Active solid-state devices (e.g. – transistors – solid-state diode – Combined with electrical contact or lead – Of specified material other than unalloyed aluminum
Reexamination Certificate
2002-04-22
2004-05-25
Flynn, Nathan J. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Combined with electrical contact or lead
Of specified material other than unalloyed aluminum
C257S700000, C257S773000, C257S774000, C257S691000
Reexamination Certificate
active
06740975
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wiring substrate, and more particularly to a wiring substrate in which a signal wiring layer and a conductor layer are electromagnetically coupled via a dielectric layer for the purpose of transmitting a signal.
2. Description of the Related Art
There are known wiring substrates configured such that a dielectric layer(s), a signal wiring layer(s), a solid conductor layer(s), and other layers are arranged in an alternating fashion. Among such wiring substrates, those involving transmission of a high-frequency signal may assume a strip line structure or a microstrip line structure in which a signal wiring layer and a solid conductor layer are electromagnetically coupled so as to serve as a signal transmission path.
When a wiring substrate is configured such that the dielectric layers and the large-area conductor layers, such as solid conductor layers, are arranged in an alternating fashion, a number of circular or rectangular through holes are often formed in the conductor layers at predetermined intervals, such as in a lattice pattern, or meshed so as to directly connect the upper and lower dielectric layers through the through holes (or mesh).
There are many conventional reasons for connecting the upper and lower dielectric layers directly together through the through holes provided in the conductor layer. The first reason is that the bonding strength between dielectric layers, which are formed from ceramics (e.g., alumina) and/or resins (e.g., epoxy resin), is greater than that between a dielectric layer and a conductor layer, which is formed from metal (e.g., tungsten, molybdenum, copper, or silver). Therefore, the direct bonding of the dielectric layers through the through holes of the conductor layer increases the strength of the wiring substrate. The second reason is that a conductor layer has poorer gas release properties than a dielectric layer, which is especially important during firing, curing, thermal treatment, and other processes performed during manufacture of the wiring substrate. Therefore, during the course of such treatments or processes, a conductor layer having a large area is likely to hinder diffusion of gas released from an underlying dielectric layer or conductor layer. This gas release phenomenon may generate blisters between a dielectric layer and a conductor layer in contact with the dielectric layer, which reduces the bonding strength between the layers of the wiring substrate.
For these reasons, the conventional wisdom is to provide a conductor layer, which is electromagnetically coupled with a signal wiring layer, with a number of through holes to maintain the bonding strength with a dielectric layer and to prevent the generation of blisters.
Although the conventional wiring substrate is generally thought to be acceptable, it is not without shortcomings. Namely, when a conductor layer (which is to be electromagnetically coupled with a signal wiring layer) has a number of through holes formed therein or is meshed as shown in
FIG. 6
, for example, the through holes are located at portions of the conductor layer that face the signal wiring layers. That is, and with reference to
FIG. 6
, when a conductor layer
2
having through holes
2
B and signal wiring layers
4
(shown in phantom) are arranged together, the conductor layer
2
and the signal wiring layers
4
overlap. However, the through holes
2
B and the signal wiring layers
4
also overlap.
The signal wiring layers
4
differ in characteristic impedance between the case where the conductor layer
2
and the signal wiring layers
4
overlap and the case where the through holes
2
B and the signal wiring layers
4
overlap. Thus, when a single signal wiring layer
4
is examined for characteristics along its longitudinal direction, the signal wiring layer
4
shows local variations in characteristics, such as characteristic impedance. As a result, a signal transmitted through the signal wiring layer
4
suffers reflection, distortion, and other similar defects that potentially result in delays or errors in the signal transmission.
SUMMARY OF THE INVENTION
The present invention overcomes the shortcomings associated with conventional wiring substrates. An object of the present invention is to provide a wiring substrate that exhibits improved bonding strength and reliability, and in which variations in characteristics, such as characteristic impedance, are restrained with respect to signal wiring layers.
To achieve the above objects, the present invention provides a wiring substrate having a layered structure comprising a conductor layer having a plurality of through holes formed therein, a dielectric layer in contact with the conductor layer; and a signal wiring layer positioned in opposition to the conductor layer via the dielectric layer. The signal wiring layer is adapted to transmit a signal. The conductor layer includes (1) a wiring correspondence region corresponding to the signal wiring layer as projected onto the conductor layer in a thickness direction of the conductor layer, and (2) a wiring noncorrespondence region, which is the remaining portion of the conductor layer not including the wiring correspondence region. The through holes are arranged in the wiring noncorrespondence region of the conductor layer.
In the wiring substrate of the present invention, the through holes are formed in the wiring noncorrespondence region of the conductor layer remaining after removal of the wiring correspondence region. Since the signal wiring layer is electromagnetically coupled with the wiring correspondence region, in which no through holes are provided, the signal wiring layer is not prone to characteristic variations in electromagnetic coupling as observed along the longitudinal direction of the signal wiring layer. Thus, local variations in characteristics, such as characteristic impedance, can be restrained with respect to the signal wiring layer.
Notably, no particular limitation is imposed on the configuration of the wiring substrate so long as the signal wiring layer and the conductor layer are arranged in layers in such a manner as to face each other via the dielectric layer. The present invention includes, for example, the following configurations: a conductor layer underlies a signal wiring layer via a dielectric layer; and a signal wiring layer underlies a conductor layer via a dielectric layer. The present invention further includes a configuration, such as a strip line structure, in which a signal wiring layer is sandwiched between two conductor layers.
Preferably, the above-described wiring substrate is configured such that at least one of the through holes are formed in a side portion of the wiring noncorrespondence region, which is located in the vicinity of and along the wiring correspondence region.
As mentioned previously, the configuration in which the through holes are not formed in the wiring correspondence region restrains variations in characteristic impedance and like characteristics. Also, in the wiring substrate of the present invention, the through holes are formed in a side portion of the wiring noncorrespondence region which extends along the wiring correspondence region. Thus, regions in the vicinity of the through holes (i.e., the regions including the wiring correspondence region) exhibit, among other properties, good gas release properties and strong bonding between the upper and lower dielectric layers, thereby preventing generation of blisters in the wiring correspondence region of the conductor layer, and thus imparting high reliability to the wiring substrate.
Preferably, the above-described wiring substrate is configured such that the through holes formed in the side portion are elongated in a direction substantially parallel to the length direction of the signal wiring layer rather than along a direction perpendicular to the length direction.
In the wiring substrate of the present invention, the through holes have an elongated shape; thus, the through holes formed
Flynn Nathan J.
Greene Pershelle
NGK Spark Plug Company Limited
Sughrue & Mion, PLLC
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