Electricity: conductors and insulators – Conduits – cables or conductors – Preformed panel circuit arrangement
Reexamination Certificate
1996-12-05
2001-02-06
Kincaid, Kristine (Department: 2831)
Electricity: conductors and insulators
Conduits, cables or conductors
Preformed panel circuit arrangement
C361S771000, C361S767000, C361S768000, C174S255000
Reexamination Certificate
active
06184479
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a multilayer printed circuit board and to a method of fabricating the multilayer printed circuit board. More particularly, the present invention relates to a multilayer printed circuit board where a plurality of wires in different wiring layers are electrically connected by via-holes and to a method of fabricating this multilayer printed circuit board.
2. Background of the Invention
As electronic equipment becomes smaller and more compact, the printed circuit board on which such electronic equipment is mounted must have a larger number of wires in a limited space. This requirement can be achieved by increasing the number of circuit layers (also called wiring layers herein) of the printed circuit board and making the board into a multilayer circuit board. A plurality of such circuit layers are then connected to each other through via holes.
A now conventional process for fabricating a multilayer printed circuit board is described with reference to FIG.
1
. As shown in
FIG. 1
, a layer of electrically conductive material
11
is first formed on an electrically insulating substrate
10
(FIG.
1
(
a
)). Then, a layer of photoresist
12
is blanket-coated onto the electrically conductive layer
11
and is selectively exposed to actinic radiation and developed (FIG.
1
(
b
)). The electrically conductive layer
11
is selectively etched using the patterned photoresist layer
12
as an etch mask to form a desired lower wiring layer
13
(FIG.
1
(
c
)) and then the patterned photoresist layer
12
is stripped (FIG.
1
(
d
)). Next, a photosensitive dielectric layer
14
is formed on the lower wiring layer
13
(FIG.
1
(
e
)). This dielectric layer
14
is selectively exposed to actinic radiation and then developed to form one or more photo-via holes
15
, thereby uncovering selected areas of the lower wiring
13
(FIG.
1
(
f
)). Thereafter, using any of a variety of metal deposition techniques such as electroless plating or sputtering, an upper wiring layer
16
is formed on the dielectric layer
14
and metal is deposited onto the sidewalls and bottoms of the photo-via holes
15
(FIG.
1
(
g
)). By repeatedly performing these steps, a plurality of electrically connected wiring layers is readily formed.
A multilayer printed circuit board formed as described above is connected to an external circuit through a metal wire such as a gold wire. This is achieved by bonding the gold wire to the board.
FIG. 2
is a diagram showing a gold wire bonded to the printed circuit board. As shown in FIG.
2
(
a
), a wire
16
has been conventionally connected to a predetermined connection area on an upper wiring layer
15
formed on a photosensitive dielectric layer
14
. In normal wire bonding, the wire
16
is pressed against the connection area using a force of about 50 g to 150 g, while ultrasonic vibration is applied to the connection area so that the gold wire is fixed to the connection area by thermocompression bonding.
One type of photosensitive dielectric material is, for example, epoxy resin having photosensitivity. One such photosensitive epoxy resin is sold (in what is initially liquid form) under the trade name PROBIMER-52 by Ciba Geigy of Japan. Although there are other such materials, this photosensitive resin is susceptible to deformation by high temperature or high pressure. Therefore, when bonding the wire
16
to the bonding pad
15
overlying the photosensitive resin layer
14
, there are some cases where the photosensitive resin layer
14
is softened by thermocompression bonding through application of heat and application of pressure and therefore a depressed portion occurs in the direction where the wire
16
is pressed, as shown by reference character A in FIG.
2
(
a
). In addition to the softening caused by heat or pressure, there is the possibility that the size of the depressed portion will be increased by the ultrasonic vibration applied to the bonding pad
15
, when bonding is performed.
If wire bonding is performed under the above-described circumstances, there is the possibility that, at the portion B clamped by the capillary head
17
of a bonding machine at the edge of the depressed portion A, the wire
16
will be excessively locally stressed and cut, as shown in FIG.
2
(
b
). On the other hand, if the temperature, applied when bonding is performed, is set to a low temperature in order to prevent the formation of such a depressed portion of the photosensitive resin layer
14
, then there will be the problem that a sufficient connection strength cannot be obtained between the wire and the pad and therefore a reduction in reliability of the connection will be caused.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to prevent a photosensitive resin layer from being depressed by external heat, pressure, or ultrasonic vibration.
Another object of the present invention is to perform wire bonding without reducing the strength of the connection between the wire and the pad.
To achieve the above objects and in accordance with one aspect of the present invention, there is provided a multilayer printed circuit board having a substrate, a first conductive circuit layer formed on the substrate, a photosensitive dielectric layer formed on the first conductive circuit layer, and a second conductive circuit layer formed on the photosensitive dielectric layer and electrically connected to the first conductive circuit layer through a plurality of photo-via holes formed in the photosensitive dielectric layer. In the multilayer circuit board, the second conductive circuit layer has a wiring area where a plurality of wires are arranged and a pad area to which external heat or pressure is to be applied in order to connect a conductive wire. Significantly, the thickness of the second conductive circuit layer in at least part of the pad area is greater than that of the second conductive circuit layer in the wiring area because, at least in this part of the pad area, the second conductive circuit layer extends into the photosensitive dielectric layer.
In accordance with another aspect of the present invention, there is provided a multilayer circuit board having a substrate, a first conductive circuit layer formed on the substrate, a photosensitive dielectric layer formed on the first conductive circuit layer, and a second conductive circuit layer formed on the photosensitive dielectric layer which is electrically connected to the first conductive circuit layer through a plurality of photo-via holes formed in the photosensitive dielectric layer. In the multilayer circuit board, the second conductive circuit layer has a wiring area where a plurality of wires are arranged and a pad area to which external heat or pressure is to be applied in order to connect a conductive wire. Significantly, the thickness of the photosensitive dielectric layer existing under at least part of the pad area is less than that of the photosensitive dielectric layer existing under the wiring area, excluding the photo-via holes, because the former thickness is reduced by the degree to which the second conductive layer extends into the photosensitive dielectric layer.
In accordance with still another aspect of the present invention, there is provided a method of fabricating a multilayer printed circuit board, comprising the steps of forming a first conductive circuit layer having a predetermined wiring pattern on a substrate; forming a photosensitive dielectric layer on the first conductive circuit layer; selectively exposing and developing the photosensitive dielectric layer so that in a contact-hole area a first photo-via hole is formed having a depth which extends to the first conductive circuit layer, and in at least part of a pad area, a second photo-via hole is formed having a depth which does not extend to the first conductive circuit layer; in at least the contact-hole area and the pad area, selectively forming a second conductive circuit layer; and bonding a conductive wire to the second
Okabe Shuhichi
Sakurai Keizo
Cuneo Kamand
Fraley Lawrence R.
International Business Machines - Corporation
Kincaid Kristine
Scully Scott Murphy & Presser
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