferred
embodiments of the present invention provide a multilayer ceramic
substrate in which breakage of a bottom wall portion defining the
bottom of a cavity is less likely to occur and in which undesirable
deformations, such as warpage, are effectively suppressed, and a
method for producing the multilayer ceramic substrate.
[0010] A preferred embodiment of the present invention provides a
multilayer ceramic substrate which includes a cavity, including a
peripheral wall portion including a first ceramic layer including a
through hole arranged to define the cavity, and a bottom wall
portion including second ceramic layers not including a through
hole. In the multilayer ceramic substrate, the bottom wall portion
includes at least two types of ceramic layers, the at least two
types of ceramic layers including a high thermal expansion
coefficient layer having a relatively high thermal expansion
coefficient and a plurality of low thermal expansion coefficient
layers having a relatively low thermal expansion coefficient,
wherein at least a portion of the high thermal expansion
coefficient layer is sandwiched between a first low thermal
expansion coefficient layer and a second low thermal expansion
coefficient layer of the plurality of low thermal expansion
layers.
[0011] An outer surface of the bottom wall portion is preferably
defined by the first low thermal expansion coefficient layer, and a
surface of the bottom wall portion arranged in contact with the
peripheral wall portion is preferably defined by the second low
thermal expansion coefficient layer.
[0012] The peripheral wall portion preferably includes a high
thermal expansion coefficient layer having a thermal expansion
coefficient greater than that of the second low thermal expansion
coefficient layer, and a third low thermal expansion coefficient
layer having a relatively low thermal expansion coefficient
defining the outermost layer of the peripheral wall portion.
[0013] The bottom wall portion preferably further includes a first
constraining interlayer arranged in contact with the second low
thermal expansion coefficient layer. In this case, the first
constraining interlayer preferably includes an inorganic material
powder which is not substantially sintered at a firing condition at
which ceramic material included in the low thermal expansion
coefficient layer is sintered, and the inorganic material powder is
solidified by permeation of the ceramic material included in the
low thermal expansion coefficient layer. However, the first
constraining interlayer is not limited to being sandwiched between
the low thermal expansion coefficient layers, and other
arrangements may be provided.
[0014] The peripheral wall portion preferably further includes a
second constraining interlayer arranged along a surface of the
peripheral wall portion in contact with the bottom wall portion. In
this case, the second constraining interlayer includes an inorganic
material powder which is not substantially sintered at a firing
condition at which a ceramic material included in the low thermal
expansion coefficient layer is sintered, and the inorganic material
powder is solidified by permeation of the ceramic material included
in the low thermal expansion coefficient layer.
[0015] A through hole included in the second constraining
interlayer preferably does not extend outwardly from an inner
peripheral edge of the through hole in the first ceramic layer, the
through hole included in the first ceramic layer of the peripheral
wall portion is arranged in contact with the second constraining
interlayer, and at least a portion of the inner peripheral edge
defining the through hole in the second constraining interlayer is
preferably located inwardly of the inner peripheral edge defining
the through hole in the first ceramic layer of the peripheral wall
portion arranged in contact with the second constraining
interlayer.
[0016] Another preferred embodiment of the present invention
provides a method for producing a multilayer ceramic substrate
including a peripheral wall portion including a first ceramic layer
including a through hole arranged to define a cavity, and a bottom
wall portion including a plurality of second ceramic layers not
including a through hole, wherein the bottom wall portion includes
at least two types of ceramic layers, the at least two types of
ceramic layers including a high thermal expansion coefficient layer
having a relatively high thermal expansion coefficient and a
plurality of low thermal expansion coefficient layers having a
relatively low thermal expansion coefficient, and at least a
portion of the high thermal expansion coefficient layer is
sandwiched between a first low thermal expansion coefficient layer
and a second low thermal expansion coefficient layer of the
plurality of low thermal expansion coefficient layers.
[0017] The method for producing a multilayer ceramic substrate
according to this preferred embodiment of the present invention
includes the steps of preparing a first ceramic green layer
including the through hole, the first ceramic green layer to be
subjected to firing to form the first ceramic layer, and the first
ceramic green layer including a low-temperature sintering ceramic
material, preparing, as second ceramic green layers to be subjected
to firing to form as the second ceramic layers, a high thermal
expansion coefficient green layer to form the high thermal
expansion coefficient layer, a first low thermal expansion
coefficient green layer to form the first low thermal expansion
coefficient layer, and a second low thermal expansion coefficient
green layer to form the second low thermal expansion coefficient
layer, each of the green layers including a low-temperature
sintering ceramic material, producing a stacked composite body
including a raw stacked body formed by stacking the first ceramic
green layer and the second ceramic green layer, and an outer
constraining layer provided on both principal surfaces of the raw
stacked body, the outer constraining layer including an inorganic
material powder which is not substantially sintered at a firing
condition at which the low-temperature sintering ceramic material
is sintered; firing the stacked composite body at a firing
condition at which the low-temperature sintering ceramic material
is sintered; and then removing the outer constraining layers from
the stacked composite body.
[0018] The production method according to a preferred embodiment of
the present invention is preferably used to produce a multilayer
ceramic substrate in which an outer surface of the bottom wall
portion is defined by the first low thermal expansion coefficient
layer, and a surface of the bottom wall portion arranged in contact
with the peripheral wall portion is defined by the second low
thermal expansion coefficient layer. In this case, the raw stacked
body preferably further includes, as the second ceramic green
layer, a first constraining interlayer arranged in contact with the
second low thermal expansion coefficient green layer. The first
constraining interlayer preferably includes an inorganic material
powder which is not substantially sintered at a firing condition at
which the low-temperature sintering ceramic material is sintered,
and the inorganic material powder is solidified by permeation of
the ceramic material included in the low thermal expansion
coefficient green layer as a result of the firing step.
[0019] In addition, the raw stacked body preferably further
includes, as the first ceramic green layer, a second constraining
interlayer arranged along a surface of the peripheral wall portion
in contact with the bottom wall portion. In this case, the second
constraining interlayer includes an inorganic material powder which
is not substantially sintered at a firing condition at which a
ceramic material included in the low thermal expansion coefficient
layer is sintered, and the inorganic material powder is solidified
by permeation of the ceramic material included in the low thermal
expansion coefficient layer as a result of the firing step.
[0020] More preferably, in the raw stacked body, the through hole
in the second constraining interlayer is made smaller than the
through hole in the first ceramic green layer of the peripheral
wall portion arranged in contact with the second constraining
interlayer.
[0021] According to various preferred embodiments of the present
invention, the bottom wall portion of the cavity has the stack
structure including at least a portion of the high thermal
expansion coefficient layer sandwiched between the first and second
low thermal expansion coefficient layers, and the first and second
low thermal expansion coefficient layers have compression stress
generated during a cooling process after the firing. As a result,
the strength at the bottom wall portion can be improved, thus
making the bottom wall portion less likely to be broken.
[0022] In particular, when the outer surface of the bottom wall
portion is defined by the first low thermal expansion coefficient
layer, and when the surface of the bottom wall portion in contact
with the peripheral wall portion is defined by the second low
thermal expansion coefficient layer, the compression stress
generated in the first and second low thermal expansion coefficient
layers will act over substantially the entire thickness of the
bottom wall portion, thereby enabling the strength of the entire
bottom wall portion to be effectively improved.
[0023] The peripheral wall portion including the high thermal
expansion coefficient layer and the third low thermal expansion
coefficient layer defining the outermost layer of the peripheral
wall portion improves the strength of the entire multilayer ceramic
substrate, and greatly suppresses warpage caused by the stress
difference between the front and back of the multilayer ceramic
substrate.
[0024] When the first constraining interlayer is arranged in
contact with the second low thermal expansion coefficient layer,
shrinkage at the interface between the bottom wall portion and the
peripheral wall portion is greatly suppressed during the firing,
thereby preventing undesirable deformations, such as warpage of the
multilayer ceramic substrate and cracks.
[0025] The suppression of undesirable deformations, such as warpage
of the multilayer ceramic substrate, increases the degree of design
freedom of the multilayer ceramic substrate including a cavity.
[0026] In the method for producing a multilayer ceramic substrate
according to various preferred embodiments of the present
invention, the stacked composite body to be subjected to firing
includes the raw stacked body to form the multilayer ceramic
substrate as well as the outer constraining layers, and shrinkage
of the raw stacked body is greatly suppressed during the firing. As
a result, the dimensional accuracy of the multilayer ceramic
substrate is improved, and undesirable deformations, such as
warpage, are prevented.
[0027] When the raw stacked body includes the first constraining
interlayer, shrinkage is suppressed at a boundary between the
bottom wall portion and the peripheral wall portion, undesirable
deformations and cracks are prevented which may be caused at this
site, and the dimensional accuracy is further improved.
[0028] In addition, when the raw stacked body further includes the
second constraining interlayer arranged along the surface of the
peripheral wall portion in contact with the bottom wall portion in
the multilayer ceramic substrate, shrinkage is suppressed at a
boundary between the bottom wall portion and the peripheral wall
portion, and deformations and cracks which may occur at this
location are effectively prevented.
[0029] In addition, when the through hole in the second
constraining interlayer is made smaller than the through hole in
the first ceramic green layer of the peripheral wall portion in
contact with the second constraining interlayer, it is possible to
increase the possibility that at least a portion of the inner
peripheral edge defining the through hole in the second
constraining interlayer can be located inwardly of the inner
peripheral edge defining the through hole in the first ceramic
green layer of the peripheral wall portion in contact with the
second constraining interlayer, without locating the inner
peripheral edge defining the through hole in the second
constraining interlayer outwardly of the inner peripheral edge
defining the through hole in the first ceramic green layer of the
peripheral wall portion in contact with the second constraining
interlayer, even if a deviation of the location of the through
holes occurs between the second constraining interlayer and the
first ceramic green layer in contact with the second constraining
interlayer during the production of the raw stacked body.
Therefore, modifications, cracks, defects, etc., can be suppressed
and prevented with more certainty in the bottom wall portion of the
cavity after the firing.
[0030] The above and other elements, features, steps,
characteristics and advantages of the present invention will become
more apparent from the following detailed description of the
preferred embodiments with reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 is a cross-sectional view illustrating a functional
module 31 including a multilayer ceramic substrate 1 according to a
first preferred embodiment of the present invention.
[0032] FIG. 2 is a cross-sectional view schematically illustrating
a stacked composite body 41 produced in the process of
manufacturing the multilayer ceramic substrate 1 shown in FIG.
1.
[0033] FIG. 3 is a cross-sectional view illustrating a functional
module 31 including a multilayer ceramic substrate 1a according to
a second preferred embodiment of the present invention.
[0034] FIG. 4 is a cross-sectional view schematically illustrating
a stacked composite body 41a produced in the process of
manufacturing the multilayer ceramic substrate 1a shown in FIG.
3.
[0035] FIG. 5 is a cross-sectional view schematically illustrating
a multilayer ceramic substrate 1b according to a third preferred
embodiment of the present invention.
[0036] FIG. 6 includes cross-sectional views schematically
illustrating multilayer ceramic substrates 61 to 65 according to
Comparative Examples 1 to 3 and Examples 1 and 2, which were
produced in an experimental example performed to confirm an
advantageous effect of a preferred embodiment of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0037] FIG. 1 is a cross-sectional view illustrating a functional
module 31 including a multilayer ceramic substrate 1 according to a
first preferred embodiment of the present invention.
[0038] The multilayer ceramic substrate 1 includes a cavity 3
provided therein. The multilayer ceramic substrate 1 includes a
bottom wall portion 4 defining the bottom of the cavity 3 and a
peripheral wall portion 5 defining the periphery of the cavity
3.
[0039] The multilayer ceramic substrate 1 includes a stack
structure including a plurality of first ceramic layers 2a
including a through hole arranged to define the cavity and a
plurality of second ceramic layers 2b which do not include a
through hole, and the peripheral wall portion 5 includes the first
ceramic layers 2a, whereas the bottom wall portion 4 includes the
second ceramic layers 2b. The stacking of the first ceramic layers
2a including the through hole and the second ceramic layers 2b
which do not include a through hole define the concave cavity 3 in
the multilayer ceramic substrate 1. In addition, the bottom wall
portion 4 includes a first high thermal expansion coefficient layer
6 which has a relatively high thermal expansion coefficient and
first and second low thermal expansion coefficient layers 7 and 8
which have relatively low thermal expansion coefficients, wherein
at least a portion of the first high thermal expansion coefficient
layer 6 is sandwiched between the first and second low thermal
expansion coefficient layers 7 and 8. In particular, in the present
preferred embodiment, the outer surface of the bottom wall portion
4 includes the first low thermal expansion coefficient layer 7, and
the surface of the bottom wall portion 4 in contact with the
peripheral wall portion 5 includes the second low thermal expansion
coefficient layer 8.
[0040] The peripheral wall portion 5 includes a second high thermal
expansion coefficient layer 9, which has a higher thermal expansion
coefficient than the second low thermal expansion coefficient layer
8, and a third low thermal expansion coefficient layer 10 which has
a relatively low thermal expansion coefficient defining the
outermost layer.
[0041] In addition, the bottom wall portion 4 preferably includes a
first constraining interlayer 11 arranged in contact with the
second low thermal expansion coefficient layer 8. In the present
preferred embodiment, the first constraining interlayer 11 is
preferably sandwiched between two of the second low thermal
expansion coefficient layers 8. In addition, a second constraining
interlayer 12 is preferably provided along the surface of the
peripheral wall portion 5 in contact with the bottom wall portion
4. It is to be noted that the first constraining interlayer 11 may
preferably be arranged so as to be sandwiched between the second
low thermal expansion coefficient layer 8 and the first high
thermal expansion coefficient layer 6.
[0042] The multilayer ceramic substrate 1 preferably includes a
plurality of wiring conductors. The wiring conductors preferably
define, for example, passive elements, such as capacitors or
inductors, or wiring connections, such as an electrical connection
between elements, and typically include conductor films 13 to 16
and via hole conductors 17, as shown in FIG. 1.
[0043] The conductor films 13 are preferably arranged inside the
multilayer ceramic substrate 1. The conductive films 14 and are
preferably arranged on principal surfaces of the multilayer ceramic
substrate 1. The conductor films 16 are preferably arranged on the
bottom of the cavity 3. The via hole conductors 17 are preferably
arranged to pass through specific ones of the ceramic layers 2a and
2b in the thickness direction while being electrically connected to
respective ones of the conductor films 13 to 16.
[0044] Chip components 18 and 19 electrically connected to the
external conductor films 14 are preferably mounted on one principal
surface of the multilayer ceramic substrate 1. FIG. 1 shows bump
electrodes 20 arranged to electrically connect the chip component
19 to the external conductor films 14.
[0045] In addition, a chip component 21 electrically connected to
the cavity bottom conductor films 16 is preferably mounted inside
the cavity 3. FIG. 1 shows bump electrodes 22 arranged to
electrically connect the chip component 21 to the cavity bottom
conductor films 16.
[0046] As described above, the chip components 18, 19, and 21 are
preferably mounted on the multilayer ceramic layer 1 to define the
functional module 31. The external conductor films 15 arranged on
the other principal surface of the multilayer ceramic substrate 1
preferably define electrical connections arranged to mount the
functional module 31 onto a motherboard, not shown.
[0047] The multilayer ceramic substrate 1 described above is
preferably manufactured, for example, as follows.
[0048] FIG. 2 shows a cross-sectional view illustrating a stacked
composite body 41 produced in the process of manufacturing the
multilayer ceramic substrate 1. The stacked composite body 41
includes a raw stacked body 42 to be subjected to firing to define
the multilayer ceramic substrate 1, and first and second outer
constraining layers 43 and 44 provided on the both principal
surfaces of the raw stacked body 42. It is to be noted that the
conductor films 13 to 16 and the via hole conductors 17 are omitted
in FIG. 2.
[0049] With reference to FIG. 2 along with FIG. 1 for explanations,
the raw stacked body 42 includes a bottom wall portion 4 defining
the bottom of a cavity 3 and a peripheral wall portion 5 defining
the periphery of the cavity 3, as in the multilayer ceramic
substrate 1.
[0050] The bottom wall portion 4 of the raw stacked body 42
includes second ceramic green layers defining the second ceramic
layers 2b, a first high thermal expansion coefficient green layer
46 to define the first high thermal expansion coefficient layer 6,
a first low thermal expansion coefficient green layer 47 to define
the first low thermal expansion coefficient layer 7, and a second
low thermal expansion coefficient green layer 48 to define the
second low thermal expansion coefficient layer 8. The peripheral
wall portion 5 of the raw stacked body 42 includes first ceramic
green layers to define the first ceramic layers 2a, a second high
thermal expansion coefficient green layer 49 to define the second
high thermal expansion coefficient layer 9 and a third low thermal
expansion coefficient green layer 50 to define the third low
thermal expansion coefficient layer 10. These green layers 46 to 50
include a low-temperature sintering ceramic material.
[0051] In addition, the raw stacked body 42 includes a first
constraining interlayer 11 defining the second ceramic green layer
and a second constraining interlayer 12 defining the first ceramic
green layer. The constraining interlayers 11 and 12 include an
inorganic material powder which is not substantially sintered at
firing conditions which sinter the low-temperature sintering
ceramic material.
[0052] It is to be noted that while each of the green layers 46 to
50 typically includes multiple layers, as is obvious from the
multiple ceramic layers 2a and multiple ceramic layers 2b shown in
FIG. 1, the interfaces between these multiple layers are not shown
in FIG. 2. In addition, each of outer constraining layers 43 and 44
may preferably include multiple layers.
[0053] While the raw stacked body 42 is typically formed by
stacking multiple ceramic green sheets, the raw stacked body 42 may
instead be formed by repeatedly applying a ceramic slurry.
[0054] The outer constraining layers 43 and 44 are stacked on both
principal surfaces of the raw stacked body 42, and subjected to
pressure bonding, thereby providing the stacked composite body 41.
Further, the second outer constraining layer 44 arranged on the
cavity 3 side is provided with a through hole 51 in communication
with the cavity 3.
[0055] Next, the stacked composite body 41 is fired at the firing
conditions for sintering the low-temperature sintering ceramic
material described above. In this firing step, the inorganic
material powder included in the constraining interlayers 11 and 12
and the outer constraining layers 43 and 44 are not sintered, and
the constraining interlayers 11 and 12 and the outer constraining
layers 43 and 44 do not substantially shrink. Therefore, the
shrinkage suppression effect produced by the constraining
interlayers 11 and 12 and the outer constraining layers 43 and 44
acts on the raw stacked body 42 until the sintered multilayer
ceramic substrate 1 is obtained. As a result, undesired
deformations, such as warpage, are prevented from occurring in the
obtained multilayer ceramic substrate 1, and the dimensional
accuracy is significantly improved.
[0056] Next, the outer constraining layers 43 and 44 are preferably
removed from the fired stacked composite body 41, by applying, for
example, ultrasonic cleaning or blast processing. The fired outer
constraining layers 43 and 44 are porous, and thus can be easily
crushed and removed.
[0057] The constraining interlayers 11 and 12 include, as a result
of the firing step, the inorganic material powder that is
solidified by permeation of the material, i.e., a glass component,
included in the low thermal expansion coefficient green layer 48
and/or the high thermal expansion coefficient green layer 49
adjacent to the constraining interlayers 11 and 12. It is to be
noted that the constraining interlayers 11 and 12 must have a
thickness which allows for the solidification caused by the
permeation of this material.
[0058] The multilayer ceramic substrate 1 is obtained in the manner
described above. The bottom wall portion 4 includes a stack
structure including at least a portion of the first high thermal
expansion coefficient layer 6 sandwiched between the first and
second low thermal expansion coefficient layers 7 and 8.
Accordingly, a compression stress is generated in the first and
second low thermal expansion coefficient layers 7 and 8 during a
cooling process after the firing step, thereby improving the
mechanical strength of the bottom wall portion 4.
[0059] In addition, in this preferred embodiment, the third low
thermal expansion coefficient layer 10 is also preferably provided
in which compression stress is generated during the cooling process
after the firing step. Therefore, it is possible to suppress
undesired deformations, such as warpage, caused by the stress
difference between the front and back of the multilayer ceramic
substrate 1.
[0060] In the preferred embodiment described above, the low thermal
expansion coefficient layers 7, 8, and 10 each preferably have a
thickness of about 10 .mu.m to about 100 .mu.m, for example. The
reasons are as follows.
[0061] The stress caused by the differences in thermal expansion
coefficient acts at the interfaces between each of the low thermal
expansion coefficient layers 7, 8, and 10 and each of the high
thermal expansion coefficient layers 6 and 9. More specifically, a
compression stress acts on the low thermal expansion coefficient
layers 7, 8, and 10, and this compression stress is decreased with
increasing distance from the interfaces. On the other hand, a
tensile stress acts on the high thermal expansion coefficient
layers 6 and 9, and this tensile stress is decreased with
increasing distance from the interfaces. This is because the stress
is relaxed with increasing distance from the interfaces. When the
distance from the interface exceeds about 100 .mu.m, the
compression stress will not significantly act on the interfaces,
and the effect of the compression stress will not be sufficiently
produced. Accordingly, each of the low thermal expansion
coefficient layers 7, 8, and 10 preferably has a thickness of 100
about .mu.m or less, for example.
[0062] On the other hand, when the thicknesses of each the low
thermal expansion coefficient layers 7, 8, and 10 are less than
about 10 .mu.m, the high thermal expansion coefficient layers 6 and
9 that is decreased in strength by the action the tensile stress
will be present in near-outer surface regions located at less than
about 10 .mu.m from the outer surfaces of the respective low
thermal expansion coefficient layers 7, 8, and 10. Therefore, the
near-outer surface regions of the respective high thermal expansion
coefficient layers 6 and 9 are likely to be crushed, and
accordingly, each of the low thermal expansion coefficient layers
7, 8, and 10 preferably have a thickness of about 10 .mu.m or
greater, for example.
[0063] While the thicknesses of the high thermal expansion
coefficient layers 6 and 9 are appropriately determined depending
on the thickness of the entire multilayer ceramic substrate 1 and
the respective thicknesses of the low thermal expansion coefficient
layers 7, 8, and 10, the high thermal expansion coefficient layers
6 and 9 preferably have a thickness of about 10 .mu.m to about 100
.mu.m, for example, after firing step.
[0064] In addition, the respective thicknesses of the first and
second low thermal expansion coefficient layers 7 and 8 sandwiching
the first high thermal expansion coefficient layer 6 are preferably
less than the thickness of the first high thermal expansion
coefficient layer 6, because the compression stress can be utilized
in an efficient manner. Likewise, the respective thicknesses of the
second and third low thermal expansion coefficient layers 8 and 10
sandwiching the second high thermal expansion coefficient layer 9
are preferably less than the thickness of the second high thermal
expansion coefficient layer 9. In addition, the first to third low
thermal expansion coefficient layers 7, 8, and 10 are shown as
having the same or substantially the same thickness as each other
in FIG. 1, these thicknesses may preferably differ from each other
in accordance with the design of the multilayer ceramic substrate
1, such as the balance between the bottom wall portion 4 and the
peripheral wall portion 5 and the diameter of the cavity 3.
[0065] It is to be noted that while the low thermal expansion
coefficient layer 7 is shown in FIG. 1 as including three ceramic
layers 2b, the thickness of the low thermal expansion coefficient
layer 7 refers to the total thickness of the three ceramic layers
2b. The same applies to the respective thickness of the other low
thermal expansion coefficient layers 8 and 10 and the respective
thicknesses of the high thermal expansion coefficient layers 6 and
9.
[0066] The difference in thermal expansion coefficient is
preferably set in the range of about 1.0 ppmK.sup.-1 to about 4.3
ppmK.sup.-1 between the low thermal expansion coefficient layers 7,
8 and 10 and the high thermal expansion coefficient layers 6 and
9.
[0067] It has been discovered that the difference in thermal
expansion coefficient of about 1.0 ppmK.sup.-1 or greater
significantly reduces the warpage of the bottom wall portion 4.
More specifically, it has been discovered that the amount of
warpage is decreased with an increase in the difference in thermal
expansion coefficient in a range in difference in thermal expansion
coefficient of less than about 1.0 ppmK.sup.-1, and is
substantially constant at about 1.0 ppmK.sup.-1 or greater. This is
believed to be because the stress acting in an in-plane direction
which causes warpage of the multilayer ceramic substrate 1 is
relatively small, as compared to the stress acting in an in-plane
direction of front and back surfaces due to the difference in
thermal expansion coefficient, resulting in correction of the
warpage.
[0068] On the other hand, the difference in thermal expansion
coefficient of about 4.3 ppmK.sup.-1 or less can, with more
certainty, prevent defects, such as delamination and voids, which
are caused by the difference in thermal expansion coefficient at
boundary sections between the low thermal expansion coefficient
layers 7, 8 and 10 and the high thermal expansion coefficient
layers 6 and 9.
[0069] The material defining the low thermal expansion coefficient
layers 7, 8 and 10 preferably includes glass including SiO.sub.2
and MO (provided that the MO is at least one selected from CaO,
MgO, SrO, and BaO), with SiO.sub.2:MO=23:7-17:13, for example, and
the material defining the high thermal expansion coefficient layers
6 and 9 preferably includes glass containing SiO.sub.2 and MO, with
SiO.sub.2:MO=19:11-11:19, for example.
[0070] More preferably, the SiO.sub.2 included in the glass
included in the material defining the low thermal expansion
coefficient layers 7, 8 and 10 is about 34 weight % to about 73
weight %, for example, and the SiO.sub.2 included in the glass
included in the material about the high thermal expansion
coefficient layers 6 and 9 is about 22 weight % to about 60 weight
%, for example.
[0071] The preferable compositions and their contents as described
above is suitable for the purpose of using a borosilicate glass
based material to set to about 1.0 ppmK.sup.-1 or more for the
difference in thermal expansion coefficient between the low thermal
expansion coefficient layers 7, 8 and 10 and the high thermal
expansion coefficient layers 6 and 9 and set to about 75 weight %
or greater for the ratio by weight of the common component, for
example. The ratio by weight of the common component set to about
75 weight % or greater provides a sufficient joining force between
each of the low thermal expansion coefficient layers 7, 8 and 10
and each of the high thermal expansion coefficient layers 6 and
9.
[0072] The SiO.sub.2 component included in the glass contributes to
reducing the thermal expansion coefficient, and the MO component
contributes to increasing in thermal expansion coefficient.
[0073] In addition, since the deposition of a moderate amount of
crystal from the glass in the process of firing is advantageous in
terms of mechanical strength characteristics, the glass composition
is preferably closer to the composition of the deposited crystal.
For example, in the case of
SiO.sub.2-MO--Al.sub.2O.sub.3--B.sub.2O.sub.3 based glass, a
MAl.sub.2Si.sub.2O.sub.8 or a MSiO.sub.3 crystal are likely to be
deposited, and the ratio between SiO.sub.2 and MO is thus
preferably adjusted to so as to be closer to the composition of the
crystal. Therefore, the glass composition for the low thermal
expansion coefficient layers 7, 8 and 10 preferably has a ratio
between SiO.sub.2 and MO closer to about 2, for example, in order
to reduce the thermal expansion coefficient, and the glass
composition for the high thermal expansion coefficient layers 6 and
9 preferably has a ratio between SiO.sub.2 and MO closer to about
1, for example, in order to increase the thermal expansion
coefficient.
[0074] The glass composition for the high thermal expansion
coefficient layers 6 and 9 preferably has an MO ratio greater than
that of the low thermal expansion coefficient layers 7, 8 and 10,
and is thus susceptible to corrosion during a plating process after
the firing, but is insusceptible to fatal damage because the high
thermal expansion coefficient layers 6 and 9 are not exposed to the
surface.
[0075] If the glass includes too much SiO.sub.2 in the low thermal
expansion coefficient layers 7, 8 and 10 in order to increase the
difference in thermal expansion coefficient, the glass viscosity
will not be sufficiently decreased during the firing, thereby
causing failures during sintering. A glass including too much MO
results in the inability to ensure a sufficient difference in
thermal expansion coefficient.
[0076] In addition, if the glass includes too much MO in the high
thermal expansion coefficient layers 6 and 9 in order to increase
the difference in thermal expansion coefficient, the moisture
resistance is decreased, thereby causing insulation failures. A
glass including too much SiO.sub.2 results in the inability to
ensure a sufficient difference in thermal expansion
coefficient.
[0077] As described above, the ratio between SiO.sub.2 and MO in
the glass is preferably selected to fall within the range described
above, for example, for each of the low thermal expansion
coefficient layers 7, 8 and 10 and the high thermal expansion
coefficient layers 6 and 9.
[0078] The glass included in the material defining the low thermal
expansion coefficient layers 7, 8 and 10 preferably includes about
34 weight % to about 73 weight % of SiO.sub.2, about 14 weight % to
about 41 weight % of MO, about 0 weight % to about 30 weight % of
B.sub.2O.sub.3, and about 0 weight % to about 30 weight % of
Al.sub.2O.sub.3, for example, and the glass included in the
material defining the high thermal expansion coefficient layers 6
and 9 preferably includes about 22 weight % to about 60 weight % of
SiO.sub.2, about 22 weight % to about 60 weight % of MO, about 0
weight % to about 20 weight % of B.sub.2O.sub.3, and about 0 weight
% to about 30 weight % of Al.sub.2O.sub.3, for example.
[0079] The B.sub.2O.sub.3 provides the glass with a moderate
viscosity in order to promote smooth sintering during the firing.
If the glass includes too much B.sub.2O.sub.3, then the viscosity
will be decreased too much, which causes over firing pores to be
formed in the surface, thereby resulting in insulation failures. On
the other hand, if the glass includes too little B.sub.2O.sub.3,
the viscosity will be increased, thereby resulting in sintering
failures.
[0080] The Al.sub.2O.sub.3 is a component defining the deposited
crystals in the low thermal expansion coefficient layers 7, 8 and
10. If the glass includes too much or too little Al.sub.2O.sub.3,
the deposition of crystals will be less likely to occur.
[0081] In addition, the Al.sub.2O.sub.3 improves the chemical
stability of the glass, and thus, the plating resistance and the
moisture resistance are improved in the high thermal expansion
coefficient layers 6 and 9 which include a relatively large amount
of MO. The Al.sub.2O.sub.3 makes an intermediate contribution
between SiO.sub.2 and MO to the thermal expansion coefficient, and
the glass including too much Al.sub.2O.sub.3 results in an
inability to ensure a sufficient difference in thermal expansion
coefficient.
[0082] The material defining the low thermal expansion coefficient
layers 7, 8 and 10 preferably includes 30 weight % to about 60
weight % of Al.sub.2O.sub.3 as a filler, for example, and the
material defining the high thermal expansion coefficient layers 6
and 9 preferably includes about 40 weight % to about 70 weight % of
Al.sub.2O.sub.3 as a filler, for example.
[0083] The Al.sub.2O.sub.3 filler contributes to increasing the
mechanical strength. The material including too little
Al.sub.2O.sub.3 filler results in insufficient mechanical strength.
In particular, if the high thermal expansion coefficient layers 6
and 9 on which tensile stress acts have an insufficient mechanical
strength, the high thermal expansion coefficient layers 6 and 9
will likely break, rendering them unable to produce the effect from
the low thermal expansion coefficient layers 7, 8 and 10 reinforced
by compression stress. Therefore, the high thermal expansion
coefficient layers 6 and 9 include more Al.sub.2O.sub.3 filler than
the low thermal expansion coefficient layers 7, 8 and 10 so as to
increase the mechanical strength and withstand a larger difference
in thermal expansion coefficient, and further to produce the effect
from the reinforced low thermal expansion coefficient layers 7, 8
and 10.
[0084] The Al.sub.2O.sub.3 filler provides an intermediate
contribution between the glass in the low thermal expansion
coefficient layers 7, 8 and 10 and the glass in the high thermal
expansion coefficient layers 6 and 9 to the thermal expansion
coefficient, and the material including too much Al.sub.2O.sub.3
results in the inability to ensure a difference in thermal
expansion coefficient.
[0085] It is to be noted that besides Al.sub.2O.sub.3, for example,
other ceramics such as ZrO.sub.2 may be used as the filler.
[0086] It is to be noted that it is not necessary that the first to
third low thermal expansion coefficient layers 7, 8 and have the
same composition as each other or have the same thermal expansion
coefficient as each other, and it is also not necessary that the
first and second high thermal expansion coefficient layers 6 and 9
have the same composition as each other or have the same thermal
expansion coefficient as each other.
[0087] More specifically, when the thermal expansion coefficients
for each of the first and second low thermal expansion coefficient
layers 7 and 8 are less than the thermal expansion coefficient of
the first high thermal expansion coefficient layer 6, the thermal
expansion coefficient of the first low thermal expansion
coefficient layer 7 and the thermal expansion coefficient of the
second low thermal expansion coefficient layer 8 may differ from
each other. In addition, when the thermal expansion coefficients
for each of the second and third low thermal expansion coefficient
layers 8 and 10 are less than the thermal expansion coefficient of
the second high thermal expansion coefficient layer 9, the thermal
expansion coefficient of the second low thermal expansion
coefficient layer 8 and the thermal expansion coefficient of the
third low thermal expansion coefficient layer 10 may differ from
each other. Accordingly, as long as the conditions described above
are satisfied, the respective thermal expansion coefficients can be
freely determined, and as a result, the degree of design freedom of
the cavity 3 is improved.
[0088] FIGS. 3 and 4 are diagrams describing a second preferred
embodiment of the present invention, which respectively correspond
to FIGS. 1 and 2. In FIGS. 3 and 4, the same reference numerals are
assigned to elements corresponding to the elements shown in FIGS. 1
and 2, and redundant descriptions are omitted.
[0089] A multilayer ceramic substrate 1a according to the second
preferred embodiment includes an inner peripheral edge of a second
constraining interlayer 12a defining a through hole is located
inwardly of an inner peripheral edge of a first ceramic layer 2a of
a peripheral wall portion 5 in contact with the second constraining
interlayer 12a, as shown in FIG. 3.
[0090] It is to be noted that it is not necessary that the entire
inner peripheral edge of the second constraining interlayer 12a
arranged to define a through hole (hereinafter, referred to as a
"first through hole for cavity formation") is located inwardly of
the inner peripheral edge of the first ceramic layer 2a in contact
with the second constraining interlayer 12a arranged to define a
through hole (hereinafter, referred to as a "second through hole
for cavity formation"). More specifically, all that is required is
that at least a portion of the inner peripheral edge for defining
the first through hole for cavity formation is located inwardly of
the inner peripheral edge for defining the second through hole for
cavity formation.
[0091] It is important that the inner peripheral edge for defining
the first through hole for cavity formation is not located
outwardly of the inner peripheral edge for defining the second
through hole for cavity formation. For example, if a portion of the
inner peripheral edge for defining the first through hole for
cavity formation is not located inwardly of the inner peripheral
edge for defining the second through hole for cavity formation, the
portion must be located at least in the same position as the inner
peripheral edge for defining the second through hole for cavity
formation.
[0092] More specifically, when the first and second through holes
for cavity formation have a substantially quadrilateral shape, even
when the inner peripheral edge for defining the first through hole
for cavity formation is located inwardly of the inner peripheral
edge for defining the second through hole for cavity formation on
only two sides of the quadrilateral, the inner peripheral edge for
defining the first through hole for cavity formation cannot be
located outwardly of the inner peripheral edge for defining the
second through hole for cavity formation for the other two sides of
the quadrilateral, and must be located at least in the same
location as the inner peripheral edge for defining the second
through hole for cavity formation.
[0093] When the multilayer ceramic substrate 1a according to the
second preferred embodiment is to be produced, the first through
hole for cavity formation is smaller than the second through hole
for cavity formation in a raw stacked body 42 provided in a stacked
composite body 41a, as shown in FIG. 4, such that at least a
portion of the inner peripheral edge for defining the first through
hole for cavity formation is located inwardly of the inner
peripheral edge for defining the second through hole for cavity
formation.
[0094] According to the second preferred embodiment, a step of
stacking green sheets is performed in many cases for the production
of the raw stacked body 42, and in this stacking step, even if a
deviation in position of through holes for cavity formation is
undesirably caused between the second constraining interlayer 12a
and a first ceramic green layer in contact with the second
constraining interlayer 12a, it is possible to increase the
likelihood that at least a portion of the inner peripheral edge for
defining the first through hole for cavity formation can be located
inwardly of the inner peripheral edge for defining the second
through hole for cavity formation, without locating the inner
peripheral edge for defining the first through hole for cavity
formation outwardly of the inner peripheral edge for defining the
second through hole for cavity formation. Therefore, the second
preferred embodiment prevents deformations and cracks in the bottom
wall portion 4 of the cavity 3 with greater certainty after
firing.
[0095] FIG. 5 is a cross sectional view illustrating a multilayer
ceramic substrate 1b according to a third preferred embodiment of
the present invention. While the multilayer ceramic substrate 1b is
schematically shown in FIG. 5 as compared with FIG. 1, the same
reference numerals are assigned to elements corresponding to the
elements shown in FIG. 1, and redundant descriptions are
omitted.
[0096] The multilayer ceramic substrate 1b shown in FIG. 5 includes
no constraining interlayer. The remaining configuration preferably
is substantially the same as the multilayer ceramic substrate 1
shown in FIG. 1 or the multilayer ceramic substrate 1a shown in
FIG. 3.
[0097] Another preferred embodiment of the present invention is the
multilayer ceramic substrate 1, 1a, or 1b that includes no third
low thermal expansion coefficient layer 10.
[0098] Next, an experimental example will be described which was
performed to confirm the advantageous effects of preferred
embodiments of the present invention. In this experimental example,
multilayer ceramic substrates 61 to 65 according to each of
Comparative Examples 1 to 3 and Examples 1 and 2 were produced as
shown in the cross-sectional views of FIG. 6. In FIG. 6, the same
reference numerals are assigned to elements corresponding to the
elements shown in FIG. 1, and redundant descriptions are
omitted.
[0099] The multilayer ceramic substrate 65 according to Example 2
has the structure of the multilayer ceramic substrate 1 shown in
FIG. 1.
[0100] As compared to the multilayer ceramic substrate 65 according
to Example 2, the multilayer ceramic substrate 61 according to
Comparative Example 1 is configured such that all of the ceramic
layers defining a bottom wall portion 4 are low thermal expansion
coefficient layers 7 and all of ceramic layers defining a
peripheral wall portion 5 are low thermal expansion coefficient
layers 10.
[0101] The multilayer ceramic substrate 62 according to Comparative
Example 2 is configured such that all of the ceramic layers
defining a bottom wall portion 4 are high thermal expansion
coefficient layers 6 and all of the ceramic layers defining a
peripheral wall portion 5 are high thermal expansion coefficient
layers 9.
[0102] The multilayer ceramic substrate 63 according to Comparative
Example 3 is configured such that a bottom wall portion 4 includes
no second low thermal expansion coefficient layers 8 and high
thermal expansion coefficient layers 6 are provided for the second
low thermal expansion coefficient layers 8.
[0103] The multilayer ceramic substrate 64 according to Example 1
includes no third low thermal expansion coefficient layer 10 and a
high thermal expansion coefficient layer 9 provided for the third
low thermal expansion coefficient layer 10.
[0104] In this experimental example, the thermal expansion
coefficient was set to about 5.3 ppmK.sup.-1 for the low thermal
expansion coefficient layers 7, 8, and 10. In addition, in order to
form the low thermal expansion coefficient layers 7, 8, and 10,
green sheets having a thickness of about 50 .mu.m were produced,
and an appropriate number of the green sheets were stacked to
provide a desired thickness as will be described later.
[0105] The green sheets for the low thermal expansion coefficient
layers 7, 8, and 10 include a borosilicate based glass powder and a
ceramic powder at a ratio by weight of about 60:40, which were
obtained by adding about 50 parts by weight of an organic solvent,
about 10 parts by weight of a butyral based binder, and about 1
part by weight of a plasticizer to about 100 parts by weight of the
total of the glass powder and the ceramic powder to provide a
slurry, removing air bubbles from this slurry, then forming the
slurry into the shape of a sheet in accordance with a doctor blade
method, and drying the formed sheets. Powder including about 46
weight % of SiO.sub.2, 30 weight % of B.sub.2O.sub.3, about 14
weight % of CaO, about 5 weight % of Al.sub.2O.sub.3, and about 5
weight % of TiO.sub.2 was used as the borosilicate based glass
powder, and Al.sub.2O.sub.3 powder was used as the ceramic
powder.
[0106] The thermal expansion coefficient was set to about 7.7
ppmK.sup.-1 for the high thermal expansion coefficient layers 6 and
9. In addition, in order to form the high thermal expansion
coefficient layers 6 and 9, green sheets having a thickness of
about 50 .mu.m were produced, and an appropriate number of the
green sheets were stacked to provide a desired thickness as will be
described later.
[0107] The green sheets for the high thermal expansion coefficient
layers 6 and 9 include a borosilicate glass powder and a ceramic
powder at a ratio by weight of about 70:30, which were obtained by
adding an organic solvent, a butyral based binder, and a
plasticizer at the same ratio as in the case of the low thermal
expansion coefficient layers described above to about 100 parts by
weight of the total of the glass powder and the ceramic powder, and
going through the same operations. Powder including about 40 weight
% of SiO.sub.2, about 5 weight % of B.sub.2O.sub.3, about 40 weight
% of CaO, about 5 weight % of MgO, and about 10 weight % of
Al.sub.2O.sub.3 was used as the borosilicate glass powder, and
Al.sub.2O.sub.3 powder was used as the ceramic powder.
[0108] In order to form the constraining interlayers 11 and 12,
green sheets having a thickness of about 10 .mu.m were produced,
and in order to form outer constraining layers, not shown in FIG.
6, green sheets having a thickness of about 100 .mu.m were
produced. The green sheets for the constraining interlayers 11 and
12 and the outer constraining layers include about 100 parts by
weight of an alumina powder, about 10 parts by weight of a butyral
based binder, and about 1 part by weight of a plasticizer, which
were obtained through substantially the same operations as in the
case of the low thermal expansion coefficient layers.
[0109] As a conductive paste for conductor films and via hole
conductors, not shown in FIG. 6, a conductive paste was used that
includes about 48 parts by weight of a silver powder, about 3 parts
by weight of an ethyl cellulose binder, and about 49 parts by
weight of an organic solvent, terpenes, and this conductive paste
was applied to specific green sheets in order to form the conductor
films 13 to 16 and the via hole conductors 17 as shown in FIG.
1.
[0110] Next, the various types of green sheets were stacked to the
number of sheets shown in the column "The Number of Green Sheets
Used" of the following Table 1, thereby producing raw stacked
bodies defining the multilayer ceramic substrates 61 to 65, and
outer constraining layers were formed on the top and bottom of the
raw stacked bodies to produce stacked composite bodies. In this
case, the outer constraining layers were formed by stacking four of
the green sheets having a thickness of about 100 .mu.m on each of
the bottom and top of the raw stacked bodies.
TABLE-US-00001 TABLE 1 The Number of Green Sheets Used Comparative
Bottom Wall Low Thermal Expansion 18 Example 1 Portion Coefficient
Layer Constraining Interlayer 1 Peripheral Low Thermal Expansion 18
Wall Portion Coefficient Layer Constraining Interlayer 1
Comparative Bottom Wall High Thermal Expansion 18 Example 2 Portion
Coefficient Layer Constraining Interlayer 1 Peripheral High Thermal
Expansion 18 Wall Portion Coefficient Layer Constraining Interlayer
1 Comparative Bottom Wall Low Thermal Expansion 3 Example 3 Portion
Coefficient Layer High Thermal Expansion 15 Coefficient Layer
Constraining Interlayer 1 Peripheral Low Thermal Expansion 3 Wall
Portion Coefficient Layer High Thermal Expansion 15 Coefficient
Layer Constraining Interlayer 1 Example 1 Bottom Wall Low Thermal
Expansion 6 (3 + 3) Portion Coefficient Layer High Thermal
Expansion 12 Coefficient Layer Constraining Interlayer 1 Peripheral
High Thermal Expansion 18 Wall Portion Coefficient Layer
Constraining Interlayer 1 Example 2 Bottom Wall Low Thermal
Expansion 6 (3 + 3) Portion Coefficient Layer High Thermal
Expansion 12 Coefficient Layer Constraining Interlayer 1 Peripheral
Low Thermal Expansion 3 Wall Portion Coefficient Layer High Thermal
Expansion 15 Coefficient Layer Constraining Interlayer 1
[0111] Then, the stacked composite bodies were fired at a
temperature of about 870.degree. C. for about 10 minutes. Next, the
porous outer constraining layers attached to the surfaces of the
stacked composite bodies fired were removed using an ultrasonic
cleaning machine to obtain the multilayer ceramic substrates 61 to
65 according to Comparative Examples 1 to 3 and Examples 1 and
2.
[0112] Next, in order to compare the mechanical strengths of the
respective multilayer ceramic substrates 61 to 65 against drop
impacts, the following test was performed.
[0113] Each of the multilayer ceramic substrates 61 to 65 was
mounted on a mounting board with solder, and the mounting board was
attached to the inside of a substantially rectangular
parallelepiped housing and dropped toward a concrete block. In this
case, dropping while sequentially facing the respective six
surfaces of the housing downward was referred to as 1 cycle, and
this test was performed up to 10 cycles. The bottom wall portions 4
of the respective multilayer ceramic substrates 61 to 65 were
evaluated to determine whether the bottom wall portions were
crushed or cracked. The results are shown in Table 2.
TABLE-US-00002 TABLE 2 The Number of Cycles Condition of Bottom
Implemented Wall Portion Comparative 4 Crushed Example 1
Comparative 4 Crushed Example 2 Comparative 7 Cracked Example 3
Example 1 10 Not Crushed or Cracked Example 2 10 Not Crushed or
Cracked
[0114] As is clear from Table 2, the bottom wall portions 4 of the
multilayer ceramic substrates 61 and 62 were crushed in the fourth
cycle in Comparative Examples 1 and 2. In addition, in Comparative
Example 3, the bottom wall portion 4 was cracked in the seventh
cycle while complete crushing was prevented.
[0115] In contrast, no crushing or cracking occurred in 10 cycles
in Examples 1 and 2.
[0116] While preferred embodiments of the present invention have
been described above, it is to be understood that variations and
modifications will be apparent to those skilled in the art without
departing the scope and spirit of the present invention. The scope
of the present invention, therefore, is to be determined solely by
the following claims.
* * * * *