Electricity: conductors and insulators – Conduits – cables or conductors – Preformed panel circuit arrangement
Reexamination Certificate
2001-05-25
2002-10-29
Paladini, Albert W. (Department: 2827)
Electricity: conductors and insulators
Conduits, cables or conductors
Preformed panel circuit arrangement
C174S261000, C361S760000
Reexamination Certificate
active
06472609
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a printed-wiring substrate having connection terminals, and more particularly, to a printed-wiring substrate having connection terminals on each of main and back faces thereof.
2. Description of the Related Art
Conventionally, a printed-wiring substrate having connection terminals on each of main and back faces thereof has been known.
FIG. 11
shows an example of a printed-wiring substrate
101
.
The printed-wiring substrate
101
generally has a plate-like shape, and has a main face
101
A and a back face
101
B. A large number of main-face-side connection terminals
103
are formed on the main face
101
A; and a large number of back-face-side connection terminals
105
are formed on the back face
101
B. An IC chip
111
indicated by a broken line in
FIG. 11
can be mounted on the main face
101
A by solder-bonding connection terminals
113
of the IC chip
111
to the main-face-side connection terminals
103
. Similarly, a motherboard
121
, indicated by a broken line in
FIG. 11
, can be connected to the back face
101
B by solder-bonding connection terminals
123
of the motherboard
121
to the back-face-side connection terminals
105
.
To prevent oxidation, each of the main-face-side connection terminals
103
of the printed-wiring substrate
101
, which are formed of copper connection pads, is covered with an Ni plating layer having a thickness of about 3 to 7 &mgr;m, and is further covered with a thin Au plating layer having a thickness of about 0.05 &mgr;m.
The reason for thinning the Au plating layer is that if the Au plating layer is thick, an inter-metallic compound is produced when the main-face-side connection terminals
103
and the connection terminals
113
of the IC chip
111
are soldered together, resulting in deteriorated solderability and connection reliability. Specifically, during the solder-bonding process, Au-Sn, which is a hard and brittle inter-metallic compound, is produced at the interface between solder and the main-face-side connection terminals
103
due to diffusion of Au contained in the Au plating layer of the main-face-side connection terminals
103
and Sn contained in solder. This reduces the bonding strength between the printed-wiring substrate
101
and the IC chip
111
. Accordingly, the Au plating layer is preferably rendered as thin as possible in order to solve the problem caused by production of an inter-metallic compound, to thereby enhance connection reliability with the main-face-side connection terminals
103
.
Similarly, each of the back-face-side connection terminals
105
of the printed-wiring substrate
101
, which are formed of copper connection pads, is covered with an Ni plating layer having a thickness of about 3 to 7 &mgr;m, and is further covered with a thin Au plating layer having a thickness of about 0.05 &mgr;m.
A printed-wiring substrate
201
as shown in
FIG. 12
has recently been developed as an improvement of the above-described printed-wiring substrate
101
.
In the printed-wiring substrate
201
, main-face-side connection terminals
203
are formed on the main face
201
A; and back-face-side connection terminals
205
are formed on the back face
201
B. An IC chip
211
can be mounted on the main face
201
A by solder-bonding connection terminals
213
of the IC chip
211
to the main-face-side connection terminals
203
. Similarly, a motherboard
221
can be connected to the back face
201
B not by means of solder, but through use of a socket or the like which establishes mechanical contact between connection terminals
223
of the motherboard
221
and the back-face-side connection terminals
205
.
3. Problems to be Solved by the Invention
When the printed-wiring substrate
201
shown in
FIG. 12
is used, the back-face-side connection terminals
205
are forcedly brought into contact with the connection terminals
223
of the motherboard
221
, and the contact may cause exfoliation of the Au plating layer formed on the surfaces of the back-face-side connection terminals
205
. In such a case, the Ni plating layer is exposed from portions at which the Au plating layer has exfoliated. This may result in problems such as an increase in the contact resistance between the back-face-side connection terminals
205
and the connection terminals
223
of the motherboard
221
.
SUMMARY OF THE INVENTION
The present invention has been achieved in view of the foregoing. It is therefore an object of the invention to provide a printed-wiring substrate capable of increasing connection reliability with an electronic component to be mounted on the main face as well as the connection reliability with another substrate to be connected to the back face of the printed-wiring substrate. Another object of the present invention is to provide a method for fabricating the printed-wiring substrate.
The above first object of the present invention has been achieved by providing a printed-wiring substrate having a substantially plate-like shape, a main face and a back face, and comprising main-face-side connection terminals for solder-bonding to connection terminals of an electronic component which is to be mounted on the main face; and back-face-side connection terminals for connecting, through mechanical contact, to connection terminals of another substrate which is to be connected to the printed-wiring substrate on the back-face side thereof, wherein the surfaces of the main-face-side connection terminals are coated with a main-face-side Au plating layer; and the surfaces of the back-face-side connection terminals are coated with a back-face-side Au plating layer which is thicker than the main-face-side Au plating layer.
In the printed-wiring substrate of the present embodiment having main-face-side connection terminals for solder-bonding to connection terminals of an electronic component and back-face-side connection terminals for connecting, through mechanical contact, to connection terminals of another substrate, the main-face-side Au plating layer is rendered relatively thin, and the back-face-side Au plating layer is rendered relatively thick.
Accordingly, only a small amount of inter-metallic compound is produced from Au of the main-face-side connection terminals and Sn contained in solder when the main-face-side connection terminals and the connection terminals of the electronic component are soldered together. Therefore, the solderability of the main-face-side connection terminals is enhanced, so that the reliability of connection between the main-face-side connection terminals and the connection terminals of the electronic component is improved. In addition, since the back-face-side Au plating layer has an increased thickness, the problem of exfoliation of the back-face-side Au plating layer with resultant exposure of a base layer and increased contact resistance does not occur even when the back-face-side connection terminals are connected to the connection terminals of another substrate through mechanical contact. Therefore, the connection reliability between the back-face-side connection terminals and the connection terminals of another substrate can be improved as well.
Moreover, in the above-described printed-wiring substrate, the main-face-side Au plating layer preferably has a thickness of not less than 0.03 &mgr;m but not greater than 0.12 &mgr;m; and the back-face-side Au plating layer preferably has a thickness of not less than 0.2 &mgr;m.
In the present embodiment, since the main-face-side Au plating layer preferably has a thickness of not less than 0.03 &mgr;m but not greater than 0.12 &mgr;m, oxidation of the main-face-side connection terminals can be reliably prevented. In addition, the amount of inter-metallic compound (e.g., Au—Sn inter-metallic compound) produced at the time of soldering the main-face-side connection terminals and the connection terminal of the electronic component can be reduced. Therefore, the solderability between the main-face-side connection terminals and the connection terminals of the electroni
Iba Masahiro
Kimura Kazuo
Sato Kazuhisa
Wakako Hisashi
NGK Spark Plug Co. Ltd.
Paladini Albert W.
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