Electricity: conductors and insulators – Conduits – cables or conductors – Preformed panel circuit arrangement
Reexamination Certificate
1999-12-23
2004-01-06
Talbott, David L. (Department: 2827)
Electricity: conductors and insulators
Conduits, cables or conductors
Preformed panel circuit arrangement
C029S847000, C029S853000
Reexamination Certificate
active
06674017
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a multilayer-wiring substrate for surface-mounting electronic and/or mechanical components, and particularly to such a substrate and method for making the same wherein at least two wiring conductors that sandwich an insulating layer are electrically connected by a via-hole conductor penetrating the insulating layer. More specifically, the present invention relates to a method for fabricating a multilayer-wiring substrate having wiring conductors formed alternatingly with insulating resin layers and having a via-conductor penetrating through the insulating resin layer(s) for establishing electrical continuity between the wiring conductors separated by the insulating layer(s).
2. Description of the Related Art
Some conventional steps for fabricating a multilayer-wiring substrate are described hereinafter referring to FIGS.
5
(
a
)-
5
(
b
). A wiring conductor
14
is formed on a surface of an insulating resin layer
12
and then a photosensitive-resin layer
13
is coated on the insulating resin layer
12
and the wiring conductor
14
. Next, the photosensitive-resin insulator layer
13
is exposed to light through a photomask and is then developed to thereby form a via-hole
15
so as to expose a surface of the wiring conductor, based on a photolithography method. Subsequently, another wiring conductor
18
is formed on a surface of the insulating resin layer
13
by, e.g., electroless copper plating or copper electroplating. At the same time, a via-conductor
17
is formed along an inner wall of the via-hole
15
to establish electrical connection between these wiring conductors
14
,
18
, as seen in FIG.
5
(
b
).
SUMMARY OF THE INVENTION
However, uniform development of a number of via-holes to be formed over an entire surface of a multilayer-wiring substrate by the photolithography method is difficult. In a certain portion of the multilayer-wiring substrate the via-holes may be accurately formed in the substrate, whereas, in another portion of the multilayer-wiring substrate, resin to be removed may remain on the bottoms of the via-holes due to inconsistent development speed of the photolithography. During the development of the via-hole, the resin which has once been dissolved in developer may accumulate on the bottom of the via-hole to prevent smooth feeding of new developer to the bottom of the via-hole. As a result, inward progress of development is hindered in a depth direction, resulting in insufficient development. As a result of such insufficient lithography, as shown in FIG.
5
(
c
), the lower conductor
14
is not exposed through a bottom resin
15
A of the via-hole
15
. As a result, the lower wiring conductor
14
and the upper conductor wiring layer
18
are not connected by the via-conductor
17
.
The bottom resin
15
A has to be removed before forming the via-conductor, for example, by etching using a resin etchant such as a potassium permanganate solution. Even so, the bottom resin
15
A is not removed completely and remains in a fragmental form in the via-hole
15
, causing poor electrical continuity by way of the via-conductor between the lower conductor
14
and the upper conductor
18
.
The present invention has been accomplished in view of the foregoing.
An object of the present invention is to provide a multilayer-wiring substrate with a via-conductor having a complete electrical connection and/or a lowest electrical impedance across lower and upper conductors separated by an insulating layer.
Another object of the invention is to provide a method for fabricating a multilayer-wiring substrate with a via-conductor having a uniform electrical continuity from a upper wiring conductor formed on a surface of an insulating layer to a lower wiring conductor formed on another surface of the insulating layer.
Still another object of the invention is to provide a method for completely removing undesired material adhered onto a wiring conductor that forms a bottom of a via-hole penetrating through an insulating resin layer so as to sufficiently expose the wiring conductor on which a solder bump or other via-conductor may be assuredly formed.
A further object of the invention is to provide a via-conductor structure penetrating straightly through at least two insulating layers of a multilayer wiring substrate.
According to a first aspect of the invention, a multilayer-wiring substrate is provided comprising:
a wiring conductor with a recess formed by etching a surface of the wiring conductor,
an insulating layer formed on a surface of the wiring conductor except over the recess;
another insulating layer formed on the other surface of the wiring conductor;
a via-hole penetrating through the insulating layer to the recess; and
a via-conductor or a solder bump formed inside the via-hole and adhered to a recessed surface of the wiring conductor.
An advantage of this multilayer wiring substrate is that an area of the wiring conductor to which a via-conductor or a solder bump adheres is increased by the recess. In other words, since a recessed surface is formed at the wiring conductor, a strong mechanical and electrical connection is attained between the wiring conductor and the via-conductor or the solder bump. The via-conductor or the solder bump is formed in the via-hole of the insulating layer and partly in the recess of the wiring conductor.
When a depth of the recess is 5-30% of a thickness of the wiring conductor, a good result is attained in such a connection. Further, when the recess is made by chemically etching the wiring conductor, a better connection strength of the via-conductor or solder bump to the wiring conductor is attained.
When the via-conductor is made by plating copper onto an inner peripheral wall of the via-hole that penetrates the insulating layer and by extendingly plating copper on an etched and recessed portion of the wiring conductor at the via-hole bottom, the best electrical connection is attained between the electrical conductors sandwiching the insulating layer. A via-conductor is formed by plating metal such as copper onto the recessed wiring conductor whereat the via-hole forms a bottom, and the via-conductor may be formed by further plating metal extendingly on an inner peripheral wall of the via-hole. The via-hole connects the wiring conductor with other electrical conductors formed on the other surface of the insulating layer.
The above mentioned multilayer-structure may be supported on an insulating core plate having a thickness of 0.5-1 mm, since the conductor, e.g., a copper layer, is about 10-25 &mgr;m in thickness and the insulating layer coated on the copper is about 30-60 &mgr;m in thickness. A plurality of the structures may be formed on the core plate, with alternatingly forming the wiring conductor and the insulating layer. Similar structure(s) may also be advantageously formed on the other side of the core plate in order to prevent warpage of the multilayer wiring substrate.
According to a second aspect of the invention, a method is provided for fabricating a multilayer-wiring substrate having a via-conductor in a via-hole, comprising:
forming an insulating layer on a wiring conductor; forming a via-hole in the insulating layer by removing the insulating layer to an extent that the insulating layer in the via-hole becomes fragmented and adheres to the wiring-conductor located at the bottom of the via-hole; then etching the wiring conductor located at the bottom of the via-hole so that the fragments are removed and a recess is formed in the conductor; and then forming a via-conductor in the via-hole by plating metal on an inner peripheral wall of the via-hole and extendingly plating the metal on a surface of the recessed portion of the wiring conductor.
Forming the via-hole in the insulator may be done by developing a photosensitive resin layer using a lithography technique or by laser-drilling the insulating layer.
An important step in the above described method according to the invention is etching the wiring conductor located at the bottom
Hasegawa Katsuhiko
Hirano Satoshi
Hisada Osamu
Kito Naoki
Yamasaki Kozo
NGK Spark Plug Co. Ltd.
Norris Jeremy
Talbott David L.
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