Stock material or miscellaneous articles – Structurally defined web or sheet – Discontinuous or differential coating – impregnation or bond
Reexamination Certificate
2001-11-28
2003-05-20
Lam, Cathy (Department: 1775)
Stock material or miscellaneous articles
Structurally defined web or sheet
Discontinuous or differential coating, impregnation or bond
C428S901000, C156S089120, C156S089160
Reexamination Certificate
active
06565956
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to multilayer ceramic wiring boards for providing various electronic circuits for use in electronic devices such as portable telephones, and to processes for producing such wiring boards.
BACKGROUND OF THE INVENTION
With compact electronic devices such as portable telephones, it is conventional practice to arrange a plurality of circuit elements constituting the device into a multilayer ceramic wiring board, as integrated into a single chip and mount the wiring board on the main substrate.
As shown in
FIG. 19
, the multilayer ceramic wiring board comprises superposed ceramic layers
7
each having a plurality of circuit elements
71
formed on the surface thereof and providing an inductor or capacitor. Such circuit elements
71
are interconnected by a continuity channel (hereinafter referred to as a “via hole”)
73
extending through the ceramic layer
7
to provide an electric circuit such as a filter.
In producing the multilayer ceramic wiring board, sheets (hereinafter referred to as “green sheets”) comprising a ceramic-incorporating material which contains a ceramic powder and a binder resin are prepared first by coating a surface of a plastic film with the ceramic-incorporating material in the form of a sheet and drying the material. A pattern of circuit elements in accordance with the design of the contemplated electronic circuit is subsequently printed on the surface of each green sheet using a conductive mixture material containing an electrically conductive powder and a binder resin. The printed green sheets are then placed over one another to prepare a laminate, which is thereafter pressed hot and thereby consolidated. The pressed laminate is further fired, and the fired laminate is finally cut into a plurality of multilayer ceramic wiring boards.
In recent years, it is required with increasing severity to compact electronic devices. In order to compact the multilayer ceramic wiring board itself, studies are underway to reduce the thickness of the ceramic layers
7
which are components of the wiring board.
As shown in section in
FIG. 20
, however, the boundary between the circuit element
71
and the ceramic layer
7
in the conventional multilayer ceramic wiring board is likely to become wavy or undulate in the region C indicated although the boundary should originally be straight, such that if the ceramic layer
7
is made thinner, the surface of the circuit element
71
will come into contact with the surface of other circuit element
72
or
72
in the wavy boundary region to make a short circuit.
Further as shown in
FIGS. 21 and 22
, it is likely that projections
95
will be formed on the surfaces of circuit elements
71
to extend toward other circuit elements
71
. In the case where the ceramic layer
7
has a reduced thickness, the outer end of the projection
95
will come into contact with that of the projection
95
formed on the surface of other circuit element
71
, giving rise to the problem of causing a short.
SUMMARY OF THE INVENTION
A first object of the present invention is to prevent formation of projections extending from the surfaces of the circuit elements on the multilayer ceramic wiring board. A second object of the invention is to prevent the waviness of the boundary between the circuit element and the ceramic layer in the multilayer ceramic wiring board.
To fulfill these objects, we have conducted intensive research and found that the formation of projections from the circuit elements on the conventional multilayer ceramic wiring board is attributable to the state of the ceramic powder dispersed over the surface of the green sheet and the particle size of the electrically conductive powder contained in the conductive mixture material, and that the wavy boundary between the circuit element and the ceramic layer is attributable to the state of the ceramic powder dispersed throughout the green sheet. With reference to
FIG. 23
, the surface of the conventional green sheet has interstices
82
formed between the ceramic particles
8
since the ceramic particles
8
are dispersed unevenly. The interstices
82
are filled with the binder resin or solvent. Further with reference to
FIG. 24
, the conductive mixture material contains the conductive powder
9
comprising particles of varying sizes. The mean particle size of the conductive powder
9
contained in the conductive mixture material was found to be approximately the same as that of the ceramic powder constituting the green sheet. When a pattern of circuit elements was printed on the surface of each green sheet
80
with the conductive mixture material
91
as shown in
FIG. 25
in the conventional process for producing the conventional multilayer ceramic wiring board with use of conventional green sheets and the conventional conductive mixture material, it was found that conductive particles
9
a
of small size were trapped in interstices
82
,
82
,
82
formed in the surface of the green sheet
80
at the boundary (region D) between the green sheet
80
and the conductive mixture material
91
as shown in FIG.
26
. The green sheets
80
each bearing the print of conductive mixture material were then placed over one another into a laminate, which was pressed hot, whereby the interstices in each green sheet were filled with conductive particles
9
a
of small size as shown in FIG.
27
. The conductive particles
9
a
filling the interstices of the green sheets form projections on the surfaces of the circuit elements obtained by firing the pressed laminate.
The ceramic powder
8
was unevenly dispersed throughout the green sheet used for producing the conventional wiring board as shown in FIG.
26
. Accordingly, when the laminate of printed green sheets
80
was pressed hot, compressive stress failed to uniformly act on each green sheet
80
, consequently permitting waviness of the surface of the green sheet after hot pressing to undulate the boundary between the circuit element obtained by firing the laminate and the ceramic layer thereof. Especially when small circuit elements
72
,
72
were present in the vicinity of the circuit element
71
as shown in
FIG. 20
, the compressive stress acting on the green sheet when the laminate was pressed hot involved great variations, consequently readily permitting waviness of the boundary between the ceramic layer and the circuit element.
To accomplish the first object, the prevent invention provides a multilayer ceramic wiring board prepared from a plurality of sheets formed from a ceramic-incorporating material containing a ceramic powder and a binder resin and each having a pattern of one or a plurality of circuit elements printed on a surface thereof with use of a conductive mixture material containing an electrically conductive powder and a binder resin, by placing the sheets over one another to form a laminate, and compressing and firing the laminate. The mean particle size Rs of the ceramic powder is not greater than the value of (Rd−&sgr;d) wherein Rd is the mean particle size of the conductive powder and &sgr;d is the standard deviation obtained when the distribution of the particle sizes of the conductive powder is expressed by a normal distribution function.
The present invention further provides a process for producing the multilayer ceramic wiring board which process has the steps of:
preparing a plurality of green sheets by coating a plastic film with a ceramic-incorporating material containing a ceramic powder and a binder resin and drying the ceramic-incorporating material,
forming a hole in at least one of the green sheets through the thickness thereof at least one portion of the sheet,
printing a pattern of one or a plurality of circuit elements on a surface of each of the green sheets with use of a conductive mixture material containing an electrically conductive powder and a binder resin,
placing over one another the printed green sheets on which the pattern of circuit elements is each printed according to the printing step to form a laminate,
compressing the la
Hongo Masanori
Horimoto Shigehiro
Ogura Takashi
Armstrong Westerman & Hattori, LLP
Lam Cathy
Sanyo Electric Co,. Ltd.
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