U.S. patent application number 09/726328 was filed with the patent office on 2001-08-23 for process for fabricating a thin multi-layer circuit board.
Invention is credited to Iida, Kenji, Katayama, Hiroyuki, Katsube, Toshiro, Kurokawa, Yasunaga, Nakano, Kazuo, Ohta, Yuichiro, Ozaki, Norikazu, Sumi, Masaru, Takahashi, Yasuhito.
Application Number | 20010015008 09/726328 |
Document ID | / |
Family ID | 12247169 |
Filed Date | 2001-08-23 |
United States Patent
Application |
20010015008 |
Kind Code |
A1 |
Takahashi, Yasuhito ; et
al. |
August 23, 2001 |
Process for fabricating a thin multi-layer circuit board
Abstract
A process for fabricating thin multi-layer circuit boards which
allow the electrical conduction of remodeling pads to be easily cut
do not use etching for gold, avoid the lift-off method for forming
a thin chromium film on the wiring pattern layer, allow a defective
wiring pattern layer to be removed, and which uses uniform heating
of the substrate, from the back side thereof, at the time of
pre-baking. A barrier metal (60) is excluded from a portion where
the electrical conduction of a remodeling pad (62b) is to be cut
(FIGS. 1 to 16). Alternatively, gold-plating resist is formed in
order to avoid the etching for gold (FIGS. 17 to 19).
Alternatively, a thin chromium film is formed in advance by etching
on the wiring pattern layer (FIGS. 27 to 33). Alternatively, a
metallic barrier film (122) is formed on each of the wiring pattern
layers so that the wiring pattern layer can be removed without
affecting other wiring pattern layers (FIGS. 36 to 42).
Alternatively, the substrate is disposed over a heat-accumulating
block (138) adjacent thereto so that it is uniformly heated from
the back side thereof during the pre-baking.
Inventors: |
Takahashi, Yasuhito;
(Kawasaki-shi, JP) ; Kurokawa, Yasunaga;
(Kawasaki-shi, JP) ; Iida, Kenji; (Kawasaki-shi,
JP) ; Sumi, Masaru; (Yonago-shi, JP) ; Ohta,
Yuichiro; (Yonago-shi, JP) ; Katsube, Toshiro;
(Yonago-shi, JP) ; Nakano, Kazuo; (Kawasaki-shi,
JP) ; Ozaki, Norikazu; (Kawasaki-shi, JP) ;
Katayama, Hiroyuki; (Kawasaki-shi, JP) |
Correspondence
Address: |
STAAS & HALSEY LLP
700 11TH STREET, NW
SUITE 500
WASHINGTON
DC
20001
US
|
Family ID: |
12247169 |
Appl. No.: |
09/726328 |
Filed: |
December 1, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09726328 |
Dec 1, 2000 |
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08878198 |
Jun 18, 1997 |
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6184476 |
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08878198 |
Jun 18, 1997 |
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08359448 |
Dec 20, 1994 |
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5679268 |
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Current U.S.
Class: |
29/830 ;
29/602.1 |
Current CPC
Class: |
H05K 1/0293 20130101;
H05K 3/108 20130101; H05K 3/243 20130101; H05K 2201/09736 20130101;
H05K 3/048 20130101; Y10T 29/4913 20150115; H05K 3/4644 20130101;
H05K 3/388 20130101; H05K 3/28 20130101; Y10T 29/49144 20150115;
H05K 3/027 20130101; Y10T 29/49117 20150115; Y10T 29/4902 20150115;
Y10T 29/49126 20150115; H05K 2203/175 20130101; H05K 2203/107
20130101; H05K 3/064 20130101 |
Class at
Publication: |
29/830 ;
29/602.1 |
International
Class: |
H01F 007/06; H05K
003/36 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 25, 1994 |
JP |
06-028384 |
Claims
1. A process for fabricating thin multi-layer circuit boards by
alternately stacking wiring pattern layers (24, 66) and insulating
layers (26, 78) on an insulating plate-like substrate (10), and
electrically connecting said wiring pattern layers through vias in
said insulating layers in order to constitute a predetermined
circuit pattern by said wiring pattern layers, wherein a barrier
metal exclusion zone (76) is prepared while forming a metallic
barrier layer (60) on said wiring pattern layer in an electronic
part-mounting region, a remodeling pad layer (62b) is formed on
said metallic barrier layer neighboring said barrier metal
exclusion zone, and an electronic part-mounting pad layer (62a) is
formed neighboring said remodeling pad layer (62b).
2. A process for fabrication according to claim 1, wherein said
metallic barrier layer (60) is formed by plating nickel, and said
barrier metal extrusion zone is prepared by using a resist to
plating at the time when nickel is plated.
3. A thin multi-layer circuit board which is obtained by
alternately stacking wiring pattern layers (24, 86) and insulating
layers (26, 92) on an insulating plate-like substrate (10), and
electrically connecting said wiring pattern layers through vias in
said insulating layers in order to constitute a predetermined
circuit pattern by said wiring pattern layers, wherein a metallic
barrier layer (80) is formed on the wiring pattern layer in an
electronic part-mounting region, a barrier metal exclusion zone is
included in said metallic barrier layer, and a remodeling pad layer
(82b) and an electronic part-mounting pad layer (82a) are formed on
said metallic barrier layer, said remodeling pad layer being
arranged neighboring said barrier metal exclusion zone and said
electronic part-mounting pad layer being arranged neighboring said
remodeling pad layer.
4. A process for fabricating thin multi-layer circuit boards by
alternately stacking wiring pattern layers (24, 104) and insulating
layers (26, 106) on an insulating plate-like substrate (10), and
electrically connecting said wiring pattern layers through vias in
said insulating layers in order to constitute a predetermined
circuit pattern by said wiring pattern layers, wherein a metallic
barrier layer is formed on the wiring pattern layer in an
electronic part-mounting region, a pad-forming resist is formed in
said metallic barrier layer, a gold pad layer of a predetermined
shape is formed on said metallic barrier layer by using said
pad-forming resist, and said pad-forming resist is removed.
5. A process for fabricating thin multi-layer circuit boards by
alternately stacking wiring pattern layers and insulating layers on
an insulating plate-like substrate, and electrically connecting
said wiring pattern layers through vias in said insulating layers
in order to constitute a predetermined circuit pattern by said
wiring pattern layers, wherein a pad-forming resist is formed on
the wiring pattern layer in an electronic part-mounting region, a
metallic barrier layer is formed on the wiring pattern layer, a
gold pad layer of a predetermined shape is formed on the metallic
barrier layer using the pad-forming resist, and said pad-forming
resist is removed.
6. A process for fabricating thin multi-layer circuit boards by
alternately stacking wiring pattern layers and insulating layers on
an insulating plate-like substrate, and electrically connecting
said wiring pattern layers through vias in said insulating layers
in order to constitute a predetermined circuit pattern by said
wiring pattern layers, wherein the wiring pattern layers are formed
on the insulating layers by sequentially forming a first thin
chromium film, a copper layer and a second thin chromium film on
said insulating layers followed by etching.
7. A process for fabricating thin multi-layer circuit boards by
alternately stacking wiring pattern layers and insulating layers on
an insulating plate-like substrate, and electrically connecting
said wiring pattern layers through vias in said insulating layers
in order to constitute a predetermined circuit pattern by said
wiring pattern layers, wherein a metallic barrier film covers the
wiring pattern layer every time said wiring pattern layer is
formed.
8. A fabrication process according to claim 7, wherein said
metallic barrier film is formed by a liftoff method.
9. A fabrication process according to claim 7, wherein said
metallic barrier film is formed by etching.
10. A fabrication process according to claim 7, wherein said wiring
pattern layers are constituted by a thin chromium film and a copper
layer formed on said thin chromium film, and said metallic barrier
film covers said copper layer.
11. A fabrication process according to claim 7, wherein said wiring
pattern layers are constituted by a first thin chromium film, a
copper layer formed on said thin chromium film and a second thin
chromium film formed on said copper layer, and said metallic
barrier film covers said second thin chromium film.
12. A method of pre-baking a photosensitive polyimide resin layer
in a process for fabricating thin multi-layer circuit boards by
alternately stacking wiring pattern layers and insulating layers on
an insulating plate-like substrate, and electrically connecting
said wiring pattern layers through vias in said insulating layers
in order to constitute a predetermined circuit pattern by said
wiring pattern layers, wherein a heat-accumulating block is
arranged in an atmosphere of a predetermined temperature, and said
insulating plate-like substrate is placed over the
heat-accumulating block close thereto and is heated.
13. A pre-baking method according to claim 12, wherein a gap of
about 0.3 mm is maintained between said heat-accumulating block and
said insulating plate-like substrate.
14. A heat-accumulating block used in the pre-baking method of
claim 12, comprising a block member having a flat heating surface,
a pair of guide walls that extend in parallel along the opposing
sides of the heating surface of said block member, and rail
elements extending along the corners formed by said pair of guide
walls and by said heating surface, the lateral width of said pair
of guide walls being slightly more than the distance between the
opposing side edges on one side of said insulating plate-like
substrate.
15. A heat-accumulating block according to claim 14, wherein said
rail elements have a thickness of about 0.3 mm.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a process for fabricating a
thin multi-layer circuit board on which can be mounted many
electronic parts such as integrated circuits (ICs) and large size
integulated circuits (LSIs).
[0003] 2. Related Art
[0004] There has been known a thin multi-layer circuit board which
is obtained by forming an insulating layer on a plate-like
substrate made of a suitable insulating material and interposing at
least two wiring pattern layers in this insulating layer. The two
wiring pattern layers are connected to each other at a suitable
place through a via and the two wiring pattern layers constitute a
predetermined circuit pattern. Moreover, on the surface of the thin
multi-layer circuit board are provided electronic part-mounting
pads to which can be connected the leads of electronic parts, the
electronic part-mounting pads being connected through vias to the
wiring pattern layers in the insulating layer.
SUMMARY OF THE INVENTION
[0005] An object of the present invention is to provide a thin
multi-layer circuit board which is so constituted that, when a
remodeling pad is being used, this remodeling pad can be easily
cut, and a process for fabricating the same.
[0006] Another object of the present invention is to provide a
process for fabricating a thin multi-layer circuit board, which
does not require a process for etching gold plating at the time of
forming a pad by plating gold on the thin multi-layer circuit
board.
[0007] A further object of the present invention is to provide a
process for fabricating a thin multi-layer circuit board which is
capable of forming a thin chromium film on the wiring pattern
layers without relying upon the lift-off method when a number of
wiring pattern layers are stacked on an insulating plate-like
substrate.
[0008] A still further object of the present invention is to
provide a process for fabricating a thin multi-layer circuit board
which is capable of removing a defective wiring pattern layer
without damaging the wiring pattern layers on the lower side when a
number of wiring patterns are stacked on an insulating plate-like
substrate.
[0009] A yet further object of the present invention is to provide
a method of pre-baking a photosensitive polyimide resin layer that
is applied in the fabrication of a multi-layer circuit board, the
pre-baking method making it possible not only to uniformly heat the
insulating plate-like substrate from the back side thereof but also
to carry out the operation with excellent efficiency, as well as to
provide a heat-accumulating block used for the method of
pre-baking.
[0010] According to a first aspect of the present invention, there
is provided a process for fabricating thin multi-layer circuit
boards by alternately stacking wiring pattern layers and insulating
layers on an insulating plate-like substrate, and electrically
connecting said wiring pattern layers through vias in said
insulating layers in order to constitute a predetermined circuit
pattern using said wiring pattern layers, wherein a barrier metal
exclusion zone is prepared by forming a metallic barrier layer on
said wiring pattern layer in an electronic part-mounting region, a
remodeling pad layer is formed on said metallic barrier layer
neighboring said barrier metal exclusion zone, and an electronic
part-mounting pad layer is formed neighboring said remodeling pad
layer.
[0011] According to the first aspect of the present invention,
furthermore, there is provided a thin multi-layer circuit board
which is obtained by alternately stacking wiring pattern layers and
insulating layers on an insulating plate-like substrate, and
electrically connecting said wiring pattern layers through vias in
said insulating layers in order to constitute a predetermined
circuit pattern by said wiring pattern layers, wherein a metallic
barrier layer is formed on the wiring pattern layer in an
electronic part-mounting region, a barrier metal exclusion zone is
included in said metallic barrier layer, and a remodeling pad layer
and an electronic part-mounting pad layer are formed on said
metallic barrier layer, said remodeling pad layer being arranged
neighboring said barrier metal exclusion zone and said electronic
part-mounting pad layer being arranged neighboring said remodeling
pad layer.
[0012] In the process for fabrication and the thin multi-layer
circuit board according to the first aspect of the present
invention as described above, the barrier metal exclusion zone is
prepared neighboring the remodeling pad layer. In using the
remodeling pad, therefore, the wiring pattern layer is cut by a YAG
laser along the barrier metal exclusion zone in order to cut the
electrical connection between the remodeling pad and the wiring
pattern layer. Therefore, destruction of the insulating layer in
the thin multi-layer circuit board is minimized. Moreover, the
barrier metal is formed maintaining a sufficient thickness making
it possible to prevent the barrier metal from being corroded at the
time of soldering lead wires onto the electronic part-mounting pad
and onto the remodeling pad.
[0013] According to a second aspect of the present invention, there
is provided a process for fabricating thin multi-layer circuit
boards by alternately stacking wiring pattern layers and insulating
layers on an insulating plate-like substrate, and electrically
connecting said wiring pattern layers through vias in said
insulating layers in order to constitute a predetermined circuit
pattern by said wiring pattern layers, wherein a metallic barrier
layer is formed on the wiring pattern layer in an electronic
part-mounting region, a pad-forming resist is formed in said
metallic barrier layer, a gold pad layer of a predetermined shape
is formed on said metallic barrier layer by using said pad-forming
resist, and said pad-forming resist is removed.
[0014] According to the second aspect of the present invention,
there is provided a process for fabricating thin multi-layer
circuit boards by alternately stacking wiring pattern layers and
insulating layers on an insulating plate-like substrate, and
electrically connecting said wiring pattern layers through vias in
said insulating layers in order to constitute a predetermined
circuit pattern by said wiring pattern layers, wherein a
pad-forming resist is formed on the wiring pattern layer in an
electronic part-mounting region, a metallic barrier layer is formed
on the wiring pattern layer, a gold pad layer of a predetermined
shape is formed on the metallic barrier layer using the pad-forming
resist, and said pad-forming resist is removed.
[0015] In the process for fabrication according to the second
aspect of the present invention, the gold pad layer is formed into
a predetermined shape by using the pad-forming resist without the
need of using a highly toxic gold-etching solution, contributing to
the safely of the operation.
[0016] According to a third aspect of the present invention, there
is provided a process for fabricating thin multi-layer circuit
boards by alternately stacking wiring pattern layers and insulating
layers on an insulating plate-like substrate, and electrically
connecting said wiring pattern layers through vias in said
insulating layers in order to constitute a predetermined circuit
pattern by said wiring pattern layers, wherein the wiring pattern
layers are formed on the insulating layers by sequentially forming
a first thin chromium film, a copper layer and a second thin
chromium film on said insulating layers followed by etching.
[0017] In the process for fabrication according to the third aspect
of the present invention, the wiring pattern layers on the
insulating layers are formed by sequentially forming the first thin
chromium film, the copper layer and the second thin chromium film
on the insulating layers followed by etching. Therefore, there is
no need of employing a lift-off method for forming the second thin
chromium film on the insulating layers.
[0018] According to a fourth aspect of the present invention, there
is provided a process for fabricating thin multi-layer circuit
boards by alternately stacking wiring pattern layers and insulating
layers on an insulating plate-like substrate, and electrically
connecting said wiring pattern layers through vias in said
insulating layers in order to constitute a predetermined circuit
pattern by said wiring pattern layers, wherein a metallic barrier
film covers the wiring pattern layer every time a wiring pattern
layer is formed. The metallic barrier film can be formed by either
the lift-off method or an etching method. When each wiring pattern
layer is made up of a thin chromium film and a copper layer formed
on the thin chromium film, the metallic barrier film covers the
copper layer. When each wiring pattern layer is made up of a first
thin chromium film, a copper layer formed on the thin chromium film
and a second thin chromium film formed on the copper layer, the
metallic barrier film covers the second thin chromium film.
[0019] In the process for fabrication according to the fourth
aspect of the present invention, the metallic barrier film covers
the wiring pattern layer every time a wiring pattern layer is
formed. Therefore, a defective wiring pattern layer can be removed
by wet-etching, and the etching solution is prevented from
infiltrating into the wiring pattern layers of the lower side. It
is therefore possible to form the wiring pattern again after a
defective wiring pattern layer is removed, contributing greatly to
decreasing the cost of producing the thin multi-layer circuit
board.
[0020] According to a fifth aspect of the present invention, there
is provided a method of pre-baking a photosensitive polyimide resin
layer in a process for fabricating thin multi-layer circuit boards
by alternately stacking wiring pattern layers and insulating layers
on an insulating plate-like substrate, and electrically connecting
said wiring pattern layers through vias in said insulating layers
in order to constitute a predetermined circuit pattern by said
wiring pattern layers, wherein a heat-accumulating block is
arranged in an atmosphere of a predetermined temperature, and said
insulating plate-like substrate is placed over the
heat-accumulating block close thereto and is heated. Desirably, a
gap of about 0.3 mm is maintained between the heat-accumulating
block and the insulating plate-like substrate. Desirably, the
heat-accumulating block is constituted by a block member having a
flat heating surface, a pair of guide walls that extend in parallel
along the opposing side edges of the heating surface of the block
member, and rail elements that extend along the corners formed by
the pair of guide walls and the heating surface, and the pair of
guide walls have a lateral width which is slightly larger than a
distance between the opposing side edges on one side of the
insulating plate-like substrate.
[0021] In the pre-baking method according to the fifth aspect of
the present invention, the heat-accumulating block is disposed in
an atmosphere heated at a predetermined temperature, the insulating
plate-like substrate is placed over the heat-accumulating block
close thereto and is heated. Therefore, the insulating plate-like
substrate can be uniformly heated from the back side thereof
without being stuck to the heat-accumulating block. With the
heat-accumulating block according to the fifth aspect of the
present invention, furthermore, the insulating plate-like substrate
is placed on the rail elements from the ends on one side of the
pair of guide walls and is moved over the flat surface of the
heat-accumulating block and is pulled out by being placed on the
rail elements. Therefore, even when a plurality of such
heat-accumulating blocks are arranged to be stacked in an oven, the
insulating plate-like substrates can be easily put into, and pulled
out of, the heat-accumulating blocks.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIGS. 1 to 16 are diagrams schematically illustrating the
steps in a process for fabricating thin multi-layer circuit boards
according to a first aspect of the present invention;
[0023] FIGS. 17 to 26 are diagrams schematically illustrating the
steps in a process for fabricating thin multi-layer circuit boards
according to a second aspect of the present invention;
[0024] FIGS. 27 to 35 are diagrams schematically illustrating the
steps in a process for fabricating thin multi-layer circuit boards
according to a third aspect of the present invention;
[0025] FIG. 36 is a diagram schematically illustrating a step in a
process for fabricating thin multi-layer circuit boards according
to a fourth aspect of the present invention;
[0026] FIGS. 37 to 39 are diagrams schematically illustrating the
steps in a process for applying a metallic barrier film onto a
wiring pattern layer in the thin multi-layer circuit substrate
shown in FIG. 36;
[0027] FIGS. 40 to 42 are diagrams schematically illustrating the
steps in another process for applying a metallic barrier film onto
a wiring pattern layer in the thin multi-layer circuit substrate
shown in FIG. 36;
[0028] FIG. 43 is a diagram which schematically illustrates the
principle of a pre-baking method according to a fifth aspect of the
present invention;
[0029] FIG. 44 is a graph showing the distribution of temperature
rise of when a ceramic substrate is heated by the pre-baking method
shown in FIG. 43;
[0030] FIG. 45 is a graph showing the distribution of temperature
rise of when a ceramic substrate is heated by a pre-baking method
which is different from the pre-baking method shown in FIG. 43;
[0031] FIG. 46 is a perspective view of a heat-accumulating block
that is suited for putting into practice the pre-baking method
according to the fifth aspect of the present invention;
[0032] FIG. 47 is a diagram illustrating an end surface of the
heat-accumulating block;
[0033] FIG. 48 is a graph showing the distribution of temperature
rise of when a ceramic substrate is heated by using the
heat-accumulating blocks of FIGS. 46 and 47; and
[0034] FIGS. 49 to 72 are diagrams schematically illustrating the
steps in a conventional representative process for fabricating thin
multi-layer circuit boards.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0035] Before a detailed explanation of some embodiments of the
present invention is given, a conventional process for producing a
thin multi-layer circuit board will be described with reference to
the accompanying FIGS. 49 to 72.
[0036] Referring, first, to FIG. 49, a thin chromium film 12 is
formed on, for example, a ceramic substrate 10, and a thin copper
film 14 is formed on the thin chromium film 12. The thin chromium
film 12 and the thin copper film 14 are usually formed by
sputtering. Then, in order to form a first wiring pattern on the
thin copper film 14, a copper-plating resist 16 is formed on the
thin copper film 14 as shown in FIG. 50. By plating copper on the
copper-plating resist 16, a copper-plated layer 18 is formed on the
first wiring pattern as shown in FIG. 51. After the copper-plated
layer is formed, the copper-plating resist 16 is removed as shown
in FIG. 52 and, then, a thin chromium film 20 is formed again on
the thin copper film 14 and on the copper-plated layer 18 as shown
in FIG. 53. Referring next to FIG. 54, an etching resist 22 is
formed on the first wiring pattern. Then, by effecting the etching,
the thin copper film 14 and the thin chromium film 12 are removed
from the regions other than the first wiring pattern as shown in
FIG. 55. Thereafter, the etching resist 22 is removed to obtain a
first wiring pattern layer 24 on the ceramic substrate 10 as shown
in FIG. 56. The thin chromium film 12 has a thickness of, for
example, about 0.08 .mu., the thin chromium film 22 has a thickness
of, for example, about 0.15 .mu., and copper layers (12, 18)
sandwiched between these thin chromium films have a thickness of,
for example, about 5 .mu..
[0037] After the first wiring pattern layer 24 is formed on the
ceramic substrate 10, an insulating layer such as a photosensitive
polyimide resin layer 26 is applied thereon and is pre-baked as
shown in FIG. 57. Then, the insulating layer 26 is developed by
exposure to light through a predetermined pattern, and whereby a
via hole 28 is formed at a predetermined portion of the insulating
layer 26 as shown in FIG. 58. After the insulating layer 26 is
cured, a thin chromium film 30 and a thin copper film 32 are formed
thereon by sputtering as shown in FIG. 59. Then, in order to form a
second wiring pattern on the thin copper film 32, a plating resist
34 is formed on the thin copper film 32 as shown in FIG. 60. By
plating copper on the plating resist 34, a copper-plated layer 36
is formed on the second wiring pattern as shown in FIG. 61.
[0038] In the region, i.e., in the electronic part-mounting region
shown in FIGS. 49 to 61, nickel and gold are further plated on the
copper-plated layer 36 thereby to form a nickel-plated layer 38 and
a gold-plated layer 40. Then, the plating resist 34 is removed as
shown in FIG. 62. Referring next to FIG. 63, an etching resist 42
is formed on the second wiring pattern, followed by etching to
remove the thin copper film 32 and the thin chromium film 30 from
the regions other than the second wiring pattern as shown in FIG.
64. Thereafter, the etching resist 42 is removed to obtain a second
wiring pattern 44 on the insulating layer 26 as shown in FIG. 65.
As will be obvious from FIG. 65, the first wiring pattern layer 24
and the second wiring pattern layer 44 are connected to each other
through conducting metals in a via hole, and a predetermined
circuit pattern is constituted by these two wiring pattern layers
24 and 44. The thin chromium film 30 has a thickness of, for
example, about 0.08 .mu., the copper layers (32, 36) on the thin
chromium film 30 have a thickness of, for example, about 4 .mu.,
the nickel-plated layer 38 has a thickness of, for example, about 3
.mu., and the gold-plated layer 40 has a thickness of, for example,
about 0.6 .mu..
[0039] A gold-etching resist 46 is formed in a predetermined
pattern on the electronic part-mounting region, i.e., on the
gold-plated layer 40 as shown in FIG. 66, and then gold is removed
by etching. As shown in FIG. 67, therefore, the gold-plated layer
40 is partly removed and the nickel-plated layer 38 is exposed.
Thereafter, the gold-etching resist 46 is removed as shown in FIG.
68, and, then, a chromium lift-off resist 48 is formed on the
insulating layer 26 in order to form a thin chromium film on the
second wiring pattern layer 44. In this case, the chromium lift-off
resist 48 is applied onto the gold-plated layer 40, too, as shown
in FIG. 69. By effecting the chromium lift-off sputtering, a thin
chromium film 50 is formed on the second wiring pattern layer 44
and on the chromium lift-off resist 48 as shown in FIG. 70. The
chromium lift-off resist 48 is then removed as shown in FIG. 71,
and the thin chromium film 50 is left on the second wiring pattern
layer 44 only. Then, as shown in FIG. 72, an overcoat insulating
layer 52 composed of a polyimide resin is applied onto the
insulating layer 26 and onto the second wiring pattern layer 44,
and the surface of the gold-plated layer 40 is exposed to obtain a
thin multi-layer circuit board.
[0040] In the conventional thin multi-layer circuit board shown in
FIG. 72, the gold-plated layer 40 is divided into three exposed
pads 40a, 40b and 40c. The pad 40a serves as a part-mounting pad
for soldering the lead wires when an electronic part is mounted.
The pad 40b serves as a remodeling pad that is used for remodeling
or repairing the circuit wiring, and is used for soldering the lead
wires as required for remodeling or repairing the circuit wiring.
The pad 40c is used for only electrically connecting the
part-mounting pad 40a and the remodeling pad 40b to the wiring
pattern layers 24 and 44. Depending upon the cases, the pad 40c may
be covered with the overcoat insulating layer 52.
[0041] In the above-mentioned thin multi-layer circuit board, the
nickel-plated layer 38 works as a metallic barrier layer for
protecting the copper layers (32, 36) when a lead wire is soldered
onto the part-mounting pad 40a and/or the remodeling pad. If
described in detail, when there is no nickel-plated layer 38, the
solder material permeates into the copper layers (32, 36) at the
time of soldering, causing the electric properties and mechanical
properties to change. In order to prevent the solder material from
permeating into the copper layers (32, 36), therefore, the
nickel-plated layer 38 is necessary. The nickel-plated layer 38
must have a thickness of at least 3 .mu. so that it works as a
metallic barrier layer to a sufficient degree. When the thickness
is smaller than 3 .mu., the nickel-plated layer 38 corrodes at the
time of soldering, causing a problem in that the solder material
permeates into the copper layers (32, 36).
[0042] In the conventional thin multi-layer circuit board, however,
the presence of a metallic barrier layer, i.e., the nickel-plated
layer 38, becomes a problem at the time of remodeling or repairing
the circuit wiring. That is, to remodel or repair the circuit
wiring, the electrical connection between the remodeling pad 40b
and the wiring pattern layers 24, 44 must be cut off. This is done
by cutting the conductor layer between the remodeling pad 40b and
the pad 40c by using a YAG laser beam. Here, however, a problem
arises that the nickel-plated layer 38 cannot be desirably cut by
the YAG laser beam. In particular, it becomes difficult to cut the
nickel-plated layer 38 when its thickness exceeds 3 .mu.. The
nickel-plated layer 38 can be cut by increasing the output of the
YAG laser beam but this causes the insulating layer 26 to be
damaged to a large extent. It is not, therefore, possible to
increase the output of the YAG laser beam. The thickness of the
nickel-plated layer 38 can be decreased so that it can be easily
cut by the YAG laser beam but this is accompanied by the problem of
corrosion at the time of soldering.
[0043] Moreover, it has been pointed out that the conventional
process for producing thin multi-layer circuit boards involves the
processing for etching the gold-plated layer 40 (FIGS. 66 and 67).
That is, the processing for etching the gold-plated layer 40
requires a strongly toxic solvent, and it is desirable to exclude
such an etching processing from the steps of producing thin
multi-layer circuit boards.
[0044] It has further been pointed out that the conventional
process for producing thin multi-layer circuit boards employs a
lift-off method for forming the thin chromium film 50 on the second
wiring pattern layer 44 (FIGS. 69, 70, 71), which is another
problem. That is, after the thin chromium film 50 is formed, it is
not easy to remove the lift-off resist 48 and a relatively long
period of time is required for the removal step.
[0045] In the conventional process for producing thin multi-layer
circuit boards, furthermore, there is a problem in sequentially
stacking a number of wiring pattern layers on the ceramic
substrate. In detail, when ten wiring pattern layers, for instance,
are stacked on the ceramic substrate, up to nine wiring pattern
layers can be properly formed but the tenth wiring pattern layer
may be broken or short-circuited. If this happens, the device must
be discarded as a defective product. Attempts have been made to
remove the defective wiring pattern layer by wet-etching and to
form the wiring pattern again but this permits the etching solution
to infiltrate into the lower wiring pattern layers through the via.
Therefore, the lower wiring pattern layers are damaged, and this
makes it difficult to form the wiring pattern layer again.
[0046] A further problem in the conventional process for producing
thin multi-layer circuit boards resides in the pre-baking of the
photosensitive polyimide resin. In detail, the photosensitive
polyimide resin is used not only as an insulating material but also
as a photoresist material at the time of forming a metal-plated
layer. The polyimide resin is pre-baked when it is applied in the
process for producing thin multi-layer circuit boards. Here,
"pre-baking" means vaporizing the solvent by leaving the polyimide
resin in an atmosphere heated to a temperature of about 80.degree..
In this case, it is desirable to uniformly heat the ceramic
substrate from the back side thereof. Thus, via holes and the like
are formed with a high resolution in the polyimide resin layer by
exposure to light and development. That is, via holes and the like
can be formed accurately. For this purpose, a heat-accumulating
block made of a material having excellent heat-accumulating
property was placed in an oven and a ceramic substrate was placed
on the heat-accumulating block in order to uniformly heat the
ceramic substrate from the back side thereof. This resulted in
failure because, though the ceramic substrate is uniformly heated,
the polyimide resin that reached the back surface of the ceramic
substrate when the polyimide resin was applied causes the ceramic
substrate to stick onto the heat-accumulating block to hinder the
workability to a great extent.
[0047] Thus, in order to solve the above-mentioned problems and
attain the objects of the present invention discussed hereinbefore,
some embodiments of the first through fifth aspects of the present
invention will be presented.
[0048] An embodiment of a process for fabricating thin multi-layer
circuit boards according to a first aspect of the present invention
will now be described with reference to FIGS. 1 to 17 of the
accompanying drawings. The initial steps in the fabrication process
according to the first aspect of the present invention are
substantially the same as those of the conventional fabrication
process described earlier, and are not mentioned again.
[0049] FIG. 1 illustrates a state where a first wiring pattern
layer 24 is formed on a ceramic substrate 10, an insulating layer
such as a photosensitive polyimide resin layer 26 is applied onto
the first wiring pattern layer 24 and after pre-baked a via hole 28
is formed and, then, after the insulating layer 26 is cured, a thin
chromium film 30 and a thin copper film 32 are formed by, for
example, sputtering. On the thin copper film 32 is formed a
copper-plating resist 54 for forming a second wiring pattern as
shown in FIG. 1. When copper is plated on the copper-plating resist
54, a copper-plated layer 56 is formed on the second wiring pattern
as shown in FIG. 2 and, thereafter, the copper-plating resist 54 is
removed as shown in FIG. 3.
[0050] The diagramed region is an electronic part-mounting region.
In this region, nickel and gold are further plated on the
copper-plated layer 56. For this purpose, a plating resist 58 is
formed as shown in FIG. 4. In this case, a plating resist 58a, too,
is formed on the copper-plated layer 56 to prepare a nickel
exclusion zone on a portion thereof. After the plating resist is
formed, nickel is plated to form a nickel-plated layer 60 as shown
in FIG. 5 and, then, gold is plated on the nickel-plated layer 60
to form a gold-plated layer 62. After the plating, the plating
resist is removed as shown in FIG. 6. In this case, a zone on a
part of the surface of the copper-plated layer 56, i.e., the zone
to where the plating resist 58a is applied turns into a nickel
exclusion zone.
[0051] After the plating resist is removed, an etching resist 64 is
formed on the second wiring pattern as shown in FIG. 7. By
effecting the etching thereto, the thin copper film 32 and the thin
chromium film 30 are removed from the regions other than the second
wiring pattern as shown in FIG. 8. Then, the etching resist 64 is
removed to obtain a second wiring pattern layer 66 on the
insulating layer 26 as shown in FIG. 9. As will be obvious from
FIG. 9, the first wiring pattern layer 24 and the second wiring
pattern layer 66 are electrically connected together by conducting
metals in the via hole, and whereby a predetermined circuit pattern
is constituted by these two wiring pattern layers 24 and 66.
[0052] On the gold-plated layer 62 is formed a gold-etching resist
68 in a predetermined pattern as shown in FIG. 10 and, then, gold
is etched. As a result, the gold-plated layer 62 is partly removed
as shown in FIG. 11, and is divided into three pad layers. After,
the gold-etching resist 68 is removed as shown in FIG. 12, the
three gold-plated layers, which are the pad layers, serve as an
electronic part-mounting pad 62a, as a remodeling pad 62b and as a
connection pad 62c. Then, a chromium lift-off resist 70 is formed
on the insulating layer 26 as shown in FIG. 13 in order to form a
thin chromium film on the second wiring pattern layer 66 and, at
this time, the chromium lift-off resist 70 is also formed on the
electronic part-mounting pad 62a and on the remodeling pad 62b.
After the chromium lift-off sputtering is effected, a thin chromium
film 72 is formed on the second wiring pattern layer 66 and on the
chromium lift-off resist 70 as shown in FIG. 14. Then, the chromium
liftoff resist 70 is removed as shown in FIG. 15, and the thin
chromium film 72 is left on the second wiring pattern layer 66
only. Then, as shown in FIG. 16, an overcoat insulating layer 74
composed of a polyimide resin is applied onto the insulating layer
26 and onto the second wiring pattern layer 66 while exposing the
electronic part-mounting pad 62a and the remodeling pad 62b through
the overcoat insulating layer 74.
[0053] When the remodeling pad 62b is used for remodeling or
repairing the circuit wiring in the thus fabricated thin
multi-layer circuit board, the electric connection between the
remodeling pad 62b and the second wiring pattern layer can be
easily cut off by a low output YAG laser or a similar laser. This
is because, the nickel exclusion zone denoted by reference numeral
76 is used as a portion to be cut. According to the first aspect of
the present invention, furthermore, the nickel-plated layer 60 can
be formed maintaining a thickness of not smaller than 3 .mu. and,
preferably, maintaining a thickness of about 3.5 .mu., contributing
to the corrosion resistance of the nickel-plated layer 60 when
soldering lead wires to the electronic part-mounting pad 62a and to
the remodeling pad 62b occurs.
[0054] FIGS. 17 to 27 illustrate an embodiment of the process for
fabricating thin multi-layer circuit boards according to a second
aspect of the present invention. The initial steps in this
fabrication process are substantially the same as those of the
fabrication process according to the first aspect of the present
invention. That is, a first wiring pattern layer 24 is formed on a
ceramic substrate 10, a photosensitive polyimide resin layer 26 is
applied onto the first wiring patten layer 24 and after pre-baked a
via hole 28 is formed, and, then, after the insulating layer 26 is
cured, a thin chromium film 30 and a thin copper film 32 are formed
by, for example, sputtering, and a copper-plating layer 56 is
formed on the thin copper film 32 in order to form a second wiring
pattern, which is the same as the fabrication process according to
the first aspect of the present invention.
[0055] After the copper-plated layer 56 is formed, nickel and gold
are further plated on the copper-plated layer 56 in the electronic
part-mounting region. For this purpose, a plating resist 78 is
formed around the electronic part-mounting region as shown in FIG.
17, and on the inside thereof are formed plating resists 78a and
78b on the copper-plated layer 56. After the plating resist is
formed, nickel is plated to form a nickel-plated layer 80 as shown
in FIG. 18 and, then, gold is plated on the nickel-plated layer 80
to form a gold-plated layer 82. Thereafter, the plating resist is
removed as shown in FIG. 20.
[0056] Referring to FIG. 20, after the plating resist is removed,
an etching resist 84 is formed on the second wiring pattern and,
then, etching is effected thereto remove the thin copper film 32
and the thin chromium film 30 from the regions other than the
second wiring pattern as shown in FIG. 21. Then, the etching resist
84 is removed to obtain a second wiring pattern layer 86 on the
insulating layer 26 as shown in FIG. 22. Here, it should be noted
that the nickel-plated layer 80 and the gold-plated layer 82 have,
respectively, been divided into three by the plating resists 78a
and 78b at the time of plating, and the divided gold-plated layer
serves as the electronic part-mounting pad 82a, as the remodeling
pad 82b and as the connection pad 82c, eliminating the need of
etching the gold-plated layer 80.
[0057] Thereafter, a chromium lift-off resist 88 is formed on the
insulating layer 26, as shown in FIG. 23 to form a thin chromium
layer on the second wiring pattern layer 86 and, at this time, the
chromium lift-off resist 88 is also formed on the electronic
part-mounting pad 82a and on the remodeling pad 82b. After chromium
lift-off sputtering is effected, a thin chromium film 90 is formed
on the second wiring pattern layer 86 and on the chromium lift-off
resist 88 as shown in FIG. 24. Then, as the chromium lift-off
resist 88 is removed as shown in FIG. 25, the thin chromium film 90
is left on the second wiring pattern layer 86 only. Thereafter as
shown in FIG. 24, an overcoat insulating layer 92 composed of a
polyimide resin is applied onto the insulating layer 26 and onto
the second wiring pattern layer 86 while the electronic
part-mounting pad 82a and the remodeling pad 82b are exposed
through the overcoat insulating layer 92.
[0058] The aforementioned fabrication process has a feature in that
there is no need to effect the etching for the gold-plated layer 82
and, hence, the use of a highly toxic etching solution can be
avoided. According to this embodiment, furthermore, the remodeling
pad 82b is disposed neighboring the nickel exclusion region and,
hence, the electrical connection between the remodeling pad 62b and
the second wiring pattern layer 86 can be easily cut by a low
output YAG laser or the like at the time of remodeling or repairing
the circuit wiring as in first aspect of the present invention. In
the fabrication method according to the second aspect of the
present invention, i.e., in the fabrication which excludes the
gold-etching processing, however, the nickel-plated layer 80 may be
extended without being divided. In this case, after the
nickel-plated layer is formed in an extending manner, plating
resists (78a, 78b) as shown in FIG. 17 are formed on the
nickel-plated layer in the electronic part-mounting region.
[0059] FIGS. 27 to 36 illustrate an embodiment of the process for
fabricating thin multi-layer circuit boards according to a third
aspect of the present invention. The initial steps in this
fabrication process are substantially the same as those of the
fabrication process according to the first aspect of the present
invention. That is, a first wiring pattern layer 24 is formed on a
ceramic substrate 10, a photosensitive polyimide resin layer 26 is
applied onto the first wiring pattern layer 24 and is pre-baked to
form a via hole 28, and, then, after the insulating layer 26 is
cured, a thin chromium film 30 and a thin copper film 32 are formed
by, for example, sputtering, and a copper-plating layer 56 is
formed on the thin copper film 32 in order to form a second wiring
pattern, which are the same as in the fabrication process according
to the first aspect of the present invention.
[0060] After the copper-plated layer 56 is formed as shown in FIG.
27, a thin chromium film 94 is formed by, for example, sputtering
on the thin copper film 32 and on the copper-plated layer 56. After
the thin chromium film 94 is formed, a resist 96 is formed around
the electronic part-mounting region as shown in FIG. 28 and,
besides, resists 96a and 96b are formed on the thin chromium film
94 on the inside of the electronic part-mounting region. After the
resist is formed, the etching is effected to remove the thin
chromium film 94 from the electronic part-mounting region as shown
in FIG. 29. Then, in the electronic part-mounting region, a
nickel-plated layer 98 is formed as shown in FIG. 30. Thereafter, a
gold-plated layer 100 is formed by plating gold on the
nickel-plated layer 98 and, then, plating resists 96, 96a and 96b
are removed as shown in FIG. 31.
[0061] After the plating resists are removed, an etching resist 102
is formed on the second wiring pattern as shown in FIG. 32. By
effecting the etching thereto, the thin chromium film 94, the thin
copper film 32 and the thin chromium film 30 are removed from the
regions other than the second wiring pattern as shown in FIG. 33.
Thereafter, the etching resist 102 is removed to obtain a second
wiring pattern layer 104 on the insulating layer 26 as shown in
FIG. 34. As the case of the embodiment (FIGS. 17 to 26) according
to the second aspect of the present invention, the nickel-plated
layer 98 and the gold-plated layer 100 are, respectively, divided
into three by the plating resists 96a and 96b at the time of
plating, and the divided gold-plated layers serve as an electronic
part-mounting pad 100a, as a remodeling pad 100b and as a
connection pad 100c, eliminating the need of effecting the etching
for the gold-plated layer 100. Then, as shown in FIG. 35, an
overcoat insulating layer 106 composed of a polyimide resin is
applied onto the insulating layer 26 and onto the second wiring
pattern layer 104 while exposing the electronic part-mounting pad
100a and the remodeling pad 100b through the overcoat insulating
layer 92.
[0062] The aforementioned fabrication process has a novel feature
in that the thin chromium film 94 is formed on the second wiring
pattern layer 104 formed on the insulating layer 26 without relying
upon the lift-off method. This embodiment has not only the feature
according to the first aspect of the present invention in that the
electrical connection between the remodeling pad 100b and the
second wiring pattern layer 104 is easily cut by a low output YAG
laser or the like at the time of remodeling or repairing the
circuit wiring but also the feature, according to the second aspect
of the present invention, that there is no need to etch the
gold-plated layer 100. In addition, it should be understood that
the feature according to the third aspect of the present invention,
i.e., forming the thin chromium film on the insulating layer
without relying upon the lift-off method, by itself constitutes an
invention.
[0063] FIG. 36 illustrates, in the form of an intermediate product,
a thin multi-layer circuit board fabricated by the process for
fabrication according to a fourth aspect of the present invention.
This intermediate product is in a stage in which wiring pattern
layers and polyimide resin layers are alternately stacked on a
ceramic substrate 108. That is, on the ceramic substrate 108 is
formed a first wiring pattern layer 110 which is covered with a
first polyimide resin insulating layer 112. On the first polyimide
resin insulating layer 112 is formed a second wiring pattern layer
114 which is covered with a second polyimide resin insulating layer
116. On the second polyimide resin insulating layer 116 is formed a
third wiring pattern layer 118 which is covered with a third
polyimide resin insulating layer 120. The first to third wiring
pattern layers 110, 114 and 118 are connected together through vias
formed in the polyimide resin insulating layers 112, 116 and 120,
thereby to constitute a predetermined circuit pattern. The wiring
patterns 110, 114 and 118, respectively, include thin chromium
films 110a; 110b, 114a; 114b, 116a; 116b, as well as copper layers
110c, 114c and 118c formed among the thin chromium films. The
wiring pattern layers 110, 114, 118 and the polyimide resin
insulating layers 112, 116, 118 can be formed through a variety of
processes mentioned already.
[0064] In this embodiment, the thin chromium films 110b, 114b and
118b are covered with a metallic barrier film 122 every time the
wiring pattern layers 110, 114 and 118 are formed, the metallic
barrier film 122 being comprised of, for example, titanium nitride,
platinum or the like. The metallic barrier film 122 can be
incorporated at the time of forming the wiring pattern layers 110,
114 and 118 relying, for example, upon the etching as shown in
FIGS. 37 to 39. Referring to FIG. 37, a first thin chromium film
110a is, first, formed by sputtering on the ceramic substrate 108
and, then, copper is sputtered thereon. Next, a copper layer 110c
is formed by plating copper on a region that corresponds to the
first wiring pattern, then, a second thin chromium film 110b is
formed by sputtering and a metallic barrier film 122 is formed
thereon by sputtering. Referring next to FIG. 38, an etching resist
124 is formed on a predetermined region and the etching is effected
to obtain a first wiring pattern layer 110 as shown in FIG. 39. The
metallic barrier film 122 of titanium nitride or platinum is very
stable against an etching solution for chromium or copper. When the
metallic barrier film 122 is composed of titanium nitride,
therefore, the etching solution will comprise concentrated
hydrofluoric acid, concentrated nitric acid and water. When the
metallic barrier film 122 is composed of platinum, the etching
solution will be aqua regia or a diluted aqua regia solution.
[0065] As shown in FIGS. 40 to 42, furthermore, the metallic
barrier film can also be formed by the lift-off method. That is, a
first thin chromium film 110a is formed by sputtering on the
ceramic substrate 108 and, then, copper is sputtered thereon. Next,
a copper layer 110c is formed by plating copper on a region
corresponding to the first wiring pattern, and the first thin
chromium film 110a and the copper layer 110c are formed into a
predetermined wiring pattern by etching. Then, a lift-off resist
126 is formed thereon as shown in FIG. 40. Referring next to FIG.
42, a thin chromium film 110b and a metallic barrier film 122 are
formed by sputtering. Then, the lift-off resist 126 is removed to
obtain a first wiring pattern layer 110 on the ceramic substrate
120.
[0066] In the process for fabrication according to the fourth
aspect of the present invention, a defective wiring pattern layer
that is formed in the step of alternately stacking the wiring
pattern layers and the polyimide resin insulating layers, can be
formed again. For instance, in case a defect is found after the
third wiring pattern layer 118 is formed, the third wiring pattern
layer 118 is removed without at all damaging the second wiring
pattern layer 116. In detail, the metallic barrier film 122 of the
third wiring pattern layer 118 is removed by using the
above-mentioned etching solution and, then, the second thin
chromium film 118b, copper layer 118c and first thin chromium film
118a are removed by using a known etching solution. In this case,
the second wiring pattern layer 114 has been covered with the
metallic barrier film 122, and the etching solution for chromium
and copper does not permeate into the second wiring pattern layer
114 through the via. Accordingly, the third wiring pattern layer
118 that has been removed can be formed again.
[0067] In the embodiment according to the fourth aspect of the
present invention, the metallic barrier film 122 covers the second
thin chromium films 110b, 114b and 118b of the wiring pattern
layers 110, 114 and 118. As required, the metallic barrier film 122
may be formed between the copper layers 110c, 114b, 118b and the
second thin chromium films 110b, 114b, 118b. In this case, the
second thin chromium films 110b, 114b and 118b may be damaged at
the time of removing the defecting wiring pattern layer, but the
damage does not extend to the copper layers 110c, 114b and 118b. By
strictly managing the etching time, furthermore, damage to the
second thin chromium films can be minimized.
[0068] In the process for fabricating thin multi-layer circuit
boards as described above, the photosensitive polyimide resin is
used not only as an insulating material but also as a resist
material. When the polyimide resin is applied, the ceramic
substrate is put into the oven and is pre-baked. It was mentioned
already that in this case, the ceramic substrate is uniformly
heated from the back side thereof. A fifth aspect of the present
invention is directed to improving the pre-baking method.
[0069] FIG. 43 illustrates the principle of the pre-baking method
according to the fifth aspect of the present invention, wherein
reference numeral 128 denotes a heat-accumulating block which is
made of, for example, aluminum. The heat-accumulating block 128 is
left to stand in an oven (not shown) having an atmospheric
temperature of, for example, about 80.degree.. The
heat-accumulating block 128 has a flat upper surface, and a pair of
copper strips 130 are disposed along the opposing side edges on the
surface thereof. A ceramic substrate 132 coated with the polyimide
resin is placed on the pair of copper strips 130, and a gap of
about 0.3 mm is maintained between the upper surface of the
heat-accumulating block 128 and the ceramic substrate 132.
Therefore, the ceramic substrate 132 is gradually heated from the
back side thereof by receiving radiant heat. At this moment, the
rise in the temperature is measured at five points at equal
distances along the diagonal of the ceramic substrate 132. The
results are shown in a graph of FIG. 44 from which it will be
obvious that the temperature rises are nearly the same at all of
the above-mentioned five points. FIG. 45 illustrates similar
temperature rises in the case when the ceramic substrate 132 is
placed on the upper surface of the heat-accumulating block. It can
be regarded that the temperature rise is substantially the same at
five points maintaining an equal distance along the diagonal lines
of the ceramic substrate 132. That is, it is desired to place the
ceramic substrate 132 directly on the upper surface of the
heat-accumulating block in order to uniformly heat the ceramic
substrate 132. As mentioned earlier, however, it is not possible to
place the ceramic substrate 132 directly on the heat-accumulating
block 128.
[0070] FIGS. 46 and 47 illustrate a heat-accumulating block 134
adapted to putting the pre-baking method of the present invention
into practice. The heat-accumulating block 134 comprises a block
member 138 having a flat heating surface 136, a pair of guide walls
140 that extend in parallel along the opposing sides of the heating
surface 136 of the block member 138, and rail elements 142 that
extend along the corners formed by the guide walls 140 and the
heating surface 136. The lateral width of the pair of guide wall
portions 140 is slightly broader than the distance between the
opposing side edges of the insulating plate-like substrate. With
this constitution, the ceramic substrate 132 is placed on the rail
elements 142 from the ends on one side of the pair of guide walls
140, and is pushed; i.e., the ceramic substrate 132 can be easily
moved on the flat surface 136 of the heat-accumulating block 134
and can be easily pulled out along the rail elements 142. Even when
a plurality of heat-accumulating blocks 134 are arranged in a
plurality of stages to be stacked in the up-and-down direction in
the oven, the ceramic substrates can be easily put into, and taken
out from, the heat-accumulating blocks 134, enabling the pre-baking
processing to be quickly executed.
[0071] FIG. 48 is a graph showing the results of the measurement of
the temperature rise distribution in the manner described above in
the case when the ceramic substrate 132 is heated by being placed
on the heat-accumulating block 134. It will be obvious from this
graph that the ceramic substrate 132 is heated substantially
uniformly though the temperature rise distribution is slightly
disturbed immediately after the start of the heating.
* * * * *